Safety and environmental concerns of CVD copper precursors
09/01/1998
Safety and environmental concerns of CVD copper precursors
Bob Zorich, Mary Majors, Schumacher Division of Air Products and Chemicals Inc., Carlsbad, California
Copper CVD has become one of the semiconductor industry`s most important new processes, enhancing the speed and functionality of integrated circuits. The precursors, however, are relatively new and there is limited published safety information on their handling and abatement. We will discuss aspects such as flammability, by-product generation, and toxicology of one of the main copper precursors and describe a new technique for capturing the effluent from the CVD copper reactor for eventual reprocessing.
Copper deposition has become one of the most important and rapidly growing areas in integrated circuit manufacturing. Deposition of copper can be accomplished by several means, with chemical vapor deposition (CVD) becoming widespread. Like many chemicals in the semiconductor industry, however, the precursor materials have toxic properties, and contain copper and flourine components that are potentially unfriendly to the environment. In the event of an accident, operators may be exposed to both the precursor and its by-products. Also, since 100% utilization of the chemical is not possible, both by-products and unreacted precursor materials exit the process chamber. Studies on the toxicity and flammability of these materials examined their impact on fab area personnel, followed by development of abatement procedures to prevent the release of copper and fluorine into the environment. We will describe abatement procedures for the CVD precursor, trimethylvinylsilyl hexafluoroacetylacetonate copper (I) [Cu(hfac)(TMVS)], and discuss the results of the toxicology tests and the specialized procedures for obtaining them.
Chemistry of copper CVD
There have been numerous reports on the deposition of high-quality CVD copper films from Cu(I) organometallic precursors. Most of these reports use the copper precursor "CupraSelect," also known as Cu(hfac)(TMVS), chemical formula: C10H13CuF6O2Si. This molecule combines copper in a +1 oxidation state with TMVS and hfac ligands to make a clear yellow, liquid precursor. The deposition is a disproportionation reaction: for every copper atom that is reduced to metal, another is oxidized to Cu(hfac)2 while releasing free TMVS:
2 Cu+1(hfac)(TMVS)(g) -> Cu0(s) + Cu+2(hfac)2 (g) + 2 TMVS(g)
The process does not use plasma energy, so there is minimal decomposition of the by-products in the process chamber, simplifying the handling of effluent. As a result, the exhaust contains only unreacted Cu(hfac)(TMVS), as well as Cu(hfac)2 and TMVS vapors.
Generally, chemicals in this class have properties minimizing their impact on the environment and the health of the individuals handling them. First, Cu(hfac)(TMVS), the material studied, is compatible with industrial standard stainless steel delivery vessels. Mechanical damage to valves may occur for a variety of reasons (e.g., overtightening of fittings or mishandling of face seals on VCR connections), leading to small chemical releases. Large scale failures are unlikely, however, limiting the quantity of material released in a spill.
Second, the volatility of the precursor, while high enough for CVD processing, is so low that significant concentrations of Cu(hfac)(TMVS) will not be present in the atmosphere in the event of a spill. This helps to reduce the exposure levels of individuals involved in cleaning up spills.
Finally, Cu(hfac)(TMVS) does not ignite easily, even though its decomposition by-product, TMVS, is flammable. The slow decomposition rate, however, minimizes the concentration of the TMVS, reducing fire potential.
Flammability tests
Cu(hfac)(TMVS) reacts very slowly with oxygen and moisture in the air and will not ignite easily. It decomposes at such low temperatures and low energies that it is not a primary factor of the flammability properties. Instead, one of the decomposition by-products, TMVS, is the key component in any fire involving copper precursors. It has a high vapor pressure (~275 torr at room temperature), and will be consumed completely to form H2O, CO2, and SiO2. TMVS has a flash point of -19?C, and a lower explosive limit (LEL) of 0.5%, which places pure TMVS in the same flammability category as acetone. Since Cu(hfac)(TMVS) decomposes slowly, releasing only small quantities of TMVS at any one time, a well-ventilated area can prevent accumulation of dangerous levels of TMVS.
Cu(hfac)(TMVS) is often supplied as a blend including low concentrations of TMVS as a stabilizing agent to improve process characteristics. This TMVS can evaporate quickly, so there must be extra ventilation during a spill of a precursor blend. The other by-product, Cu(hfac)2, is a nonflammable solid with an extremely low vapor pressure (2 mtorr at room temperature).
Copper CVD by-products
While copper CVD produces far less effluent than processes such as electroplating, there is still a substantial volume of waste generated during the process. Usually, waste is on the order of 0.5 g/processed wafer. In a typical wafer fabrication environment, this can mean generation of up to 25 kg of by-products each month. The primary elements of concern are copper and fluorine, neither of which can be released into the environment. If they are simply captured or processed by a burn box technique, they must be sent to toxic waste-disposal facilities in a form that is not easy to handle and costly to dispose of. The IC manufacturer must bear the cost of transport and disposal using this approach to abatement.
