New regs on sub-atmospheric gas sources reduce risk, improve safety
Ion implant has traditionally been hazardous due to the toxic materials and high-pressure cylinders. However, sub-atmospheric gas sources (SAGs) have been proven to help improve safety. Due to the growing adoption of SAGs, the National Fire Protection Association (NFPA) recently published definitions and guidelines for their use. This article describes the two primary types of SAGs and their differences as they relate to safety and efficacy.
Al Brown, RUSHBROOK, Strathaven Scotland; Karl Olander, ATMI, Danbury, CT USA
Historically, ion implant has been considered a hazardous area in a semiconductor fab. As recently as 15 years ago, implanters were isolated in a corner of the facility to minimize potential exposure to the toxic gases used during implant. High-pressure cylinders located in a confined area inside the implanter presented a challenge for semiconductor manufacturers.
Over the years, the semiconductor industry learned to prepare for and prevent gas leaks. Strategies to control the risk include source isolation, ventilation, gas-detector technology, improved gas delivery components and systems, and extensive personnel training. It is also common to install treatment systems designed to prevent discharges-to-atmosphere above allowable limits.
The introduction of sub-atmospheric pressure gas sources (SAGs) in the mid-1990s enabled semiconductor manufacturers to overcome the risk posed by hazardous materials inside the tool. Environmental health & safety (EHS) professionals, insurance underwriters, and loss control engineers endorsed the potential safety benefits of SAGs.
Fabs under pressure
Strategies emerged to reduce the risk inherent in using highly toxic gases for ion implantation. One involved removing pressure from the system; without pressure, there is no release driver. The second uses embedded mechanical controls to mitigate cylinder pressure. Both require a vacuum to be in place before gas is delivered from the cylinder to the tool. Recently, NFPA published definitions of these two approaches and recommendations for their use. Here, we explore recently released standards that establish and define Type 1 and Type 2 SAGs and examine adoption trends and safety/economic benefits.
NFPA 318 defines SAG types
Adopting new technology involves many groups: end-users/engineers, the ESH and fire service communities, and risk underwriters. Given the divergence in the underlying technologies used to achieve sub-atmospheric pressure delivery, adopting code language that governs their use has been challenging.
The terms Type 1 and Type 2 were established in late 2008, when NFPA 318 approved a dual definition [126.96.36.199.1-2] to differentiate SAGs based on gas-storage pressure. The 2009 edition provides the following definitions:
- 188.8.131.52.1 Sub-atmospheric 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.
- 184.108.40.206.2 Sub-atmospheric Gas Delivery System (Type 2 SAGs). A gas source package that stores compressed gas and delivers gas sub-atmospherically and includes a container (e.g., gas cylinder and outlet valve) that stores gas at a pressure greater than 14.7 psia at NTP and delivers gas at a pressure of less than 14.7 psia at NTP.
While both types of systems reduce the risk of using highly toxic gases in ion implanters, there are some fundamental differences.
Type 1 sub-atmospheric gas sources
The first SAGs operated by reversibly adsorbing dopant gases within a high surface area carbon matrix (Fig. 1). The adsorbed gas, in a lower energy state, exhibits a significant vapor-pressure reduction. With surface areas of 1200m2/gr, sorbent loadings (the saturation of gas into the sorbent) of 30???50% by weight can be stored at a final pressure of 650torr at 20??C. A vacuum, afforded by the process, provides the motive force to displace the gas/solid equilibrium and convey the gas to the point of use.
Figure 1. Arsine and phosphine adsorption isotherms on high-density carbon at 70??F.
Removing the pressure component effectively eliminated the prospect of accidental gas leaks from this SAG type, allowing semiconductor facilities to move away from inefficient solids vaporization and standardize on 100% pure gases. Results include greater process throughput and reduced operating and capital costs. The term gas source was coined to describe the new sub-atmospheric pressure delivery methodology.
Figure 2. Typical container configurations for Type 1 and Type 2 SAGs and traditional high-pressure cylinders.
In 1999, industrial property insurer FM Global, recognizing the inherent risk reduction offered by SAGs, modified its Property Loss Prevention Guide for Semiconductor Facilities (Data Sheet 7-7), to advocate “the use of sub-atmospheric gas sources instead of high-pressure cylinders whenever process compatibility will allow.”
Over 130,000 Type 1 SAGs have been deployed for use within the semiconductor industry, with no reported safety incidents. The science behind Type 1 SAGs has continued to evolve and improve; most recently, advancements in sorbent technology have doubled delivery capacities, helping reduce cylinder change-out frequency.
Type 2 sub-atmospheric gas sources
Over the past 15 years, alternative dopant packaging technologies have also been developed. Examples include on-demand gas generators and vacuum-initiated cylinders. The latter uses pressure-control devices embedded within a traditional compressed gas cylinder, effectively transforming it into a smarter and safer gas delivery vessel. NFPA now refers to these cylinders as Type 2 SAGs.
Figure 3. Typical dopant pressures for Type 1 and Type 2 containers from 70?????130??F.
