Advanced Gates Present New ESH Issues

The International Technology Roadmap for Semiconductors (ITRS) indicates that new metals and dielectric films will be required for future semiconductor technology generations (130 nm and beyond). Researchers are investigating high-k materials as possible gate dielectric materials, along with complementary metal films for the gate electrode. High-k materials may also be used for embedded elements. For these applications, the type of film material and the geometry of the semiconductor device will result in the need for advanced chemical vapor deposition (CVD) processes to produce the films.

Transition metal oxides and silicates are high-k materials that may potentially replace SiO₂ in gate applications. Metals such as titanium, tantalum, zirconium, hafnium, strontium and yttrium have been identified as possibilities for gate dielectric; and metals such as titanium, tungsten and tantalum may be used for complementary metal electrodes in advanced gate stack as well as in barrier and conducting film applications. The CVD reactions will utilize metal organic, metal halide, metal nitrato and other precursors, presenting challenges in determining environment, safety and health (ESH) properties, and characterizing and handling process emissions.

	As shown in this schematic, several analytical techniques are used in a CVD chamber to characterize process emissions.

When a material is identified for a development application, it undergoes an ESH review. The chemical components are considered as well as the properties, such as toxicity, flammability and reactivity. Predicted reaction products and material incompatibilities are identified where possible. To conduct a process hazard analysis, the tool and process in which the material will be used, along with other process components, must be evaluated. Compatibility with existing tool exhaust and toxic/hazardous gas monitoring (TGM) must be verified. Process emissions characterization is also a key component of the ESH analysis.

In many cases, specific TGM sensors for these CVD precursors are not available. Also, the material may be reactive to air or moisture so that, if the material were released, it would break down rapidly upon exposure to the humidified fab air. Therefore, the monitoring can be based on a breakdown product of the precursor or an adduct material.

Once the components for monitoring have been determined, the locations for monitoring and the concentrations to trigger alarms must be determined. The locations can depend on the fab and tool layout, facilities layout, and placement of the source gases and precursor materials. Since hazardous gases and precursors are typically located in an exhausted enclosure, a likely point for monitoring would be the exhaust exit of that cabinet. Thus, if the cylinder or ampoule had a leak, the hazardous material (or decomposition product) could be detected and the delivery of the gas or precursor to the process tool discontinued. Other areas to monitor could be where the gas/precursor delivery lines enter the tool or the worker area around the tool. A release detected at these points could trigger a tool and/or material delivery shutdown or even an evacuation of workers. The concentration that triggers an alarm depends on the material. It could be based on the threshold limit value (TLV) of the material; or, in some cases, the immediate danger to life and health (IDLH) concentration. A review of the specific hazard of the material and the location of the material source and tool will be factors.

There are several types of monitors available. Some monitors are based on a cassette tape formulated to react with a specific gas or group of gases. If the gas is present in the air sample, the reaction of the gas with the tape will produce a stain that is proportional to the gas concentration. The stain is optically measured and the concentration level reported to the fab monitoring system. Tape monitors are available for mineral acid gases, such as HCl, HBr and HF; hydrides, such as silane, arsine and phosphine; and other compounds, such as trimethylamine. Other compounds can be detected, such as dichlorosilane as HCl and SiF₄ as HF. These tape systems can have multiple sampling points in an area. Another similar type of monitor for materials classified as toxic or highly toxic uses a hydrogen flame for detection. The toxic substance causes a characteristic color change in the flame, which is analyzed by optical filters. This type of monitor is useful for highly toxic hydrides, organometallic and inorganic toxics.

Some broad-spectrum monitors include air composition monitors and combustible gas analyzers. An air composition monitor scans one area at a time, but can detect multiple components in each sampling event. It is well suited for gas/ampoule cabinets, tools and storage areas to alert of a potential leak. It is also suitable for the workplace area should a leak spread to the breathing zone. Although it can detect multiple compounds, each compound has to be programmed into the system, which has component-specific analytical methods assigned to each detection point. For materials that are not commonly used, a method may have to be developed and programmed into the system.

