Improved ion implant exhaust management reduces energy, capital costs

How Texas Instruments got greener, safer and saved money.

BY STEVEN BALLANCE Texas Instruments, Dallas, TX, KARL OLANDER and JOE SWEENEY, Entegris, Billerica, MA

Over the last decade, considerable efforts have been put forth by manufacturers and suppliers to help reduce costs, consumption of natural resources, and where economically viable or by mandate, to become more green in fab operations. In the early 2000s, Texas Instruments (TI) outlined an opportunity to re-think its approach around one of the largest energy and cleanroom air consumption areas in the fab—ion implant operations.

In comparison to other manufacturing tools in the fab, ion implanters require the largest exhaust volume, typically using 2500 CFM in total ventilation, split between the gas box [400+ CFM] and the containment shell enclosure [2000+ CFM]. The energy cost to replace this volume of air equates to about $8,000 per tool and, with up to 30 implanters in a typical fab, the operating costs can reach up to $240K annually. In addition, the investment needed to replace this volume of clean, highly conditioned air is substantial and requires large infrastructure expenditures (FIGURE 1).

Ion Implant 1

In the late 2000s, TI provided the industry with a glimpse of what was possible around air handling and energy reduction in its implant centers. The initial concept, implementation and projected results had been years in the making and were first published in August, 2009 by Solid State Technology, as provided by Steve Russo, then a senior member of TI’s technical staff.

In the article, Russo explained the operating protocols for handling the highly toxic materials utilized in the ion implant process, which are traditionally stored within the tool itself. Now, after years of development and modification, a bigger picture, along with intriguing data, has emerged.

Recycling the shell exhaust

The 2009 article described how TI recycled the implanter shell exhaust within the fab, reducing the make-up air requirement by 80% [2000 CFM per tool]. Fab air is drawn through the implanter shell to dissipate heat from the process and provide dilution in the event of a process leak. This volume of air is treated as general exhaust, and traditionally expelled from the fab using blowers on the roof.

The successful implementation of the first phase, led to the recycle of the shell exhaust on more than 60 ion implant tools across three fabs without incident. Whereas the initial installation included ductwork to convey shell exhaust to the roof (if needed in an emergency), subsequent facilities were built on the premise of continuously returning the shell exhaust to the fab. In practice, the reconfigured exhaust systems amounted to a $57,000 capital cost avoidance per process tool. FIGURE 2 illustrates these cost savings projections.

Ion Implant 2

Recycling the shell exhaust has resulted in avoiding $1.7 million in capital for exhaust and make-up air infrastructure, as well as, reducing annual energy cost by $470,000. The lower energy usage equates to reduced CO2 emissions of 6,500 metric tons. FIGURE 3 illustrates the new design configuration for shell exhaust recycle.

Ion Implant 3

The role of sub-atmospheric pressure gas sources

In redesigning the implant exhaust configuration, Russo and his team he relied on using only the safest gas packaging technology— sub-atmospheric gas sources, or SAGS.

These packages deliver gases below atmospheric pressure, greatly reducing the likelihood of a gas leak and providing the basis to redirect the shell exhaust back into the fab.

It is interesting to note that around the same time Russo published his first article on his new design, the National Fire Protection Agency (NFPA) adopted the SAGS classification for gas packages into the standard. The NFPA classified gas packages that store and deliver gas sub-atmospherically as SAGS Type I and packages that store gas under pressure but deliver gas sub-atmospher- ically as SAGS Type II. Both SAGS systems share a common feature—they require a process vacuum in order to deliver the toxic gas, virtually eliminating accidental gas releases (FIGURE 4).

Ion Implant 4

The initial planning for re-configuring the shell exhaust system in the new design was done to take full advantage of the safety profile of the SAGS packages. Using traditional high pressure delivery systems in the new design wouldn’t have been prudent because of the higher gas leak potential and lower safety profile. Exclusively using SAGS technologies enabled the exhaust reduction program approach. Continuous efforts and success rely on doing everything possible to see that gas delivery is always sub-atmospheric and TI has taken precautions to ensure the gas delivery systems are consistently performing in this way.

Gas box exhaust reductions

The process of lowering implanter shell exhaust began over 12 years ago, and since then most TI tools have been fitted with this design. On its continued quest for reduced energy and costs, TI identified the gas box as being the next best opportunity.

The gas box exhaust, potentially containing hazardous materials, is sent through a scrubber before being released. Scrubbed (or acid) exhausts, therefore, consume more resources than shell exhaust and contribute more to the costs of fab operations.

