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An Amateur Explanation Of The FBR Process

In this short article I will try to make a rudimentary example of how an FBR process works, keep in mind this is written by an amateur.

First things first, not all of the readers might know exactly what a FBR granular product is? FBR stands for Fluidized bed reactor, in the solar industry it is used to make granular polysilicon. These are small circular shaped silicon of high purity that is used to melt into bigger blocks of silicon called ingots. Another such starting material for ingots is called Simens chunks. These are irregular in size and is broken into chunks because the original form is bigger rods that usually don't fit into a melting crucible to make ingots so they are broken down into chunks.

These blocks of silicon (ingots) are then cut into thin slices called wafers, then treated to become solar cells that produce current when the suns irradiance hits the cell. Finally the cells are connected together into a solar module that transfers the current of the cells into electricity to the end user who can put it on his/her roof and connect to inverters and to the grid to have power.

The basic principle of the FBR granular polysilicon process is that of introducing tiny seed material, for example quartz, or anything that has a purity of silicon is dropped into a tank from the top of the tank. At the same time from the bottom of the tank you introduce gases. One gas should deposit silicon onto the seed, while the other could be a regulation gas to keep the silicon deposition optimal. In a section (usually the middle) you have such warmth that the gas forms several small bubbles (more heat = more bubbles) and the heat is enough to deposit silane onto the seed material, this achieves two things: 1. the particles have silicon deposited on them from the gas, 2. the bubbles makes the seed particles bounce around inside the tank making them have silicon deposited on all of the surface of the particles. When you want to stop the production you add cooling gases that also cleans the walls.

So yeah, it is kinda like a popcorn machine….

While FBR have many advantages like its granular small shape there are some problems, the main problems is a lower grade of purity, usually just solar grade, and the other problem is dust creation.

Lower grade of purity: Anything the seed particles is in contact with (or gases introduced to the chamber) that is not silicon will make the final product contaminated. Just bouncing off a wall inside the tank could make metal that the tank is made of deposited onto the seed material. To counter this REC is now doing several patents, basically what all of them is trying to achieve is to lessen contamination when a seed particle hits something, be it by adding a protective layer to the tubes outside the tank, or to add protective layers inside the tank itself. MEMC who already produce at higher purity levels seems to have solved this in another way by having 3 different tank reactors, doing the process in 3 steps is probably more costly but it allows them electronic grade quality on the granular product.

Dust creation: The other problem is that the seed particles are tiny in size and some of them never gets to form big enough sizes to become granules. These "sand" particles sticks to the walls, the tubes and just about everywhere inside the tank and is generally considered a low value product. This is one of the reasons for introducing another gas (for example hydrogen) to the process as to make just the optimal amount of the silicon depositing gas (for example silane gas) inside the tank with the right pressure, this causes the least amount of dust creation. Other workarounds is to melt the dust into something more useful (REC is having patents on this) or to coat the tank in such a layer that dust will not be formed or stuck onto the tank itself. The dust removal is a major part of why MEMC has high quality FBR, they take great care in the first tank to allow all the seed to grow to a certain size before introducing it to a 2nd tank where the seed grows into a granule. This causes less impurity on the seed as moving it from the first tank to the 2nd tank obviously causes less dust creation in the 2nd tank. Then finally a 3rd tank is used to finish the product and dust is cleansed of the product.

Fbr example