CHAPTER VI - ELECTRONICS
Emission Sources
Solvent air emission sources in the electronics industry include the six major sources. Since each has different characteristics, each has potentially a different cost effective solvent emission control technology to apply to it. Summarized below are the characteristics of each source and the control technologies which are most cost-effective for each. Note that there may be more than one technology which is cost-effective for each source, and that is one reason for use of the problem solving methodology. As an example of this there are five different supposedly cost effective solvent emission control technologies for semiconductor air pollution sources. In each case, for that particular example, the customer may have actually selected that control technology which is unique for this particular solvent emission control situation. This difference in technology selected may be more due to cost differences in local utility rates, different air flow rates and solvent composition/concentration perceived differences in secondary pollutant (wastewater discharges, solid wastes, NOx emissions) risks and other very valid reasons. In summary, what is most cost-effective may be a very subjective matter, but an objective evaluation of the alternatives with costs assigned to all risk items should be done to make sure what control technology is chosen makes the most sense.
Bob Kenson of Kenson Associates and a McIlvaine Consultant-by-Phone has summarized electronic applications as follows:
1. Precision Cleaning
These sources are the cleaning of electronic components or semiconductor materials and assemblies. Chlorinated solvents are used for their unique deep cleaning ability to remove greases, oils and particulates. Usually the solvents are in high concentration because of the use of a vapor cloud to enhance cleaning. The chlorinated solvents are high cost, so recovery by carbon adsorption is preferred to allow reuse after their condensation and recapture in the decanter/phase separator. There are three carbon adsorption technologies to consider, and selection of each depends upon the specific application details. For non-chlorinated solvents, granular carbon systems with steam desorption are usually the most cost-effective. For chlorinated solvents, activated carbon fiber systems with steam desorption are usually the most cost-effective. For water soluble or water sensitive solvents, nitrogen desorbed granular carbon system are usually the most cost effective.
2. Solvent Storage Tanks and Drums
Although these solvent emissions are low in airflow, they are high in solvent concentration. If their boiling point is above 200°F, then condensing the solvents is very cost effective and the solvent can be reused after recovery. If it is water soluble, then an adsorber/gas scrubber might be used. In this latter case, however, the solvent is difficult to reuse unless it is usable in water or is separable from water by distillation. Carbon adsorption can also be used, and in some cases non-regenerable carbon systems are used where there is low value in the recovered solvent for reuse.
3. Semiconductor Manufacturing
These sources can involve the combined exhausts of several process steps in the semiconductor manufacturing process. They can include precision cleaning (chlorinated solvents), negative photoresist developing (non-chlorinated solvents) and photoresist stripping (phenol, chlorinated solvents) process steps. Airflows are large, but solvent concentrations are low and there is not reuse value if they are recovered. All technologies for control of these complex solvent emissions have drawbacks, so three different technologies can be applied with probably equal cost effectiveness. Carbon adsorption (granular or carbon fiber) with steam regeneration, regenerative thermal incineration and rotary concentrator plus incineration or recovery have all been applied with success.
4. Negative Photoresist
This process usually involves low concentrations of non-chlorinated solvents with no reuse potential. It is obvious that incineration is the most cost-effective solvent emission control technology for use here. The type used depends on airflow, solvent stream characteristics and process operation cycle. For low air flows, both recuperative thermal and catalytic incineration systems have been successfully applied. For high airflows, regenerative thermal incinerators appear to be most cost-effective. The most optimum control technology requires careful analysis of capital and operating costs of the alternative incineration technologies.
5. Circuit Encapsulation
This process is continuous and low air volume and again no solvent reuse is possible. The same three incineration systems mentioned in the negative photoresist process are potentially cost effective for this process as well.
6. Enclosure Painting/Coating
Although this is only tangentially related to electronics and semiconductor manufacturing, this is an important solvent emission source. The high air volume, low solvent concentration characteristic of this proces makes it difficult to find any cost effective emission controls. Since these are mixed solvents, recovery is usually not practical but incineration is. Both regenerative thermal incineration and rotary concentration plus incineration have been applied and the most cost-effective choice depends on the application details. At airflows above 50,000 CFM and over 4,000 hours/year operation, the rotary concentrator has very low operating costs and reasonable capital costs relative to regenerative thermal incineration. The higher the solvent concentration, the more favorable it is to use regenerative thermal incineration.
7. Effluent Gases
The safe disposal of effluent gases produced by the semiconductor industry deserves special consideration. Exhaust gas conditioning is becoming the best term to describe exhaust gas treatment; and depending upon the particular process, type of process equipment, or process gases being utilized, vacuum pump fluid mist eliminators, scrubbers, controlled combustion systems, and/or trapping devices should be utilized to condition exhaust gases.
Typical processes used for wafer processing such as epitaxial deposition, plasma etching, and diffusion use specialty gases and liquids including silane, arsine, phosphine, and metal organics, to name only a few.
The characteristics of these chemicals and the by products of their use include corrosion, spontaneous ignition, and toxicity, the latter sometimes lethal even at low concentrations. Almost all process gases are toxic, some, such as arsine, phosphine, and silane are pyrophoric; and others such as carbon tetrachloride are suspected human carcinogens. Hydrogen chloride and boron trichloride are highly corrosive. These characteristics and especially the diversity of the characteristics involved makes exhaust conditioning complex and dictates proper exhaust handling.
