Polysilicon deposition reactors

In the polysilicon deposition process, trichlorosilane (TCS) gas is decomposed to polycrystalline silicon in a chemical vapor deposition reactor, a large water-cooled steel chamber.

Heated silicon seed rods, called filaments, act as nucleation sources for the trichlorosilane gas. The filaments, assembled into U-rods, are loaded into a clean reactor, securing the ends of the filaments into graphite chunks/electrodes inserted into the baseplate.

The bell jar is then lowered onto the baseplate by a hoist system. After a thorough purge cycle, the filaments are pre-heated by graphite heating rods, and then heated by electrical resistance to a temperature about 1100 degrees C. Trichlorosilane (TCS), heated to a vapor, is carried into the reactor in a hydrogen gas carrier stream.

Polysilicon deposition reactors in production

A gas distribution system, a series of nozzles, is designed to produce uniform polysilicon deposition across the diameter and throughout the length of the rod.

The decomposition temperature, gas flow and reactor vent system are designed for maximum deposition of silicon from the trichlorosilane. The decomposition equation is not quantitatively defined, but is recorded as:

SiHCl₃   =>    Si  +   SiCl₄  +   H₂ ,  with several other chlorosilanes, (SiH₂Cl₂, SiHCl, SiCl₂, HCl, and silane polymers also formed. About 10 kg of trichlorosilane are required to produce 1 kg of polysilicon, so the exhaust gases must be recycled for the process to be cost-effective.

The elemental silicon content of the TCS molecule is 20.7%. Traditionally, first pass conversion efficiency from TCS is 8 to 11%. Conversion efficiencies depend on source gas mixture, reactor design and operating conditions (Si:Cl ratio, pressure, temperature). With an advanced reactor design for solar grade polysilicon, conversion efficiency can range up to ~20%.

Polysilicon deposition reactors in the plant

Deposition Reactor Design and Performance

 The deposition reactor consists of a steel baseplate, a large water-cooled specialty steel bell jar, and electrical, gas, vacuum, and water utilities.

The baseplate has feed-through holes for electrical connections to the filament and the pre-heat rods, a gas distribution system, a vacuum system and the gas venting system.

The water-cooled steel bell jar is raised and lowered by a hoist system to provide access to the reaction chamber.

The bell jar has view ports for operator observation of the process and for a pyrometer that monitors temperature of the growing polysilicon rod and adjusts power to maintain deposition temperature.

Gas flow is increased as rod diameter increases. Electrical power, gas flows, water flows, temperature, and pressure are measured and controlled by a computer system.

The computer system is configured for process safety, with identification and warning of electrical shorts, gas leaks, over-temperature and over-pressure limits, and automatic process shutdown.

Reactor design and performance is evaluated by the characteristics of the polysilicon rods produced, deposition rate, electrical power consumption, conversion efficiency, production rate, and number of aborted run cycles.

A special fast-rate deposition reactor has been designed for deposition of solar grade polysilicon. This reactor uses high pressure, up to 90 psi, to accomplish deposition rates about 30 kg/hr.

Reactors are designed for production of 36 rods or 48 rods, with poly rods at 125 mm diameter x 2 meters length. These reactors can produce 150 metric tons per year (MTY) of solar grade polysilicon and are available commercially.

Photo Gallery of polysilicon deposition reactors (chemical vapor deposition reactors)

	Polisilicon deposition reactor in early stages of fabrication
	A high-volume polysilicon production plan will have a corresponding quality of polysilicon deposition reactors.

Key Words:  polysilicon  (polycrystalline silicon) deposition, chemical vapor deposition, thermal decomposition of trichlorosilane, polysilicon rod production