In addition, by-products can damage the equipment used in the manufacturing process. For example, unreacted Cu(hfac) (TMVS) that contacts the hot vanes of a typical rotary vane vacuum pump will decompose to form copper metal. This plating action will cause significant wear and tear on the vacuum pump, reducing the pump lifetime as well as degrading pump efficiency during operation. Another concern is that the hfac component of the effluent can attack oxidized metal surfaces, and may be aggressive enough to attack other materials of construction of the vacuum pump, the exhaust lines, and the waste-handling system. This leads not only to decreased component lifetime, but to more serious failures with potential chemical exposure of the IC fabrication area personnel and equipment, as well as causing damage to the processing equipment itself.
Therefore, a properly designed copper abatement system provides a number of benefits. First, the removal and containment of toxic effluents can possibly trap the by-products for eventual recovery and reprocessing. This reduces both the environmental impact of the by-products and reduces costs involved in toxic waste processing for removal and destruction of the by-products.
A good abatement program also provides the copper precursor vendor an opportunity to reuse the same ligands and undeposited copper in the preparation of subsequent batches of the precursor.
A final benefit is a significant reduction in costs and downtime associated with pump and exhaust handling systems by eliminating corrosion and plating problems. We recommend that the chip manufacturer fully explore the disposal options available in its area prior to determining which copper process to use.
Copper capture and abatement
One of the advantages of Cu(hfac)(TMVS) and related compounds is the nondestructive nature of disproportionation reactions. That is, unlike plasma and some high-temperature processes, the ligands are not decomposed, remaining essentially whole. This provides an opportunity not present in many other processes, as the effluent can be used in subsequent chemical reactions to form compounds with easier handling requirements. It then becomes an issue of supplying an appropriate "secondary reactor" that is sufficiently controlled to permit the desired chemical reaction to take place.
Before looking at the abatement process in detail, we want to point out that even though cryogenics can trap the effluent from the reactor, it can limit the effectiveness of the process. These include process pressure variations as cold traps fill and the potential for backstreaming of particles from the trap back into the reaction chamber. Other issues include safety questions, as the trap will capture flammable TMVS that will be transported back to the recycler. In addition, cryogenic trapping poses the threat of liquid oxygen condensation, which can be a source of violent reactions with other trap contents. Alternatively, recycling techniques can be used to convert the unreacted copper precursor to materials that are easily handled, but in the process completely destroy the molecule. Either way, the cost-effectiveness of the recycling process is lower when compared to the recommended procedure.
Equivalent processes can be designed for similar copper precursor molecules, but we will continue to use Cu(hfac)(TMVS) as the example in this discussion. In this case, the copper and fluorine atoms are contained in a single molecule, Cu(hfac)2. Under atmospheric pressure, this material is a solid at room temperature, a liquid at 80-100?C, and a gas at temperatures above ~100?C. It is generally noncorrosive and does not decompose easily, so that it does not lead to the plating and corrosion problems noted earlier.
As a result, the basic theory of this abatement procedure is to force the effluent to form Cu(hfac)2, to keep the material at a reasonably high temperature, thus preventing condensation as it travels through the vacuum pump, and finally, to chill the material in a controlled chamber or vessel until it condenses. The Cu(hfac)2 can then be abated by removing the trap or by liquefying the Cu(hfac)2 and draining it into a separate vessel for removal.
The specific equipment for this task may vary; however, experimentation has shown an effective process as follows (Fig. 1):
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Figure 1. Recovery and abatement apparatus for CVD copper process.
The effluent from the process chamber is directed into a hot (~200?C) reaction chamber, where the copper precursor will decompose readily to form copper metal, Cu(hfac)2 vapor, and TMVS vapor. After leaving the initial high-temperature process, the lines to the pump must remain heated to prevent condensation of the Cu(hfac)2 in the lines.
The Cu(hfac)2 and TMVS vapors travel through the lines and the vacuum pump unchanged. Removing the Cu(hfac)(TMVS) vapors from the exhaust stream means that there is no decomposition in the pump, thus lengthening the pump`s life span by minimizing metal deposition in the critical rotary vane region. Generally, there will be little loss of exhaust materials in the vacuum pump itself, since the inner workings usually run at temperatures above the condensation point of either Cu(hfac)2 or TMVS. Pump purges must also avoid over-cooling the pump to prevent condensation.
A low-temperature cold trap placed downstream of the vacuum pump then captures the Cu(hfac)2. This trap can run at around room temperature or slightly below, as Cu(hfac)2 solidifies at temperatures below approximately 80?C. The Cu(hfac)2 is essentially pure and can be easily transported and reprocessed. The Cu(hfac)2 trap can be removed, or it can be heated to above 80?C where it becomes a liquid and can be drained into a transport reservoir for shipping.
