Single Crystal Growth of Silicon by Float Zone Method

WaferPro
WaferPro
Jul 22, 2017 · 3 min read

Silicon is one of the most plentiful solid element in the world, being 2nd only to oxygen as well as it composes over 25% of the world’s crust. It rarely happens in elemental form, basically all of it is presenting as compounds.

Float-zone silicon is a high purity option to crystals grown by the Czochralski procedure. The concentrations of light impurities, like carbon and also oxygen, are very low. One more light impurity, nitrogen, really helps to manage microdefects and produces a renovation in mechanical strength of the wafers, as well as is currently being intentionally added throughout the development phases.

The float zone technique

The float Zone (FZ) process is based upon the zone melting concept as well as was discovered by Theuerer in 1962. The manufacturing occurs under vacuum or in an inert gaseous environment. The procedure begins with a high purity polycrystalline rod and also a monocrystalline seed crystal which are held face-to-face in an upright position and also are rotated.

Having a radio frequency field both are partly melted. The seed is raised from below to get contact with the drop of melt formed at the pointer of the poly rod. A necking procedure is executed to develop a dislocation free crystal prior to the neck is enabled to expand in size to form a taper and also get to the wanted size for steady state development. As the fused zone is moved along the polysilicon rod, the fused silicon solidifies right into a single Crystal as well as, at the same time, the component is cleansed. Regular oxygen and also carbon levels in FZ silicon are below 5 1015 cm-3. FZ crystals are doped by including the doping gas phosphine/diborane to the inert gas for n- and also p-type, specifically. Compared to CZ growth, the silicon molten Zone is not touching any materials other than ambient gas, that could only have doping gas. FZ silicon can conveniently get a lot higher pureness and also increased resistivity.

Moreover numerous zone refining can be done on a rod to even further minimize the impurity concentrations. Once more the efficient segregation coefficient k has a crucial role. Boron, as an example, has an equilibrium segregation coefficient of k0 = 0.8. In comparison to this phosphorus can’t be segregated (k0 = 0.35) however as well vaporizes from the melt at a relatively high rate. This is the reason that on the one hand it is less complicated to create even more uniform p-type FZ compared to n-type FZ as well as alternatively high resistivity p-type silicon can just be acquired from polysilicon with low boron content. Dopants with a minimal k0 just like Sn can be presented by pill doping — gaps are drilled right into the ingot into that the dopant is included — or by vaporizing a dopant layer overall ingot prior to the float zoning procedure.

Also read more about “Float Zone Wafer — Characteristics and applications

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