Next-generation CVD aluminum precursors pose new handling challenges
Clark E. McGrew, Matheson Gas Products, Longmont, Colorado
Among newer materials explored for advanced CVD processes are aluminum compounds in place of more traditional precursors such as tungsten hexafluoride (WF6). New processing materials can bring with them new processing challenges (and often handling and safety issues as well). While WF6 reacts with air and water to form poisonous compounds like HF, aluminum precursors tend to be pyrophoric and ignite upon reaction with air and water. With knowledge of their physical and chemical properties, and proper handling, these new aluminum precursors can be safely incorporated into CVD processes.
Under current practice, device contacts and vias - the submicron vertical connections between different levels of horizontal aluminum wiring - are filled with tungsten metal by CVD  from the precursor WF6. As the need for increased device speed grows, the resistivity of tungsten becomes the limiting factor in its continued use. Metals such as aluminum or copper with lower resistivity will have to replace tungsten in order to reach the goals of the Semiconductor Industry Association Roadmap .
Both aluminum and copper are currently being investigated for tungsten replacement. While copper can carry a higher current density than aluminum, it has several drawbacks: there is a lack of suitable volatile copper compounds for deposition; etch materials and chamber clean technology need further development; and copper precursors are more expensive and less stable (leading to a shorter shelf life) than aluminum precursors. Aluminum metallization technology does not share these drawbacks and has some key advantages over copper. High-quality aluminum vias are readily fabricated by CVD and are compatible with existing aluminum PVD and etch technology. For these reasons, aluminum CVD is likely to be the next step in the advancement of metallization technology .
The process precursors of aluminum CVD introduce new safety and handling issues that need to be well understood by everyone involved. This article provides a brief overview of the current aluminum CVD candidates and outlines safe handling practices. It covers the physical properties, chemistry, and safety-related hazards of aluminum precursors in comparison with more traditional precursors, such as WF6.
Materials issues - physical properties
The candidates for aluminum CVD precursors can be broken down into two main categories: aluminum alkyls and alane complexes. Aluminum alkyls are complexes in which aluminum is directly bonded to an organic structure or ligand. The alane complexes consist of alane (AlH3) bonded generally to a tertiary amine ligand.
Aluminum alkyls - such as triisobutylaluminum (TIBA), trimethylaluminum (TMA), and aluminum alkyl hydrides, such as dimethylaluminum hydride (DMAH) - are generally liquids at room temperature and have relatively low vapor pressures when compared to WF6. The aluminum alkyls tend to associate, existing as dimers or trimers, as is the case of DMAH (Fig. 1a). Sterically hindered compounds such as TIBA cannot associate because of the larger organic ligands, and, therefore, they exist as monomers. The organic structure of the ligands plays an important role in the selection of an aluminum CVD precursor because it determines the decomposition mechanism, deposition rate, and, ultimately, film purity. Generally, as the size of the ligand increases, the volatility of the precursor decreases. This affects the deposition rate and overall throughput of the process tool. Because larger ligands contain more carbon, there is increased potential for carbon incorporation, reducing film purity. Of all the candidates, DMAH appears to be the most promising.
Figure 1. a) Dimethylaluminum hydride (DMAH), b) dimethylethylamine alane (DMEAA), c) tungsten hexafluoride (WF6).
Alane complexes, such as dimethylethylamine alane (DMEAA, Fig. 1b) and trimethylamine alane (TMAA), are monomeric. They have higher vapor pressures than the aluminum alkyls but the pressures are still low when compared to WF6 (Fig. 1c). The alane complexes, however, are not as thermally stable as the alkyls. Of this class, DMEAA is becoming the precursor of choice.
Understanding these aluminum precursors for CVD can minimize the associated safety risks. The paramount criteria for maintaining safety are being informed of the physical and chemical properties of these compounds (see table) and following sound safety procedures. The aluminum precursors create substantially different safety concerns from those of WF6.
The pyrophoric nature of Al precursors is their most distinctive property from a safety point of view. Not only do the aluminum precursors ignite spontaneously in air, they can do so explosively on exposure to moisture, due to an exothermic reaction between the precursors and water. In contrast, WF6 reacts with moisture in the air to form poisonous products, chiefly hydrofluoric acid (HF).
Figure 2. Decomposition equations of aluminum alkyls.
