Saturday, July 4, 2009

When Good Gas Goes Bad: Gas Purifiers At Work

Once obsessed with metallic impurity levels, reducing variations in the delivered gas purity has become the primary focus of contamination control engineers managing gas distributions systems. When used correctly at bulk sources or at the point-of-use, gas purifiers can remove harmful impurities down to the ppt range.

Metal contaminant levels are now only one of several challenges when it comes to gas purity for semiconductor processing. Even when a gas supplier delivers the highest grade of ultra-high purity (UHP) process gas available to meet process specification, impurities from process lines and container valving within a fab can still contaminate the system before it enters the tool or point-of-use (POU).

While filters are a necessary part of any gas delivery system, filtration alone cannot remove all impurities in a gas stream. Many filters are particle size specific and can only remove individual groups of particles. Filters also cannot remove moisture, and some moisture can even be adsorbed on PTFE filters. In fact, some filters can be a substantial source of moisture contamination if not properly introduced to a dry gas line system.

To address these issues, gas purifiers are incorporated into the gas flow lines — removing, in some cases, 99.9999999% of specific impurities.

Local purifiers can maintain and ensure ultimate purity of gases at the point-of-use, even though as-delivered bulk gas purity levels appear to be sufficient for multiple generations of devices. The main focus will be reducing variations in purity rather than continuing to improve gas specifications. Figure 1 shows where purifiers could be located in a gas delivery system.

Frequent cylinder changes will increase the risk of the introduction of human errors and varying levels of contaminants to the process tools and gas stream, while exposing personnel to possible gas leaks. Gases that are used in large quantities, like bulk N2, H2, or silane (SiH4) and ammonia (NH3), used for CVD and epitaxy, are usually delivered to the fab in bulk containers (i.e. isocontainers, trailers, or tanks), located in the fab building or in an exterior storage site. Bulk delivery systems usually include gas purifiers installed in the delivery lines, positioned at various points along the way, as shown in Figure 1. These purifiers remove trace amounts of contaminants introduced from gas containers and valving or delivery lines.

For more toxic or corrosive gases such as arsine (AsH3), or phosphine (PH3) used for MOCVD and ion implantation, and HCl for etching and cleaning, the gases are usually stored in sub-atmospheric cylinders to prevent accidental releases and some have built-in filters and purifiers that also minimize inter-cylinder and intra-cylinder variability as well as residual pressure valves to eliminate back contamination.1,2

(Click Image For a Larger Version)

Lithography purge gases, such as critical clean dry air (CDA), nitrogen (N2), and helium (He), are used to keep optical surfaces clean and to provide a consistent environment for the light beam. Gas purifiers are essential to maintaining process control.

Helium is used for some lithography tools as a purge gas as well as a backfill gas for the tools’ lasers. In this application, the gas must meet the most stringent requirements for NH3 content (150 ppt), total acids (25 ppt), and organics (100 ppt), as shown in Table 1. Hydrocarbons with high molecular weight (C6 – C30 compounds) and high boiling point acids, bases, and refractories (airborne contaminants) are detrimental even in small quantities. These contaminants will adhere to exposed surfaces and leave nonvolatile residues such as carbon, SiO2, and inorganic salt deposits on lenses and mirrors, diminishing the transmissivity of the optical beam path, altering critical dimensions of fabricated devices, and ultimately damaging optical surfaces.3 If left unchecked, these contaminants can also collect on sensors and other sensitive components, causing problems with tool performance and integrity. Purifiers that convert hydrocarbons to H2O and CO2 and purifiers used to remove moisture and damaging contaminants from the purge gases will keep stepper lenses and optics assembly clean.

These purge gases (with the exception of CDA) are stored on-site in cryogenic vessels or supplied in bulk trailers, with gas purifiers commonly located within the cylinder or at the bulk source. The large bulk purifiers (>20k L/min) are often owned and operated by the gas supplier. While obtaining and delivering ultra-high purity materials is no longer a challenge for most gas suppliers, the vapor purity from these products can still vary widely. To safeguard ultimate purity, these gases all pass through a purifier before use.

Purification of ambient air in a lithography cleanroom can also keep contaminants to a minimum.4

To prevent reticle haze, built-in airborne molecular contaminant (AMC) purifiers in reticle pods can provide a constant purge of acids, bases, and organics. Haze is typically caused by ammonia, SO2, and moisture, in combination with laser light exposure. The pod can also be purged with XCDA (critical clean dry air) to prevent moisture from contributing to haze. The XCDA requires a separate purifier system.

(Click Image For a Larger Version)

The 2005/2006 edition of the ITRS shows declining tolerable levels of impurities for corrosive etchants and deposition gases at the sub-65nm technology nodes for metal impurities and at the ≤45nm nodes for H2O and O2 — contaminants especially attributed to containments and delivery lines. Figures 2 and 3 show the tightening of impurity requirements as technology nodes move forward for various deposition, etch, and purge gases.

