The FZ (float zone) gas-phase doped silicon single crystal with high purity, few defects, low compensation, and low oxygen and carbon content can be supplied by PAM-XIAMEN. It is widely used in various high-sensitivity detectors and low-loss microwave devices. To get more specifications of FZ silicon, please refer to https://www.powerwaywafer.com/silicon-wafer/float-zone-mono-crystalline-silicon.html. For all parameters, the variation of radial resistance is an important parameter index of FZ silicon single crystal. The radial resistivity variation (RRV) is the difference between the resistivity of the wafer center point and a point or several symmetrically distributed set points offset from the wafer center, and can be expressed as a percentage of the center value.

The non-uniform distribution of the resistivity of the silicon single crystal will adversely affect the uniformity of the device parameters. If the axial resistivity of the silicon is not uniform, the reverse withstand voltage, forward voltage drop, power, etc. of the devices made from different wafers will be different; while the radial resistivity variation of silicon is not uniform, it will make the large-area device current. The distribution is uneven, local overheating occurs, and local breakdown occurs, thereby reducing the withstand voltage and power indicators of the device. So what will affect the radial conduction resistance of FZ silicon?

1. What Affects Radial Resistance of Monocrystalline Silicon?

The gas-phase doping process results in resistivity drift and resistivity varies. The main factors affecting radial resistance of silicon crystals in the gas-phase doping are thermal convection, crystal rotation, pulling speed and etc. The details are as follows:

1.1 Effect of Heat Convection on Radial Resistivity Uniformity

The smaller the diameter of the quartz crucible, the shallower the melt depth, and the better the radial resistivity uniformity of the single crystal silicon. Due to the temperature gradient of the silicon melt in the quartz crucible, thermal convection is induced by the buoyancy force generated under the action of the gravitational field. The heat convection rises along the crucible wall and descends to the center of the crucible, so that the heat convection makes the temperature of the melt at the edge of the single crystal growth interface higher than the center, so that the growth interface protrudes toward the melt. The stronger the thermal convection, the more likely the interface is convex toward the melt. The interfacial facets that are convex to the melt appear in the center. Due to the facet effect, the radial resistivity appears to be lower than the edge in the middle, resulting in uneven radial resistivity. At the same time, due to the temperature oscillation generated by the turbulent nature of thermal convection, the thickness of the impurity boundary layer is different everywhere, resulting in uneven radial distribution of resistivity.

1.2 Influence of Crystal Rotation on the Uniformity of Radial Resistance

The electroactive impurities in the silicon single crystal are boron impurities and phosphorus impurities, and the resistivity and conductivity type of the single crystal are the result of the mutual compensation of the two impurities. For the P-type high-resistance single crystal, the boron impurity concentration is higher than the phosphorus impurity, while for the N-type single crystal, the phosphorus impurity concentration is higher than the boron impurity. When a single crystal grows, due to the segregation of impurities, an enriched layer of phosphorus impurities is generated in the liquid phase near the solid-liquid interface (the segregation coefficient of phosphorus is 0.35, and the coagulation coefficient of boron is 0.9). Under the action of multiple factors such as force and gravity, phosphorus impurities are distributed according to a certain law on the melt and crystal interface. Usually, the concentration of phosphorus impurities in the central region is higher than that in the edge region, so for P-type single crystal, the performance is For N-type single crystal, the resistivity of the central region is high, and the resistivity of the edge region is low.

Increasing the crystal rotation speed will increase the high-temperature liquid flow moving upward under the solid-liquid interface, inhibiting the thermal convection. When the forced convection of the crystal transfer is dominant, the growth interface changes from convex to flat, or even concave to the melt. In this way, it is beneficial to curb the appearance of facets. The facet effect will combine the impurity atoms originally adsorbed at the solid-liquid interface into the crystal, resulting in the difference of impurity segregation.

Increasing the crystal rotation reduces the thickness of the impurity diffusion boundary layer, thereby reducing the concentration difference of the impurity diffusion boundary layer, thereby reducing the difference in impurity segregation, weakening the facet effect, and improving the uniformity of single crystal radial resistivity.

1.3 Effect of Pulling Speed on Uniformity of Radial Resistivity

Increasing the pulling speed increases the solidification speed of the crystal, and as a result, a part of the crystal protruding from the growth interface will be melted, so that the interface tends to be flat, which is beneficial to suppress the appearance of facets.

2. How to Calculate RRV Value?

To calculate the radial resistance variation, we firstly should use the 2-probe method, 4-point probe method and others to test the resistivity of single crystal silicon. Then, the radial resistivity variation measurement is through the formula: (MaxR – MinR)/MinR

MaxR: the maximum resistivity value of the tested silicon ingot

MinR: the minimum resistivity value of the tested silicon ingot

Take the following radial resistance values tested by us for example:

3. FAQ of FZ Silicon Ingot

Q1: Do you start with undoped polysilicon rods and dope from gas phase during FZ crystallization or do you start with doped ingots and use the FZ crystallization primarily to recrystallize and eliminate Oxygen?

A: Dope from gas phase during FZ crystallization.

Q2: What is the radial and axial resistivity uniformity for your FZ ingots?

A: If Gas Phase Doping, RRV of FZ silicon ingot is about 20%;
If NTD, RRV is about 12%

Q3: How easy is it for you to hit a resistivity target such as 300±20 Ohmcm?

A: Not easy, We adopt NTD to meet resistivity of silicon crystal at 300±20Ωcm;
If Gas Phase Doping, we can meet the resistivity at about 300±60Ωcm.

For more information, please contact us email at victorchan@powerwaywafer.com and powerwaymaterial@gmail.com.