Getter purification

Zirconium alloys with aluminum (Zr-Al), iron (Zr-Fe), and vanadium (Zr-V-Fe) have been used for gas purification for over 30 years. The getters are typically used in a porous pellet form of various sizes. The zirconium metals are very reactive with a wide variety of gas species such as O2, H2O, CO, CO2 and N2. The chemisorption reactions are irreversible and form oxides, carbides and nitrides via the following reactions:
CO + Zr -> Zr-O + Zr-C
CO2 + Zr -> Zr-C + 2 Zr-O
N2 + Zr -> 2 Zr-N
O2 + Zr -> 2 Zr-O

Hydrogen sorption is governed by a different reaction where hydrogen is dissociatively chemisorbed and dissolved into the bulk of the metal getter:
H2O + Zr -> Zr-O + 2H (bulk)

Zirconium getters are operated at elevated temperatures, typically 350-450C, depending on the gas to be purified and impurities to be removed. The chemical reactions shown above occur on the surface of the metal, and the reaction products then diffuse into the bulk structure, providing clean surface for renewed adsorption and reaction. This reactivation process is carried out continuously at high temperatures.

The capacity of getter can be as much as 25 times that of ambient metal-supported catalyst media since getter can use the bulk of the media for adsorption while other catalysts rely only on surface reations.

Outlet purity from a getter purifier has been shown to be in the single digit ppt range under optimum operating conditions. Because of its' ability to removal virtually all impurities from rare gases, getter media is used to provide zero gas for the most sensitive analytical instrumentation.

Getter purifiers are used for rare gases including argon, helium, krypton, xenon as well as nitrogen and hydrogen. The most common application for heated getter purifiers is rare gases since it can remove nitrogen, hydrogen and hydrocarbons.

Advantage	Benefit
Longer lifetime than ambient catalyst media helps reduce maintenance frequency	Provides lower cost-of-ownership and allows use of smaller vessels to reduce cost and overall footprint
Only technology that can remove N2 and all hydrocarbons from rare gases such as argon and helium	Prevents nitride and carbide inclusions during crystal growth and sputtering processes
Heating and control system used to maintain getter temperature also provides continuous monitoring and feedback on purifier performance	Easy to confirm purifier performance

Limitation	Response
Outlet purity can decrease momentarily during spikes in flow or inlet impurity due to reduced residence time of the gas within the getter vessel.	Be sure the purifier is properly sized so that maximum expected flows are still well within the maximum flow rating of the purifier
Efficient removal of impurities such as CH4, CO and CO2 may be compromised at higher inlet levels.	Be sure the getter vessel is sized to provide adequate residence time for removal of all impurities at maximum expected inlet levels
Heating and control system increase initial purchase price and utility requirements as compared to ambient catalyst media	Costs recovered over time due to longer lifetime, reduced downtime and elimination of regeneration steps

Application	Gases Used	Process Benefit
Silicon crystal growth	Argon	Removes oxygen, carbon and nitrogen impurities to prevent crystal defects
Silicon carbide crystal growth	Argon	Removes oxygen, carbon and nitrogen impurities to prevent crystal defects
Physical Vapor Deposition and Sputtering	Argon, Nitrogen	Argon purifiers remove oxygen, carbon and nitrogen impurities to improve wafer uniformity.  Hydrogen removal helps improve process chamber pumping speeds. Nitrogen purifier helps prevent introduction of impurities during loadlock purge sequence.
Zero gas for analytical instruments	Nitrogen, Argon	Removes all impurities to provide baseline for APIMS, mass spectrometers and other ppb-level instruments
Optical Fiber Manufacturing	Helium	Removes all impurities to prevent loss of attenuation in optical fiber