Combustible gas analyzers, on the other hand, respond to most combustible gases in an air environment. One type consists of a catalytic sensor and electronic transmitter designed to operate in concentrations below the lower explosive limit (LEL) of the gases. Since it is generally calibrated to some percentage of the LEL, it can detect a broad range of combustible gases, as long as the flashpoints of the materials do not greatly exceed 110°F. Combustible monitors can be employed for flammable materials that do not have a functional group that can be monitored (e.g., halogen or amino group) or are not air- or water-reactive.

In R&D, materials and tools are handled with a conservative PPE (personal protective equipment) strategy. Furthermore, the process is only operated under engineering control such that a technically qualified individual is supervising all activity, including chamber maintenance. The process emissions are monitored to determine reaction byproducts and presence of unreacted precursor. This monitoring can also help determine if there is residual precursor or byproduct present in the chamber prior to maintenance.

Until the process is fully characterized and any potential maintenance hazards identified, a conservative chamber maintenance procedure is employed. The area around the chamber and subfab is cleared prior to opening the chamber. Maintenance personnel wear supplied air and full PPE. If available, a TGM sensor point is positioned in the chamber area and IH area monitoring is conducted. Supplemental exhaust is used during the chamber opening, if available. Emissions can be monitored during the chamber purge to ensure there is no spectral signature from residual precursor or byproduct. In some cases, special chamber wipes may be used. For water-reactive materials, other materials such as isopropanol may be used during the chamber wipe procedure.

Emissions characterization from these advanced CVD processes serves several purposes. The most important function is to identify and, where possible, quantify the byproducts of the CVD reaction. The emissions typically contain unreacted precursor, breakdown products of the precursor, and other byproducts formed from reaction with co-flow materials. This characterization data can be used to determine the level of hazard of chamber maintenance, and if point-of-use (POU) abatement will be recommended. In some cases, abatement may already be required based on the precursor itself (highly toxic or flammable) or a co-flow material (such as silane). For cases where POU abatement is added, emissions data collected before and after the abatement unit can be used to determine the abatement efficiency. Emissions data can also be used for process diagnostics; in some cases, byproduct emissions correlate with deposition parameters.

Several analytical techniques are used to characterize process emissions. The instruments can be located at various points in the exhaust. The Figure illustrates the analytical sampling schematic for a CVD chamber with POU abatement. QMS is utilized in the foreline, which is typically 1-50 Torr. The mass spectra collected here are useful for identifying precursors and process byproducts. QMS can scan all masses if the expected reaction byproducts are not known. The ITMS, in this case located post-pump, where it utilizes a novel pneumatically actuated transfer line, can trap higher mass ions and fragment them for species identification. This technique is used to determine likely reaction pathways or breakdown processes. FTIR is useful for species quantification, particularly if the species are known and calibration curves have been generated. FTIR can also detect precursors that are outside the mass range of the QMS.

In our research, before each material was evaluated in a CVD process, the material and process were analyzed to determine the appropriate controls and procedures for the CVD chamber. The properties of the precursor were considered, along with reaction products and hazardous decomposition products upon exposure to air or water. TGM strategy was based on the hazardous decomposition products since a release would be to ambient (humidified air).

The process emissions were then characterized to determine the byproducts. The Table lists the main byproducts from several advanced CVD processes (not including unreacted precursor).

POU abatement will likely minimize the ESH impact of these types of precursors and their byproducts. Of particular concern is the potential of exposure to unreacted precursor or metal compounds during exhaust duct maintenance if these compounds are not removed from the process exhaust by abatement.

Since many of the advanced CVD processes under development utilize precursors that are relatively new and not widely used, it is important to conduct an ESH analysis during the R&D phase. This analysis includes an assessment of the materials used in the process as well as the process byproducts. Emissions characterization capability is crucial for a thorough analysis. In many cases, POU abatement is warranted to minimize the overall ESH impacts of the precursors and byproducts.

This article was originally presented at the SEMI Technology Symposium (STS) Critical Technologies Conference, Austin, Texas, Oct. 17, 2001.