Current acceptable contamination levels for dielectric CVD precursors are <1000>2O and O2.5,6 These levels may be lower depending on the material being deposited. Films like silicon nitride and conductive films are much more sensitive to oxygen. In addition, new materials used to deposit hafnium oxides and zirconium oxides have been found to be particularly sensitive to moisture. In some precursors, moisture and oxygen may react and degrade the material’s assay or form other contamination byproducts, thus altering their physical and chemical properties. The result is an uncontrollable and unpredictable deposition rate.

Many of the advanced specialty gases, such as those used for atomic layer deposition (ALD) or front end dielectrics, are delivered in smaller volumes, increasing their sensitivity to contamination effects and degradation. These gases are not inert and have a greater tendency to degrade in the distribution system due to materials of construction, atmospheric contamination, thermal degradation, etc.

Some of the most demanding applications include low-temperature epitaxy and associated cleaning where deposition temperatures are lower, making the process sensitive to fluctuations in purity levels. The ITRS recommends using gases with purity levels (at the sub-main valve location) that are adequate for the process and to view the purifiers as necessary “insurance.” In doing so, the challenge to a purifier is minimal, and given the lower levels of impurities, longer purifier lifetimes can be expected.7

Regardless of how well a gas cylinder or bulk container is polished, cleaned, and dried, there is always water adsorbed on the interior wall of the vessel that will continue to desorb. As the internal pressure of the contents fall and more gas is consumed, the concentration of moisture will increase. This is particularly true for liquefied gases (HCl, Cl2, BCl3, NH3) where some volatile impurities are concentrated in the equilibrium space above the liquid and less volatile impurities accumulate in the liquid, leading to variable purity level as the cylinder is emptied.2,7,8 Moreover, chemical interactions between the walls and gas contents will generate additional impurities. For example, carbon monoxide (CO) and tungsten hexafluoride (WF6) are highly reactive with iron- and nickel-containing steel cylinders and tubing at high pressure and in the presence of moisture, forming volatile metal carbonyls such as Fe(CO)5, and Ni(CO)4. Gas purifiers are the only solution to controlling the contamination contribution from these gas containers.

For selective high aspect-ratio plasma etching of oxide films deposited over silicon nitride (Si3N4), carbon monoxide is added to fluorocarbon gases to scavenge fluorine by forming COF2, CFx, and an oxygen-rich polymer layer at the oxide-nitride interface (and reduce the etch rate of the underlying silicon-nitride.)2,7,8 Residual moisture in the carbon monoxide, if not removed by a purifier or other means, can affect process parameters. In particular, the volatile Fe(CO)5 and Ni(CO)4 formed from “moist” carbon monoxide interacting with cylinder material can be transmitted in the gas phase throughout the process lines and cause serious device contamination. In addition, contaminating metal (Fe, Ni, Al, Cu, Cr, Zn) ions from the gas container can be deposited on an etched surface and then diffuse during subsequent steps, altering resistivity and other electrical characteristics. When used together with specially designed purifiers that are built into the cylinder and/or point-ofuse purifiers, metal carbonyls and moisture can be sufficiently removed before the gas enters the process chamber.

Anhydrous liquefied HCl is a frequently used process gas for cleaning susceptors and precleaning in oxidation processes. It is also used with another corrosive halide, hydrogen bromide (HBr), in dry etching of silicon, polysilicon, and gallium arsenide (GaAs) layers prior to epitaxy. Whether supplied in bulk or in specialty cylinders, HCl is highly reactive and corrosive, and the degree of dryness in HCl delivered to the process chamber is critical to the quality of the manufactured devices. The chosen location for the purifier depends on where the gas company’s responsibility begins and ends. Otherwise, the purifier is generally located closest to the location where the cleanest gas is required. When used in cleaning, water in gaseous HCl will lead to formation of new, undesired oxide on susceptors; when used in etching, trace amounts of water can lead to reduction of HCl etch selectivity. Furthermore, small amounts of water in HCl can corrode gas distribution system and chlorinated scales of particles can migrate into process chambers as well.9 HCl can also react with iron oxides to produce iron chlorides and water, providing yet another source of moisture to the gas distribution system.

Gas purifiers can also allow further utilization of the HCl heel at the gas source before the cylinder or trailer is returned to the supplier for refill.10 POU specialty gas purifiers can ensure that the contamination levels will not spike unexpectedly farther downstream. In some cases, a chip fab may opt for an ultra-high purity product offered by the gas supplier instead of relying on a purifier. However, this usually comes at a premium prices and a purifier is still usually employed in the line as added insurance.

Inert nitrogen, oxygen, argon, and hydrogen are commonly used as purge gases or as carrier gases in both front and back-end processes. The main contamination culprits that require gas purifiers are H2O, O2, CO2, and condensible organics. As expected, both front and back end processes are more sensitive to moisture, oxygen, and hydrocarbon contamination as device feature size decreases.

