Challenges of hydrogen pumping in emerging applications

08/01/2008

Emerging applications, such as thin-film solar cell manufacturing in the solar industry and silicon/silicon germanium (Si/SiGe) epitaxy in the semiconductor industry, are using increasing levels of hydrogen gas in certain process steps. Because of its extremely small molecular size, hydrogen presents unique pumping challenges. This article discusses the factors leading to increased hydrogen use in these two processes, details the pumping challenges hydrogen presents for each, and highlights the vacuum pumping capabilities that are needed to resolve these challenges.

Regardless of the process or application, pumping hydrogen presents certain challenges because of the gas’ small molecular size and low viscosity, which is approximately half that of nitrogen. While these are not an issue when pumping at low pressures, they become a problem when the gas is compressed toward atmospheric pressure in a vacuum pump. As a result of its low viscosity, hydrogen tends to leak back through pump clearances, reducing the pump’s effective pumping speed.

Hydrogen also has a much higher thermal conductivity (7?? greater) than gases such as nitrogen. As a result, systems pumping hydrogen typically have a different thermal profile and different component dimensions than those pumping nitrogen.

A pump optimized to deal with hydrogen ideally operates with clearances set to account for the thermal properties of the gas and integrates a progressive nitrogen purge capacity to offset the challenges of pumping this lighter gas at higher pressures. Finally, since hydrogen is flammable if mixed with an oxidant such as air, the pump exhaust must be appropriately managed to avoid ignition.

Thin-film solar cell manufacturing

Manufacturing solar cells on a glass substrate using thin-film processing techniques is seeing growing adoption because of its potential to help lower the cost per watt per solar cell. This process involves the plasma enhanced chemical vapor deposition (PECVD) of silicon layers with different dopants to create the solar cells. The efficiency of this manufacturing process can be enhanced to 10% by using a layer of microcrystalline silicon to form a second p-n junction on top of a base amorphous silicon p-n junction device. Such a manufacturing technique requires a high proportion of hydrogen to silane (SiH₄), and hydrogen flow levels are still being optimized for this process. At the current time, they are typically in the range of several hundred standard liters per minute (SLM) at pressures less than 10 Torr.

To pump hydrogen and the other deposition gases, such as phosphine and silane, as well as chamber cleaning gases containing fluorine, the vacuum systems used in this process must meet certain basic requirements. They should incorporate large backing pumps and larger booster pumps to handle the high gas flows required to manufacture large panels at the low pressures needed to pump hydrogen.

They must be able to easily expand or contract pumping capacity to accommodate increases or decreases in process flows, which requires a modular combination of pump units—sometimes in multiple tiers to optimize the low pressure operation and compression ratio when pumping high flows of hydrogen—coordinated by a single controller. The single controller should be capable of coordinating the entire pumping system as if it were a single pump set from the process chamber point of view to ensure safe operations. The modules should also be designed to minimize the frequency and difficulty of maintenance operations.

Diluting the hydrogen with nitrogen can improve pumping performance. In addition, if any oxidant is present, some dilution may be required to ensure sufficient gas forward velocities to minimize (in combination with proper abatement design) the risk of flashback into the exhaust.

Finally, the pumping system must be corrosion-resistant to withstand the fluorinated gases often used in chamber cleans and robust enough to handle process by-products such as silicon-rich powder. Ideally, the pumping system should be followed by an all-dry abatement system to burn the exhaust gas, and the resulting powder-laden inert gas stream can be treated separately in a cost-effective manner.

Epitaxy

Silicon and silicon germanium epitaxy is an increasingly important process used in transistor formation at the 45nm node in semi-conductor manufacturing. As in thin-film solar cell manufacturing, the process uses a high flow of hydrogen gas. The gas helps promote the growth of crystalline silicon for silicon-on-insulator (SOI) wafers or of crystalline silicon germanium in the transistor source/drain region to provide compressive strain in p-type devices.

The pressure range in this process is greater than those in thin-film solar manufacturing. Epitaxy process pressures are typically =10 Torr, although the trend is to reduce pressure. Hydrogen flows are typically up to 100 SLM.

In expitaxy processes, the vacuum pumps used must be able to meet the challenges of hydrogen pumping and also be capable of handling the solid or semisolid process by-products such as silicon dusts and chlorosilane polymers. The solid by-product quantities produced in epitaxy processes are relatively modest, and if an abatement unit is located close to the pump and the connecting pipework is maintained at elevated temperatures using active pipework heating, then the by-products can generally be conveniently managed using combustion, combined with wet scrubbing. In some cases, however, these by-products can be unstable, and special handling of contaminated components may be recommended by the process tool manufacturer. In addition, some epitaxy process tools use hydrogen chloride (HCl)-based chamber cleaning processes. The vacuum pumping system must, therefore, be resistant to HCl as well.

Conclusion

A number of emerging applications in both the solar and semi-conductor industries require the use of hydrogen gas during the manufacturing process. Due to its small molecular size, low viscosity, and flammability, hydrogen gas poses definite challenges for vacuum pumping systems. Modular dry pumps with an integrated controller provide an excellent solution to these challenges. They must be able to pump both hydrogen and other gases at low pressures and at the high volumes required by the various processes. They must also be corrosion-resistant and able to handle the process by-products in a cost-efficient and safe manner. Appropriate abatement capability is also required to manage process gases and by-products in the exhaust stream safely and at minimal cost.

Michael Percy received his MA in physics from the U. of Cambridge. He has worked in the field of vacuum for more than 20 years. Percy is the business manager for dry pumps at Edwards Ltd., Manor Royal, Crawley, West Sussex, RH10 9LW, UK; ph +44 (0) 8459 21 22 23; e-mail [email protected].

Mike Wilders received his BSc in applied physics from the U. of Bath and is an applications manager in the pumping group at Edwards. He has worked in the fields of vacuum and semiconductor processing for more than 20 years.