Finally, the residual TMVS vapor continues on through the trap, and can either be captured separately in a cryogenic trap (with appropriate safety measures installed) or directed to a burn box. In the burn box, TMVS is simply reacted with oxygen to form SiO2, H2O, and CO2.
This four-step process produces a single molecule containing both copper and fluorine atoms. It is solid at room temperature, with low vapor pressure. Exposure by inhalation is thus unlikely, although precautions to avoid ingestion of fine particles of the Cu(hfac)2 during trap replacement and cleaning are necessary.
Toxicology testing
Due to the novelty of the chemicals in this class, little toxicology data is available. For example, Cu(hfac)(TMVS) has no published toxicity history. Tests conducted to quantify its acute toxicity included evaluations on the potential for adverse effects of the copper precursor when exposure occurs via inhalation, ingestion, or by contact with skin [1].
Together with the testing laboratory, Schumacher developed a special apparatus and a set of procedures to maximize the vapor concentration of the copper precursor in the test chamber. A nitrogen carrier gas saturated with vaporized precursor delivered the stream of Cu(hfac)(TMVS), which was then diluted with oxygen just prior to release into the inlet of the exposure chamber (Fig. 2).
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Figure 2. Atmosphere generation and exposure system.
The low vapor pressure of Cu(hfac)(TMVS) became the limiting factor in generating an atmosphere sufficiently concentrated to determine a definitive inhalation toxicity. The toxicity of a material is derived from the median lethal concentration, LC50, which is the concentration, in air, of a material that causes 50% mortality among a set of test animals inhaling the material. Using the setup in Fig. 2, the maximum obtainable concentration of Cu(hfac)(TMVS) at 20?C was 52 ppm (0.77 mg/liter) (inhalation). A total hydrocarbon analyzer monitored the atmosphere in the exposure chamber to ensure maintenance of a saturated vapor. While we noticed respiratory distress, no test animals expired during the 4-hr exposure period and the 14-day observation period that followed. This implies that Cu(hfac)(TMVS)`s LC50 (inhalation, test animals) is greater than 52 ppm. On a more practical level, it would appear that the level of copper precursor vapor equivalent to the LC50 in the test animals would be difficult to attain in the atmosphere, due both to the low vapor pressure and the decomposition of the Cu(hfac)(TMVS). However, due to the observed negative health effects on the test animals and since human exposures have not been tested, we recommend using respirators when handling Cu(hfac)(TMVS).
Cu(hfac)(TMVS)`s LD50 value via oral ingestion among test animals was a toxic 239 mg/kg. This level of toxicity is in the same general category as ammonium hydroxide, with an LD50 of 350 mg/kg. Personnel should avoid ingesting this precursor material, particularly in the event of a spill.
Acute dermal irritation/corrosion studies also indicated that Cu(hfac)(TMVS) is both corrosive to skin after a 4-hr exposure and a severe irritant. The table above shows the toxicology data and some key properties for the Cu(hfac)(TMVS) molecule. In general, a spill or release should be handled using the same principles that apply to other chemical releases. Training on the specific hazards, the appropriate personal protective equipment, and procedures for cleanup and disposal of the precursor are necessary for members of emergency response teams or other personnel who may be exposed to vapors or liquid during maintenance or a spill.
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Conclusion
While copper CVD precursors such as Cu(hfac)(TMVS) exhibit toxic effects, the routes to exposure to the environment are limited. Based on these studies and our experience, copper precursors can be safely handled using proper procedures and personal protective equipment to prevent inhalation, ingestion, or eye/skin contact. Also, the effluent materials of copper CVD processes are toxic and corrosive. However, unlike many precursor materials, especially gases and highly volatile liquids, the properties of Cu(hfac)(TMVS) permit reasonably simple control in the event of accidental releases. In addition, the effluent from copper CVD processes, particularly those using Cu(hfac)(TMVS), remain in forms that are predictable and co-reactive after their use in the CVD reactor. This allows controllable combination to form a stable compound, Cu(hfac)2, which can then be trapped for easy disposal and eventual recycling. This saves the IC manufacturer the effort and expense of waste recovery, and reduces required maintenance for the vacuum system and house exhaust, leading to greater uptime and lower overall factory maintenance costs.
Reference
1. Details of the toxicology tests can be obtained from the authors.
BOB ZORICH received his BS in physics from the University of Idaho and has been in the semiconductor industry for 18 years, working in both OEMs and wafer fabs. He is the CVD copper program manager at Schumacher and is the author of the book The Handbook of Quality Integrated Circuit Manufacturing. Schumacher, 1969 Palomar Oaks Way, Carlsbad, CA 92009; ph 760/931-9555, fax 760/929-6209.
MARY MAJORS received her BS in medical technology from Fitchburg State College. She is an EH&S specialist at Schumacher, where her primary focus is product stewardship and other customer EH&S