Thermal instability of the aluminum precursors, especially the alane complexes, can also be a safety concern. Due to the unstable nature of free alane (AlH3), the alane complexes tend to decompose, sometimes violently, to aluminum and hydrogen. Because of the amine, the alane complexes also emit toxic fumes of NOx upon decomposition . DMEAA, for instance, slowly decomposes over time at ambient temperature. The trialkyl aluminums (R3Al) and the dialkyl aluminum hydrides (R2AlH) are more thermally stable than the alane complexes. For example, DMAH can be kept for long periods under an inert atmosphere at ambient temperatures without decomposition. At temperatures above 50?C, the R3Al compounds (C2 and higher alkyls) begin to break down to R2AlH compounds. The decomposition from R2AlH then continues, usually at temperatures above 100?C (Fig. 2) . DMAH is an exception to this type of decomposition because of its inability to eliminate an alkene. Research suggests that dialkyl aluminum hydrides decompose above 150?C by heterogeneous autocatalysis. Once the hydrides start to break down, the decomposition reaction becomes self-sustaining. This decomposition leads to the formation of free alane, which further decomposes to aluminum and hydrogen.
Relatively new to the semiconductor industry, aluminum alkyls have been used in industry for over 30 years as catalysts in Ziegler-Natta polymerization of olefins to make plastics , while III-V semiconductor device manufacturers have been using these types of compounds for more than 10 years. Consequently, significant information on their safe storage and handling is available. In the plastics industry, they are often diluted in an appropriate hydrocarbon to a concentration of 25% or less. At this concentration, aluminum alkyls generally will not spontaneously ignite but will produce smoke and extensive heat . Aluminum precursors for CVD are typically sold undiluted, however, to maintain the high purity needed for semiconductor processing.
Longer alkyl (carbon) chains (lengths of C4 or greater) decrease the precursors` propensity to burn, but aluminum alkyls for CVD typically require short-chain ligands because of their higher volatility.
New technologies for storage and delivery
Liquid delivery of the aluminum CVD precursors presents certain challenges. Since both DMAH and DMEAA have certain drawbacks, research for solutions is being conducted.
DMAH is thermally stable, but its high viscosity makes traditional liquid delivery tricky. Increasing heating temperatures or the addition of an adduct to DMAH may reduce the degree of association (i.e., from trimer to monomer), thereby reducing the viscosity and increasing the delivery rate to the process tool . In DMEAA, additives can enhance thermal stability and shelf life. In alane complexes, the amine helps to increase thermal stability by reducing the amount of free alane.
Figure 3. Matheson Gas Products bubbler with patented Level Sensor .
Additives, unfortunately, do pose problems. First, the addition of materials to the aluminum precursors increases the cost of ownership on an already fairly expensive product. Second, the user is limited to direct injection liquid delivery, due to preferential distillation caused by traditional liquid delivery systems. As an alternative, Matheson`s Semi Gas Division in San Jose, California, is working with tool manufacturers to develop a new "bubbler" delivery system, designed for use with DMAH, which will eliminate the need for additives and allow for simplified precursor delivery.
A "bubbler" or "ampoule" is a container used for packaging aluminum precursors for CVD. Precursor bubblers for the semiconductor industry are usually fabricated from 316L stainless steel with an electropolished interior of 10Ra or better and high-integrity valves on inlet and outlet ports. They essentially consist of a valve and dip tube on the inlet side and another valve open to the head space above the liquid on the outlet side (Fig. 3). Bubblers provide one of the safest ways of handling aluminum precursors, and they should be the only form in which a wafer manufacturer receives delivery of material. As with all hazardous materials, the handling of aluminum precursors should only be performed by trained personnel wearing correct protective gear.
Safety and handling
Due to the hazardous nature of the compounds, users should complete a full safety and hazard analysis. Currently, Department of Transportation (DOT) regulations governing the shipping of hazardous materials help maintain personnel safety and eliminate potential damage to a bubbler during transportation. Personnel should be properly trained to handle bubblers and inspect their condition upon receipt. They should also be well prepared to receive a damaged bubbler safely. The main product of combustion for aluminum alkyls is aluminum oxide, a white powdery solid. Smoke, fire, or the presence of aluminum oxide would obviously indicate a damaged or leaking bubbler.
Operators should wear personal protective equipment (PPE) at all times when handling aluminum precursors. The following PPE is recommended when handling a bubbler :
goggles or safety glasses with solid side shields,
laboratory coat made of a flame-resistant material such as Dupont Nomex, and
leather or heavy nitrile rubber gloves.
Aluminum precursor bubblers can be safely stored in a standard flammable liquid safety cabinet located in a cool, dry area. The bottom and shelves should be lined with sand and equipped to secure the bubblers positively within the cabinet. A rubber bottle carrier filled with 1-2 in. of sand is suitable for transporting the bubbler to the process tool within a facility. When moving a bubbler outside a facility, it should be packaged according to DOT guidelines.