High-pressure cylinders delivering their contents at sub-atmospheric pressure began appearing after 2002. With a focus on vacuum-initiated delivery, these new cylinders were configured to permit flow when a minimum threshold pressure was achieved. In this manner, a cylinder at 200???1500psig discharges its contents when the downstream manifold is under vacuum (<760torr). If a pre-set threshold vacuum is not maintained, an internal valve will close. Maximum discharge rates are capped using restricted flow orifices or other flow-restricting means. As with Type 1 SAGs, the process provides the negative operating pressure required to activate the internal regulator.
Figure 4. Cumulative Type 1 and Type 2 SAG usage since introduction in 1994.
While, in theory, pressure-control devices could be located downstream of the cylinder valve, they are typically located inside the cylinder of today’s Type 2 SAGs. These cylinders also feature separate fill and discharge ports. Locating the mechanical devices within the cylinder minimizes the possibility of damage or tampering, but increases the premium on reliability. Servicing the internal components is virtually impossible given their location. Long-term reliability and sustainability of SAGs with embedded internals is being established. Certainly this delivery concept offers some notable benefits in adapting sub-atmospheric pressure delivery to a broad variety of gases. At present, there is a fleet of ~6,000 Type 2 SAGs deployed for use within the semiconductor industry.
Fire survivability differences
Recent fire testing demonstrated that during exposure to low- to intermediate-temperature fires ??? temperature exposures that could foreseeably occur during a storage fire ??? Type 1 cylinders survived significantly longer than Type 2s. In addition, the pressure wave generated during cylinder failure was significantly less on a Type 1 system.
Benefits derived from safer packaging
An immediate benefit of safer gas packaging is that users can increase the quantity of dopant stored in the implanter. Larger cylinders and increased fill densities translate into greater production efficiencies by reducing the frequency of cylinder change-outs. At present, Type 2 SAGs provide greater capacity for liquefied compressed gases.
Figure 5. Comparison of type 1 and high-pressure phosphine cylinder survivability under simulated fire conditions.
The absence of pressure in Type 1 SAGs facilitates additional operational cost savings. While Type 2 SAGs can also achieve some or all of these benefits, it comes with additional engineering controls.
Inherent benefits of Type 1 SAGs
Designating performance-based control methods helps ensure that the gas delivery is always at sub-atmospheric pressure. FM Global’s Property Loss Prevention Data Sheet 7-7 states: “Provide sub-atmospheric pressure cylinders with a pressure sensor designed to shut off the cylinder if the [delivery] pressure exceeds 1 atmosphere [760 torr].” Type 1 SAGs provide this condition inherently.
Stored pressure within Type 2 SAGs creates the potential to release the entire gas inventory into the atmosphere, though this is rare. NFPA 318 advises an emergency high-pressure shutoff be provided after the SAG Type 2 cylinder to protect the gas distribution system. Current standards on gas-box ventilation are based on anticipated worst-case release (WCR) rates. Given the very low WCR rates for Type 1 SAGs, reductions in implanter gas-box exhaust rates can be large: an estimated energy savings of $1,200???1,500 per tool, per year.
Opportunities for savings include reclassification of gas-box exhaust from “scrubbed” to “general exhaust” and/or eliminating treatment systems altogether. For new facilities, this will reduce ductwork and associated make-up air units. Given the lower risk profile of Type 1 SAGs, the level and frequency of toxic-gas monitoring can be re-examined and possibly reduced.
Today, implanters are installed in the center of the fab, reflecting their critical role in semiconductor manufacturing and the quantum leap of safety improvements in dopant delivery packaging.
Recognizing SAG types in the codes helps ensure the maximum benefit from lower-risk technologies. As code officials consider design alternatives, inclusion of specific language accurately defining SAGs and their types ensures uniform application and understanding.
Aside from implanter efficiency gains, the industry can obtain additional savings from lower-cost gas boxes and components, reduced exhaust rates, and smaller ductwork. Some of the investment historically required to prevent and contain gas releases can be avoided in the future.
Sub-atmospheric pressure gas delivery offers real safety and risk reduction benefits. With a proven 15-year service history, Type 1 SAGs have moved the industry toward inherent safety and virtually eliminated the prospect of a catastrophic event. During the next 5-year period, critical reliability data will better characterize the relative risk benefit between Type 1 and Type 2 SAGs.
Given the general concern about safety with highly toxic materials in transport and use, SAG adoption will grow.
The authors thank Rick Guevara, co-founder of Technology Risk Consulting Services LLC and Jim McManus, senior project manager with ATMI.
Al Brown received his BSc in mechanical engineering from the U. of Glasgow and is registered as a professional engineer in the UK and through FEANI as a European Engineer. He is co-founder and managing director of RUSHBROOK, 3a Bridge Street, Strathaven, ML10 6AN, Scotland; firstname.lastname@example.org.
Karl Olander received his BS and MS in chemistry from the U. of South Florida, Tampa, and a PhD in chemistry from the U. of Illinois, Urbana. He is the co-founder of ATMI.