Over the past few years, Texas Instruments and ATMI, now Entegris, providers of SAGS technologies, have teamed up to continue to look for efficiencies and safety measures in managing exhaust gas and energy usage in ion implant operations. After evaluating the energy reduction potential of the tool gas box exhaust, TI made modifications that led to reduced gas box exhaust rates of about 200 cfm, down from over 400 cfm. This resulted in an additional $800 savings per tool per year. Additional strategies to reduce gas box exhaust rates and improve overall safety are suggested below.

Building an integrated [smart] exhaust system

Today, ion implanters utilize dopant cylinders with manual valves that had their start when “lecture bottles” were first used 30 years ago—and space in the gas box was at a premium. Small cylinders and manual valves were standard. Even as solid source vaporizers were replaced, and the use of gases in larger cylinders became prevalent, the use of manual valves continued.

Interestingly, the Type 1 and Type 2 sub-atmospheric gas delivery cylinders used worldwide to supply implant dopant gases use manual valves. The presence of the manual valve presents a continuing risk because of the possibility of human error during installation and purging sequences which could result in a gas release, albeit small. Yet, there is still room to reduce risk and continue to improve safety through the application of “smart” solutions.

Ultimately, the cornerstone to minimizing the occurrence and impact of a gas leak is all about maintaining the system under sub-atmospheric conditions at all times. Operating under sub-atmospheric pressure entails the continuous monitoring of gas pressure(s) in the delivery manifolds and the ability to respond quickly if pre-set pressure thresholds are exceeded.

The use of normally closed pneumatic valves provides the means to isolate the toxic gas within the dopant cylinder should the delivery manifold deviate from sub-atmospheric pressure protocols. The normally closed condition also removes from consideration cases where valves are either poorly closed or over-torqued. Cylinder cycle purging can then be done automatically, more efficiently and without the possibility of backfilling purge gas into the cylinder.

Varying the gas box flow rate

The ability to minimize the smallest of leaks would allow the gas box to be exhausted as a function of actual risk as opposed to continuously operating at a rate needed to mitigate projected worst-case scenarios. Controlling the gas box exhaust rate using a two position damper is one possible solution.

A two-position damper can control the gas box exhaust in either a low or high flow mode. The normal or reduced exhaust condition is allowed when all of the dopant delivery cylinders are showing a sub-atmospheric pressure condition or all of the cylinder valves are closed. Interlocks initiate the high flow rate any time the gas box door is opened, such as during cylinder changes or maintenance periods, or when triggered by events such as toxic gas detection, smoke detector alarm or detection of a super-atmospheric pressure condition in the dopant delivery manifold. It is estimated that the exhaust system would operate in the low flow mode >95% of the time.

With SAGS, a nominal rate of 40 cfm can be sufficient to satisfy regulations providing a 90% reduction in gas box exhaust requirements.

Taking the next step forward

TI justified recirculating the ion implanter shell exhaust within the fab based on a thorough risk analysis built around using SAGS technology. Over the last decade, they refined the practice and proliferated it across new fab installations, significantly reducing capital require- ments for make-up air.

Developing an integrated exhaust system can ultimately reduce implant make-up air requirements by 98%— without compromising safety. Operating costs associated with the lower exhaust have been proportionately reduced,along with carbon dioxide emissions.

Further advances in exhaust/energy reduction are possible via a partnership between toolmakers, dopant suppliers and fab designers to incorporate an integrated exhaust system for ion implanters, and possibly other tools. It begins with insuring operating gas delivery is under sub-atmospheric pressure conditions all the time.

Future changes may include:

1. Adding pneumatic valve operators to the dopant cylinders

2. Variably exhausting the gas box proportional to actual risk conditions

Outstanding economic and environmental gains can continue to be made – and new standards created – if manufacturers, equipment makers and suppliers work together to envision the possibilities. As an industry, and as responsible corporate citizens, working together to pursue these types of opportunities can reduce energy consumption and exhaust while improving overall process safety.

Based on text, graphics and data originally presented at the 26th Annual IEEE/SEMI Advanced Semiconductors Manufacturing Conference (ASMC 2015), May 3-6, 2015, Saratoga Springs, New York.

STEVEN BALLANCE, P.E., is a facilities engineer at Texas Instruments, Dallas, TX. KARL OLANDER and JOE SWEENEY are with the Electronic Materials division of Entegris, Danbury, CT.

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