Connection involves preparing the bubbler and the process lines. With the valves closed, personnel should carefully loosen the inlet and outlet caps and inspect the ports for smoke or signs of oxide. With the inlet and outlet caps removed from the bubbler, and with the cleanliness of the inlet and outlet ports assured, the bubbler is ready for installation. Connection to the process tool should be completed according to the manufacturer`s instructions. Introduction of air and moisture into the process lines must be kept to a minimum. An inert gas purge through the process lines will help to keep countercurrent diffusion down. The equation:
estimates the flow rate, L, necessary to maintain the partial pressure, Po, of an atmospheric component at some desired partial pressure, P, inside the process lines. A represents the cross-sectional area of the process line, X is the length of the line, and D1, 2 is the diffusion coefficient of the impurity gas, 1, in the inert purge gas, 2. (Note: the diffusion coefficient for air in nitrogen is roughly 0.2 cm2/sec) . The product of the equation is the lowest flow rate at which a purge gas should be run and provides a starting point for determining purge flow. A cycle purge step including evacuation of the process lines to a known base pressure can remove contaminants before introduction of the aluminum precursor to the process lines.
Fire equipment and procedures
Fire is the single largest hazard associated with the use of aluminum precursors and can be successfully handled by a corporate emergency response team (ERT). However, all personnel potentially involved in handling an emergency - prior to arrival of the ERT - should clearly understand all safety concerns.
As in any fire situation, personnel should determine whether they have the ability to control it. If not, fire alarms should be activated and the building evacuated. Personnel should be properly trained in the use of firefighting equipment and procedures. The following PPE is recommended for fighting aluminum precursor fires  (Fig. 4):
SCBA: self contained breathing apparatus,
aluminized fire suit (hood, jacket, pants, gloves, and shoe covers), and
Type D fire extinguisher with an extinguishing medium such as Met-L-X. [Never use water, fire foam, or halogenated fire extinguishers on a metal alkyl or metal hydride fire.]
Wafer manufacturers must be prepared to respond to either:
fire caused by the aluminum precursor itself, or
a bubbler of aluminum precursor threatened by fire.
Fire caused by aluminum precursor leaking from the bubbler can be extinguished in two ways: stopping the source of the precursor or smothering the precursor to isolate it from its source of oxygen. Both Class D fire extinguishers and sand are effective in smothering aluminum precursor fires. If the fire is smothered the precursor is still active and must be dealt with; however, smothering can provide time to assess the situation and proceed in a controlled manner.
A CO2 extinguisher can put out or control the burning material temporarily, allowing time to close an open valve and stop the flow of a precursor. The burning precursor is only isolated from the atmosphere as long as CO2 is applied. Once the extinguisher is exhausted, the aluminum precursor will reignite immediately. In some cases, aluminum precursor fires will extinguish themselves - aluminum oxide builds up at the point of exposure, in effect sealing the leak from oxygen. Personnel should not disturb the oxide, or the fire could be reignited.
Move a leaking bubbler to a safe place or remove all flammable material from the area; whichever is safest. Use sand to contain the leaking material and extinguish the fire with a Class D extinguisher. When a bubbler containing an aluminum precursor is threatened by a fire, not from the precursor itself, move the bubbler or extinguish the fire by traditional means, whichever is the safest and most timely. If the bubbler cannot be moved, it should be kept cool since pressure may build inside if temperatures reach the decomposition point of the precursor.
Due to the reactivity of the aluminum precursors, ingestion or inhalation of the compounds is highly unlikely. The greatest health risks are thermal burns or inhalation of aluminum oxide dust caused from combustion of the precursor. A threshold limit value (TLV) of 2 mg/m3 of aluminum has been established for aluminum alkyls . Aluminum oxide has a TLV of 10 mg/m3 in air. Exposure to aluminum oxide dust can cause eye, skin, and respiratory tract irritation. Lung damage (known as Shavers` disease) due to the inhalation of finely divided aluminum oxide particles has been reported . Treatment for exposure to aluminum precursors or the products of combustion should include flushing the affected area with large amounts of water and treatment for thermal burns. Medical attention should be sought in all cases of overexposure to the precursors and their products of combustion.
The precursors discussed represent a promising class of materials for furthering development of aluminum CVD processes. Bubbler technologies for safe and cost-conscious storage and delivery are currently being developed that will help bring these new materials to production for next-generation devices.n
Nomex is a trademark of E.I. DuPont Co. Met-L-X is a trademark of Ansul Fire Protection Co., Marinette, WI.
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CLARK MCGREW received his BA degree in science from the University of Northern Iowa in 1988 and has worked in process development and process scale-up of organic and organometallic compounds for more than eight years. He is a certified hazardous materials emergency response team member and is currently development chemist for organometallic products in the Research & Development group of Matheson Gas Products; Advanced Technology Center, 1861 Lefthand Circle, Longmont, CO 80501; ph 303/678-0700, fax 303/442-0711.