For processes using novel metal oxides and liquid precursor materials for barrier and conductor metals, bulk gases are often used as a carrier gas or push gas. This is the case for several metalorganic precursors used for high capacitance oxides, i.e. hafnium, zirconium, or lanthanum oxide precursors. Also, ammonia is used as a liquid nitrogen precursor for the formation of Ta/TaN diffusion barriers in copper damascene processing.11 In this case, the liquid source must be vaporized through a liquid injection system or a temperature-controlled bubbler at the point of use before it is introduced into the process chamber, with bulk N2 as a push gas to apply pressure to push the liquid precursor through its path. The nitrogen gas typically has to be run through a purifier to minimize moisture.

Other areas that may require gas purifiers are those related to ambient environments of the wafer itself, for example, in controlled environments such as the cleanroom ambient, wafer enclosures and carriers, SMIF PODs, and FOUPs. Environmental control for gate wafer environment is especially critical, with permissible impurity levels down to 0.5 ppt (by mass) of total metals. These requirements are expected to remain unchanged through the year 2020.

The deposition of epitaxial thin film structures plays a major role in the manufacturing of many compound semiconductor devices such as LEDs, solar cells, and GaAs/InP electronic devices. Primary methods to grow III-V materials include molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), vapor phase epitaxy (VPE), and liquid phase epitaxy (LPE), requiring ultra-pure specialty Group V hydride gases such as arsine, phosphine, and ammonia.

The presence of moisture in organometallic vapor phase epitaxy processes can affect minority carrier lifetimes and compromise the photoluminescence properties of LEDs and vertical cavity surface emitting lasers12 (Figure 4). Controlling moisture level in hydride gases is complicated by the fact that as an impurity, water is highly soluble in compressed hydrides in the liquid phase.13

In addition to oxygenated impurities from the hydrides for epitaxial growth, corrosive halides are used in associated etch processes. The byproducts of moisture reacting with the anhydrous halide acids can be even more corrosive than the initial anhydrous material. Gas purifiers that help to remove trace amounts of moisture minimize these process problems.

Considering the high reactivity of the gases used and the instability of the contaminant levels, gas purifiers for corrosive halides and specialty Group V hydrides are essential to maintaining optimal device performance.


  1. B.L. Hertzler, R.M. Pearlstein, J. Irven, M. Sistern, “Progress in improving cylinder gas purity,” Solid State Technology, Nov 2000.
  2. R. Pearlstein, J. Hart, J. Irven, R. Parise, J. Van Ommeren, “Incorporating more gas control within cylinders,” Solid State Technology, Nov 2003.
  3. X. Jiang, D. Alvarex, A. Tram, J.J. Spiegelman, “Photolithography advances push purge-gas purification,” Solid State Technology, Nov 2002.
  4. S. Barzaghi, A. Pilenga, G. Vergani, S. Guadagnuolo, “Purged gas purification for contamination control of DUV stepper lenses,” Solid State Technology, Sep 2001.
  5. International Technology Roadmap for Semiconductors 2005th edition
  6. International Technology Roadmap for Semiconductors 2006 update.
  7. R. Chakraborty, K. Brown, M. Horikoshi, “Removal of metal carbonyl and moisture impurities through purification of CO gas,” Solid State Technology, Jul 2005.
  8. C. Wyse, J. Vininski, T. Watanabe, “Cylinder, purifier technologies for controlling contamination in CO,” Solid State Technology, July 2002.
  9. C.C. Dong, D.D. Christman, D.V. Roth, A. Schwartz, R.W. Ford, “New Purifier for Water Removal from Bulk Gaseous HCl,” Semiconductor International, May 2000.
  10. R. Torres, D. Fraenkel, J. Vininski, E. Hennig, T. Watanabe, V. Houlding, “High Pressure Point-of-Use Purification of Corrosive Gases. Effect on Gas Distribution Components,” Semiconductor Fabtech, 12th ed., Dec 2001?
  11. N. Li, D.N. Ruzic,R.A. Powell, “Chemically enhanced physical vapor deposition of tantalum nitride-based films for ultra-large-scale integrated devices,” JVST B: Microelectronics and Nanometer Structures, Vol. 22, Issue 6, pp. 2734-2743, Nov 2004.
  12. H.H. Funke, A.O. Wright, “Using tunable diode laser spectroscopy to detect trace moisture in ammonia,” Solid State Technology, Oct 2004.
  13. R. Torres, J. Vininski, T. Watanabe, C. Wyse, D. Lawrence, H. Funke, M. Raynor, “An Integrated Solution Approach for the Use of Ammonia in Growth of GaN Based Semiconductors,” Proceedings of CS-MAX, 2002.

Lita Shon-Roy has more than 20 years’ experience in semiconductor manufacturing. She has worked in both sales and as a process engineer in for such companies as IPEC, Air Products/Schumaker, Brooktree/Rockwell, Hughes Aircraft, and as a consultant. She holds a Master’s Degree in Electrical Engineering with specialty in Solid State Physics from USC and a Bachelor’s Degree in Chemical Engineering from UCSD. She has also completed the majority of coursework toward an MBA from SDSU.

Maggie Y. M. Lee is a freelance science editor and technology consultant. She received an A.B. in mathematical physics from Bryn Mawr College, an M.Sc. in space physics from Rice University, and completed her doctoral research in physics at Wesleyan University and Brookhaven National Laboratory.

No comments: