Storing gas on a sorbent provides an innovative, yet simple and lasting solution.

BY KARL OLANDER, Ph.D. and ANTHONY AVILA, ATMI, Inc., an Entegris company, Billerica, MA

The period following the introduction of subatmospheric pressure gas storage and delivery was punctuated by continuous technical innovation.

Even as the methodology became the standard for supplying ion implant dopants, it continued to rapidly evolve and improve. This article reflects on the milestones of the last 20 years and considers where this technology goes from here.

From the beginning, the semiconductor industry’s concern over using highly toxic process gases was evident by the large investment being made in dedicated gas rooms, robust ventilation systems, scrubbers, gas containment protocols and toxic gas monitoring. While major advances have been made in the form of automated gas cabinets and valve manifold boxes, gas line components, improved cylinder valves and safety training, the underlying threat of a catastrophic gas release remained.

Risk factors targeted

The underlying risk with compressed gases is twofold: high pressure, which provides the motive force to discharge the contents of a cylinder, and secondly, a relatively large hazardous production material inventory, which can be released during a containment breach. Pressure also is a factor in component failure and gas reactivity, e.g., corrosion. Mitigating these issues would considerably increase safety.

FIGURE 1. The stages of developing a new chemical precursor for use in commercial IC production.

Analysis of the risks suggested an on-demand, point-of-use gas generator would improve safety by both reducing operating pressure and gas inventory[1]. The challenges associated with this approach include complexity of operation and gas purity, especially in a fab or process tool setting. Chemical generation of arsine, while possible, per equation [A], also substituted a highly reactive toxic solid for arsine[2]. Considerable safety and environmental issues accompanied the operation of such a generator. An on-demand, point-of-use electrochemical approach for supplying arsine, per equation [B], would also eliminate the need for high pressure storage if the associated operational issues could be overcome. Numerous attempts at developing a commercial electrochemical generator just never proved successful[3].

[A] KAsH2 + H2O —> AsH3/H2O + KOH
[B] As(s) + 3H2O + 3e(-) —> AsH3(g) + 3OH(-)

Innovation from a simple(r) solution

Pressure swing adsorption processes utilize the selective affinity between gases and solid adsorbents, and are widely used to recover and purify a range of gases. Under optimal conditions, the gas adsorption process releases energy and produces a material that behaves mores like a solid than a gas.

Early work at reversibly adsorbing toxic materials on a highly porous substrate showed promise. In 1988, the Olin Corporation described an arsine storage and delivery system where the gas was [reversibly] adsorbed onto a zeolite, or microporous alumino- silicate, material[4]. A portion of the stored gas could be recovered by heating the storage vessel to develop sufficient arsine pressure to supply a process tool. In 1992, ATMI supplied a prototype system based on the Olin technology to the Naval Research Lab in Washington, D.C.

The breakthrough that lead to the first commercial subatmospheric pressure gas storage and delivery system occurred when ATMI reported the majority of the adsorbed gas could be supplied to the process by subjecting the storage vessel to a strong vacuum. Using vacuum rather than thermal energy simplified the process, providing the means for an on-demand system[5]. Using a sorbent had the effect of turning the gas into something more akin to a “solid.” That characteristic, coupled with the absence of a pressure driver, delivered an inherently safe condition. The vacuum delivery condition also helped define where the technology would find its first application: ion implantation[6].

Safe and efficient gas storage and delivery

In 1993, prototype arsine storage and delivery cylinders based on vacuum delivery were beta tested at AT&T in Allentown, PA[g] [f]. The system was trademarked Safe Delivery Source®, or SDS®. Papers were presented on safe storage and delivery of ion implant dopant gases the following year in Catania, Sicily at the International Ion Implant Technology Conference[7].

The goal to find a safer method to offset the use of compressed gases was realized: (1) gas is stored at low pressure (ca. 650 Torr at 21°C) and (2) the potential for large and rapid gas loss is averted. Leaks, if they occur, whether by accidental valve opening or a containment breach, would be first inward into the cylinder. Once the pressure equalizes, gas loss to the environment would be governed mainly by diffusion as the gas molecules remain associated with the sorbent. The SDS package, while not a gas generator per se, effectively functions like one.

FIGURE 2. Cutaway view of SDS3 carbon pucks within a finished cylinder.

While subatmospheric pressure operation is an artifact of having to “pull the gas” away from the sorbent, it has become synonymous with safe gas delivery. The optimization work which followed focused on reducing pressure drop in the gas delivery system by improving conductance in valves, mass flow controllers and delivery lines. A restrictive flow orifice was no longer required. The new gas sources proved to work best when in close proximity to the tool.

The years after this technology introduction also saw considerable efforts to improve the sorbent; ultra-pure carbon replaced the zeolite-based material used in the first generation SDS (SDS1), roughly doubling the deliverable quantities of gas per cylinder. These granular carbon sorbents in the SDS2 were later replaced by solid, round monolithic carbon “pucks” in SDS3 (FIGURE 2), which necessitated the cylinder be built around the sorbent[8]. This improvement again roughly doubled gas cylinder capacity.

Recognized in international standards

In 2012, the United Nations (U.N.) recognized the uniqueness of adsorbed gases and amended the Model Regulation for the Transport of Dangerous Goods by creating a new “condition of transport” for gases adsorbed on a solid and assigning a total of 17 new identification numbers and shipping names to the Dangerous Good List. Adoption is expected to occur by 2015. A few of the additions are noted here.

Arsine   – UN 2188 – compressed
Arsine, adsorbed – UN 3522 – SDS
Phosphine – UN 2199 – compressed
Phosphine, adsorbed – UN 3525 – SDS

FIGURE 3. The evolution of a SAGS Type 1 gas package.

In recent years, fire codes have been updated through the definition and classification of subatmospheric Gas Systems, or SAGS, based on the internal [storage] pressure of the gas.9 Systems based on both sub-atmospheric pressure storage and delivery are designated as Type 1 SAGS. It is important to note that the UN definition for adsorbed gases, and the resulting new classifications mentioned above, only applies to Type 1 SAGS, defined as follows:

3.3.28.5.1 Subatmospheric Gas Storage and Delivery System (Type 1 SAGS). A gas source package that stores and delivers gas at sub-atmospheric pressure and includes a container (e.g., gas cylinder and outlet valve) that stores and delivers gas at a pressure of less than 14.7 psia at NTP.

It is also worth mentioning that sub-atmospheric pressure gas delivery can also be achieved using high pressure cylinders by embedding a pressure reduction and control system. The Type 2 SAGS typically employs a normally closed, internal regulator[s] that a vacuum condition to open. This is not a definition of sub-atmospheric storage and delivery, but of sub-atmospheric delivery only.

In general, Environmental Safety and Health managers, risk underwriters and authorities having jurisdiction recognize the importance of SAGS and requires recommend their use whenever process conditions allow[10].

Also, in addition to the inherent safety, PDS employs a pneumatic operator (valve) to the cylinder which further minimizes the opportunity for human error. In an emergency, such as a toxic gas alarm, pressure excursion, loss of exhaust, etc., gas flow at the source can be quickly stopped and the cylinder isolated. Cycle/purge operations are made safer as human involvement is minimized. Human-initiated events, like over-torqueing the valve, failing to close the valve or even back-filling a cylinder with purge gas, are prevented.

Expanding the use of SAGS beyond the domain of ion implant involves successfully navigating key process factors such as operating pressure, flow rates, proximity to the tool and purity. One approach includes coupling the PDS cylinder and gas cabinet together to yield a plug and play “smart” delivery system. Unlike high pressure systems, which are more concerned with excess flow situations, knowing and controlling pressure allows a SAGS cabinet to operate at a reduced risk. This enables linking cabinet ventilation rates with the system operating pressure. During normal operating conditions, the exhaust rate could be reduced by up to 80 percent because the system is operating sub-atmospherically. Should the operating pressure exceed a preset threshold, the exhaust flow would automatically revert to a higher range or the cylinder valve would close.

The future, therefore, could see these PDS packages extended to another level by incorporating them into smart delivery systems, which will further reduce risk, maximize efficiency, improve cost of ownership and expand the footprint for SAGS into new applications like plasma doping, solar, epitaxy and etch.

Summary

During the last 20 years, the semiconductor industry undertook a large effort to develop safer gas delivery technologies to reduce risks associated with dopants used in ion implant. Many technologies were considered, including chemical and electrochemical gas generators, complexing gases with ionic liquids or mechanically controlling cylinder discharge pressure using embedded regulator devices.

In the end, storing gas on a sorbent provided an innovative, yet simple and lasting solution. Gas-sorbent interactions are well understood, reproducible and can be achieved with a minimum of moving parts. Gas release risks, driven by pressure, are all but removed from consideration. And any potential for human error continues to be a target for improvement wherever toxic gases are used.

References

1. Proc. Natl. Acad. Sci. USA 89 pp 821-826, 1992.
2. Appl. Phys. Lett., 60 1483
3. Electron Transfer Technology, US Patent 59225232
4. Olin Corporation, US Patent US4744221A
5. Advanced Technology Materials, US Patent US5518528 6. Many thanks to Dan McKee and Lee Van Horn for being the first of many early adopters.
7. Proceedings of the Tenth International Conference on Ion Implantation Technology, 1994, pp 523-526.
8. DOT-SP 13220.
9. NFPA 318, Standard for the Protection of Semiconductor Fabrication Facilities 2012 Edition. 10. SAGS in the FAB, SST reference

ATMI is a wholly owned subsidiary of Entegris, Inc. ATMI, Safe Delivery Source, SDS, Plasma Delivery Source and PDS are trademarks of Entegris, Inc. in the U.S., other countries, or both. All other names are trademarks of their respective companies.