The solar industry needs a low cost, high purity, and environmentally friendly source of silicon to achieve the rapid adoption of solar electricity and carbon-emission reduction goals needed by the world today. It is with these goals in mind that the Schumacher Process was developed.
The Schumacher Process
The Schumacher process differs from the Siemens Process in that it operates at significantly lower temperatures in a continuous loop rather than a batch process. Through the use of a fluidized bed (FB) reactor rather than a batch bell jar reactor, the Schumacher Process is able to achieve the following advantages over the incumbent manufacturing process:
- Lower capital cost to build the plant (CapEx)
- Lower cost to operate the plant (OpEx)
- Closed loop operation eliminates almost all waste (no hazardous material or explosive polymers)
Schumacher Process Diagram
The key to the Schumacher Process is the use of bromine rather than chlorine or monosilane as used in other Siemens and FBR processes. The process begins by reacting metallurgical grade (MG) silicon with hydrogen and silicon tetrabromide (SiBr4) to form a mixture of SiBr4 and tribromosilane (SiHBr3) which is separated and purified by distillation. The silicon deposition takes place in the FB reactor, a vertical column where the tribromosilane gas is injected upward from the bottom and high purity silicon is deposited on small seed particles. Offgasses from this reaction are reused in the SiBr4 synthesis stage. Once the deposition process achieves particles of the proper size, the pure silicon beads are removed from the reactor without interrupting the process.
Other FB processes use either silane (SiH4) or trichlorosilane (SiHCl3) in the FB reactor. Both silane and trichlorosilane are relatively small molecules. Therefore, SiH4 and SiHCl3 tend to easily cross from vapor to solid inside of the FB reactor. This results in vast quantities of unusable amorphous silicon dust. Some FB process engineers have attempted to reduce this phenomenon through introducing additional hydrogen into the process, but this raises the cost. Tribromosilane, on the other hand, as a result of the large size and mass of the bromine atom does not easily cross from vapor to solid, and as a result, no silicon dust is produced in the reactor.
When silicon is manufactured in an FB reactor using tribromosilane, large, dense spherical granules are routinely formed, under a wide tolerance of temperature and pressure combinations. Silane on the other hand, tends to generate silicon with a wide range of particle size that are filled with absorbed gasses. These gas pockets tend to cause the silicon beads made with silane to fly apart like popcorn during the melt stage prior to crystallization.
Schumacher Process Silicon
- Large uniform size beads (1mm diameter)
- No internal gas pockets
- No exterior oxidization
- No amorphous silicon dust
Other FB Silicon
- Narrow operating window
- Wide disparity of bead size (0.25mm to 4mm typical diameter)
- Internal gas pockets tend to pop like popcorn in melt prior to ingot manufacturing process
- Exterior oxidation makes beads harder to melt
- Large amounts of amorphous silicon dust raises cost of raw materials
Since other FB reactor silicon is difficult to melt and has a tendency pop like popcorn when heated, crystalline silicon ingot manufacturers have tended to blend bead (also known as granular) silicon with chunk silicon in their Czochralski (CZ) crystal growth pullers (monosilicon ingots) or Directional Solidification Furnaces (polysilicon ingots). In fact, they have to first melt chunk silicon in the furnace crucible, prior to adding granular silicon to the melt. This reduces the problems created by exterior oxidation and the popcorn effect found in granular silicon made from silane or trichlorosilane.
Granular silicon made with the Schumacher Process can be poured from a hopper directly into the melting crucible without the need to pre-melt chunk silicon. This reduces manufacturing time as technicians are no longer required to laboriously hand pack each crucible prior to starting the melting process. Furthermore, Schumacher Process silicon is ideal for advanced manufacturing techniques like Continuous Melt Replenishment (CMR) Czochralski processing, or Hot-Top-Off of CZ or DSS furnaces.