Wafers are thin slices of semiconductor material used for manufacturing integrated circuits and other microdevices. They serve as the base substrate for all our technologies. Depending on the application, you can choose among various types, from different materials to wafers made with varying growth methods. FZ wafers, for example, stand out as one of the highest-quality options.
Nevertheless, to understand the usability of float zone (FZ) silicon wafers, it's important to understand their conductivity and how they compare to CZ silicon. To help you understand that, we created this guide.
The ability of a material to permit electrons to move and carry electrical current is known as conductivity. It is a critically important concept in wafer manufacturing, as controlling conductivity is essential for creating transistors, diodes, and other devices. Using doping to control wafer conductivity precisely has the following effects:
On the other hand, poor conductivity can cause inconsistent performance, eventually shortening a product's lifespan and reducing yield.
Two important factors determine a substrate's conductivity:
To measure wafer conductivity, techniques such as the four-point probe are used. During this method:
This method accurately measures a wide range of doping levels and surface resistivity. Moreover, it's minimally destructive, as it can be done on test wafers or edge dies.
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If conductivity plays a big role in determining a wafer's properties, understanding different silicon conductivities allows manufacturers to pick the ideal substrate for their projects.
Float zone silicon is extremely pure silicon obtained by vertical zone melting. However, one negative aspect of float zone silicon is that its wafers are generally not greater than 150mm due to the surface tension limitations during growth.
On the other hand, most of the silicon produced commercially is Czochralski silicon. This process forgoes purity in favor of low cost, high production speed, and ideal resistance to heat stress. The typical diameter of their wafers is between 75 and 200 mm.
Moreover, CZ and FZ silicon offer different conductivity properties.
In semiconductors, a material's intrinsic purity is its degree of freedom from undesired impurities or flaws in its crystal lattice. It describes the chemical and structural cleanliness of the semiconductor material in its native (undoped) state.
Float Zone (FZ) wafers are much purer than Czochralski (CZ) wafers. FZ wafers contain very low levels of oxygen and carbon. Meanwhile, CZ wafers are grown in a quartz crucible, typically absorbing more impurities like oxygen and carbon.
Resistivity in semiconductors is a critical parameter that measures the opposite of conductivity—how much the material opposes the flow of electric current. It is a material's inherent quality unaffected by its area or length. Good conductors and insulators have resistivity values in the middle of those of semiconductor materials.
Because FZ wafers are purer, they can have much higher resistivity, which allows them to have very low base conductivity if desired. On the other hand, CZ wafers have a higher natural impurity content, which, even before deliberate doping, slightly raises their conductivity.
As a result, CZ wafers typically have higher base conductivity or lower resistivity.
In semiconductors, the term "controlled doping range" describes the range over which the concentration of dopant atoms—intentionally added impurities—can be precisely changed to alter the semiconductor material's electrical conductivity.
Due to the highly precise controllable doping levels, FZ wafers support a much wider and higher resistivity range. This makes them ideal for applications that need very low conductivity. In contrast, accidental dopants such as oxygen usually limit CZ wafers to lower resistivity values.
Conductivity uniformity in semiconductors refers to the consistency of the electrical conductivity across a semiconductor wafer's entire surface and depth. Simply put, all the wafer's components behave electrically similarly, which is essential for producing dependable, high-yield chips.
FZ wafers' consistent crystal structure and low impurity levels provide exceptional conductivity uniformity. On the other hand, because oxygen clusters can form during the crystal growth process, CZ wafers have good conductivity but may be slightly less uniform.
CZ wafers are cultivated in a quartz crucible, absorbing oxygen and other contaminants from the surrounding air. This results in accidental doping, which marginally raises conductivity.
To minimize contamination, FZ wafers are grown in an inert gas atmosphere or vacuum without a crucible. This results in higher purity and much tighter control over doping, which, unless doped purposefully, results in lower conductivity.
Float zone wafers are used primarily for applications that require very high resistivity or the absence of oxygen for good device performance. These include discrete power, MOS power, high-efficiency solar cells, and RF/wireless communication chips.
It is also used in power electronics and high-voltage components that require high breakdown voltage, low leakage currents, and precision doping.
On the other hand, CZ wafers are dominant in standard IC fabrication (CMOS, memory, logic). It's ideal for projects where:
Due to superior purity and doping control, FZ wafers can achieve much lower conductivity (higher resistivity). On the other hand, CZ wafers naturally have higher base conductivity because of higher impurity levels, especially oxygen.
In applications where conductivity must be minimized or tightly controlled, FZ is preferred. For general-purpose ICs, CZ is more economical and sufficient. Still, if you're unsure what type of wafer is best for your project, Wafer World can help. Contact us today for more information!