Conditioning equipment can be divided into two broad categories -- source and central systems.
Typical applications for a source system are deposition reactors, dry etching systems, ion implanters, and gas bottle purge. Because of the nature and low volume of the effluents existing in these systems, it is desirable and easier to scrub them as close to their source as possible, tailoring each conditioning system to the specific process. Additionally, source type systems can be interlocked to the process equipment, allowing better control and safety of the entire operation.
Source systems include controlled combustion "scrubbers." In semiconductor applications, combustion is often very effective because many of the contaminants are combustible.
A typical arrangement of an air pollution control system is shown in Figure VI-1.
8. Perfluorocarbons
Perfluorocarbons (PFCs) are widely used in the semiconductor industry in dry processing applications such as film etching and chemical vapor deposition (CVD) chamber cleaning. Because of their long atmospheric lifetimes (estimated in millenia) and strong infrared absorption characteristics, these compounds are greenhouse gases implicated in the threat of global warming. For most major greenhouse gases, semiconductor industry emissions are roughly proportional to the industry's size among all manufacturing industries. But for PFCs, the semiconductor industry contributes a deproportionate share of total emissions. If PFCs were to be subject to an outright ban like their ozone depleting chemical cousins the chlorofluorocarbons (CFCs), wholesale replacement of PFCs would present a major hurdle for semiconductor manufacturers. DuPont has decided that ".. with the lessons of the CFC-ozone issue in hand, it would be appropriate for the industry to proactively develop policies and emission control procedures to avoid such an outcome."
EPA agrees. In response to the Framework Convention on Climate Change that grew out of the Rio Earth Summit in June 1992, EPA will use some provisions of the Clean Air Act Amendments of 1990 and a product stewardship program to create a partnership with manufacturers of long-lived HFCs and PFCs such as DuPont to discourage "... selling those chemicals for emissive uses and to ensure that users of long-lived gases handle the material in an environmentally responsible manner --by capturing and destroying the gas rather than emitting it to the atmosphere."
DuPont has accordingly implemented a sales policy for the PFC C2F6, which the company markets under the tradename Zyron®116 stating that:
"After 12/31/96, DuPont will sell Zyron®116 (C2F6) only to those applications that contain, and either recover or destroy this compound subsequent to use. A minimum of 80 percent Zyron®116 emission reduction will be required by this date, assuming commercial technology is available to accomplish such reduction."
Air Products, which sells CF4 (not technically a PFC, but likely to be grouped with PFCs for regulatory purposes), has decided to adopt an advisory approach. While offering customers assistance in implementing appropriate control measures, the company will not restrict sales or use of CF4, leaving final compliance up to the end-user.
Availability of commercially viable PFC control technology is a major assumption. In support of its sales policy, DuPont has entered into cooperative studies with semiconductor manufacturers, abatement equipment manufacturers, and the semiconductor research consortium SEMATECH to develop technologies and systems for PFC abatement. A SEMATECH spokesman says that control of PFCs as a class presents special challenges, due to their unusual stability and non-reactivity. Even within this difficult class, Zyron®116 and CF4 are particularly recalcitrant. At the current state of development destruction by combustion is the only proven technology for control of PFCs. Although combustion is effective, certain difficulties make it unlikely this technology will be a widely adopted solution. The high temperatures necessary to achieve thermal destruction of PFCs (100-1100°C) are also sufficient to generate unacceptably high levels of NOx. Materials of construction for combustors are also an issue, because one of the products of thermal destruction is large quantities of extremely corrosive HF.
Other investigators, including some of the vacuum pump vendors, are working to develop absorption systems for PFCs. Some of these vendors already produce packed cartridge units and other systems for absorption of other compounds, and are trying to adapt the chemistries for absorption of PFCs. So far these efforts have met with limited success, due to the extreme non-reactivity of PFCs.
The consensus in the industry seems to be that a system using plasma-based destruction technology holds the most promise as the ultimate solution for control of PFCs. High-energy plasma abatement systems are already marketed by several companies for use within vacuum tools to destroy certain other process gases before their reactants can deposit in the internal mechanism of the vacuum pump or within the vacuum lines. A variation in plasma-based technology may hold the key to destruction of PFCs.
Commercial units for the combustion-based abatement of reactive and toxic gases used in semiconductor processing are known to be available from at least seven companies. Tests of these units, under both fab simulation and actual fab conditions, have continued to show that all are capable of achieving high efficiency (>98) for C2F6 decomposition from process effluent streams. The extent of decomposition depends on the total process gas flow rate (a major part of which is commonly N2 pump purge and gas ballast) and the fuel choice (i.e., H2 vs. natural gas) and flow rate. Unit design is important, and the efficiency of different systems at comparable process and fuel flow rates can differ significantly, particularly regarding H2 versus natural gas as the fuel choice. In general (although not universally), natural gas is the more efficient fuel, requiring about 1/3 the fuel flow rate versus H2 to produce comparable destruction efficiencies.
The following systems are available:
System Company Country
Guardian™ System MG Industries USA
CDO™ System Delatech Inc. USA
Escape™ System DAS GmbH Germany
Phoenix™System EcoSys USA
Edge™ System Alzeta Corp. USA
Flawamat™ Centrotherm USA
Toxoclean™ Toyo Sanso Japan