Edwards’ Spectra Z 3000™

Exhaust Gas Management Solutions for Thin Film Silicon Solar Cell Manufacturing

Solar cell manufacturing is becoming increasingly important as an environmentally-friendly means of generating electrical power, and the growth of thin films of microcrystalline silicon will form a key part of this. The PECVD growth of this material poses a number of significant challenges regarding the treatment of the resultant exhaust gases, in terms of removing toxic, corrosive, global warming, and flammable gases safely, at low cost, whilst also minimising the environmental impact.

There are a number of competing technologies for the manufacture of solar cells, the most widely-used being silicon wafer-based crystalline cells, Of growing interest, however, is the competing technology of thin-film silicon solar cells, where microscopically-thin layers of silicon are deposited on large-area substrates to produce low-cost solar cells.

The efficiency of simple devices is significantly inferior to that of crystalline cells (~6-8%), but by combining 2 layers of differing material properties, the resulting structure is, in effect, 2 cells-in-one with efficiencies approaching 10% [2], yet can be produced in large-area format at low cost.

To produce these twin cell structures, a conventional amorphous silicon layer is combined with a microcrystalline (mc-Si) layer; both deposited using extremely large capacity plasma-enhanced chemical vapour deposition (PECVD) equipment. Although the chemistry resembles that used to make semiconductor chips, the gas flows are orders of magnitude greater, as are the process step times.

One aspect both processes have in common is the use of high flows of silane, which is activated by the plasma in the process to deposit silicon.
Not all the silane will react in the process chamber; in fact process chambers are designed so that this does not happen in order to maintain acceptable levels of film thickness uniformity (technically so that the gas stream does not become depleted of reagents). However, this means that there will be significant flows of unreacted silane in the process exhaust.
In addition, there is the need to routinely remove the accumulated deposits of silicon from the process chamber walls; were this material to build-up above a certain critical thickness it would start to flake-off, and the fragments of solid silicon would seriously degrade the quality and properties of the solar cell structure. To avoid this situation, the entire chamber is periodically given an in-situ cleaning, which involves using a fluorine-containing gas such as NF3 or SF6 in a plasma to generate highly active fluorine species (F•) which subsequently react with the chamber wall deposits to form volatile silicon tetrafluoride gas (SiF4)

The orders of magnitude of these by-products are of the order of 20-30 slm of silane, and 100’s slm of hydrogen. In addition there is some 20 tonnes typically a year of silica as waste."

The major challenge is to deal cost effectively and in an environmentally sound manner with these high gas flows and to deal with the solid by-products.

Description

The Spectra-Z is a high-capacity combustion system that provides reliable, high-performance and cost-effective abatement of hazardous gases from solar cell processes that use large flows of silane (SiH4). The Spectra-Z employs 4-stage combustion, an effective powder handling system and can be configured with up to six process gas inlets. The Spectra-Z has low CoO, no water or oxygen required and is suitable for indoor or outdoor installation.
Applications include: PECVD, a-Si/μ-Si deposition, LPCVD
Gases abated : SiH4, PH3, TMB, NF3
Capacity: 800 slm and 3,000 slm

Innovative aspect

The conventional way to deal with these products is to burn these waste products, then wet scrub.

Edwards’ Spectra Z 3000™ provides an all-dry combustion abatement solution that meets many customers’ needs, especially in terms of minimising costs, typically discharging into a facility central. It is able to treat up to 3000 slm of exhaust gas flow and does not require any water whatsoever; it can, with the process exhaust diverted to a dedicated Thermal Processor Unit (TPU) meet the strictest emission requirements such as the German TA Luft.

The fact that the Spectra Z uses no water whatsoever, avoids the high cost and utilisation of an expensive – and in some regions – extremely scarce commodity;

It also avoids the need to deal with the sludge associated with the wet scrub method, the powder being collected dry.

By paying careful attention to the chemistry of both the deposition and chamber cleaning steps, exhaust management solutions have been developed that can meet the challenges of thin film silicon solar manufacturing whilst minimising the costs.