Maintaining ultra high purity

Ultra High Purity (UHP) gases with bulk gas piping distribution systems are widely used in the semiconductor manufacturing industry. The requirements for faster, higher capacity semiconductor components are driving semiconductor process technology limits and the accepted impurities levels in UHP gases continue to reduce due to increased demands for purity in semiconductor fab processes.

Bulk gas distribution systems are therefore complex: acceptable system integrity depends upon continuous perfect operation of numerous components within the distribution system that can provide a leak path from the atmosphere, such as bad valve seats or diaphragm seals, or capped service taps accidentally left loose.

While UHP distribution systems are generally very reliable and are always qualified at UHP levels prior to commissioning, these systems do break down and technicians inevitably make mistakes. The result can be a leak from the atmosphere of contaminants that include excess moisture, oxygen, nitrogen or carbon dioxide. This contamination can cost hundreds of thousands of dollars in scrapped wafers, and much higher figures if the contamination source is not identified and fixed quickly.

Significant advantages
The classic approach to checking gas distribution system integrity is by detection of helium using a mass spectrometer. This verifies integrity after initial fabrication, or after a process failure, to locate the leak.

This outboard leak check procedure requires helium to be flooded into the piping branch to be checked and a high sensitivity mass spectrometer detector is used to evaluate every joint, valve and fitting for helium that may be leaking. Alternatively, with the inboard leak check procedure, a hard vacuum is pulled on the piping branch and the mass spectrometer while helium is flooded along the outside.

Both techniques are effective, but present operational problems for plant operators. The piping system needs be taken out of service for some time during test and during a purge down of the system with UHP gas following the tests. Furthermore, this method can only be used to verify system integrity under a static condition and only on an occasional spot-check basis.

A non-invasive leak detection technique is a faster and easier way to identify an ambient atmospheric leak in the gas distribution system, without interrupting the normal gas supply. As this approach does not interfere with gas delivery, it can be done on a continuous real-time monitoring basis that avoids unscheduled downtime and unnecessary cost.

Oxygen – An ideal indicator
The key to successful non-invasive detection is to identify a suitable leak tracer gas that is not found in the UHP gases, but does appear in the event of a leak - and a gas species that should be detected before fabrication processes become corrupted.

Oxygen is the logical choice as a leak tracer gas. As a highly mobile molecular species with favorable surface adsorption properties, oxygen will find the leak path and readily diffuse into the UHP gas much faster than H2O. Once inside the distribution system, the poorer adhesion of O2 to the inside surfaces of the piping ensures it travels with the prevailing flow and diffuses rapidly to an analyzer sampling location.

An ultra-trace oxygen analyzer is required, such as the Servomex Delta F NanoTrace Analyzer (DF-560) which has the ppb sensitivity available to detect minute leaks in the realm of the helium mass spectrometer’s capability, but it can do so on-line in real time without the disruption of normal UHP gas delivery.

An optimized approach
Most semiconductor fabs incorporate a multi-contaminant analysis system to verify levels of key impurities (for example H2, O2, CO, CO2, particles and hydrocarbons) in each of the UHP bulk gases distributed within the fab.

Due to cost constraints, this multi-component measurement is typically the minimum of an exit purity measurement at the end-to-main in the distribution system. Problems can therefore develop if a leak develops in a line branching off of the main line. Depending on gas usage patterns, it is possible for contamination never to exit the main line or for a quick spike that propagates into the main line being too brief for slower analyzers to detect or be diluted down to below the analyzer’s detection limit. In either case, the contamination will not be detected by exit purity analyzers.
A better approach is to monitor more locations, but at less cost per point. This enables a more widespread protection of the entire distribution system, including out toward tool locations. A careful study of the piping in the distribution system will yield a map of the potential locations for continuous monitoring. By selecting points which optimize widespread protection proximity to more contamination-sensitive tools, this yields an acceptably short list of locations for continuous monitoring in each UHP gas piping network.

Once the location sample point locations have been determined, oxygen analyzers must be selected which have a reliable lower detection limit (LDL) at least 2-5 times below the O2 Impurity Specification Limit for the UHP gas of interest. Following installation, each should be operated for several days on purified source gas to characterize its zero baseline performance.

Data should then be charted with sufficient resolution to determine the peak-to-peak noise and zero the analyzer’s performance. The analyzer’s trend data while sampling purified UHP gas should then be compared with trend data taken while sampling the normal process UHP gas, enabling the user to establish the contaminant signature.

Provided the analyzer has an adequately low detection limit, comparison of trending data taken over time versus the typical contaminant signature will uncover a potential leak well before risk to any fab production process has occurred. The trending data can be used to determine warning alarm setpoints, which can be set below the UHP gas product specification limits, but outside of the actual contaminant signature range.

An alarm trip at this level should prompt further investigation: A consequential breach of the distribution system will show up as an out-of-spec condition, but it will be caught early and readily localized by the network of on-line oxygen analyzers.

Locating the leak within the system
Once a leak in the gas distribution system has been uncovered by the on-line oxygen analyzers, the leak source can be located by using an additional portable analyzer for spot-checking. First, the data from the on-line analyzers can be correlated with data on pressure and flow changes within the system to identify a smaller area where the leak may reside. Analyzers that are not detecting abnormal oxygen levels are assured to be upstream of the leak, minimizing the area of the system to be spot-checked.

In the absence of pressure and flow data, review the process tools’ gas usage patterns. A tool cycle will cause local supply pressure to drop slightly when cycling on, and increase slightly when cycling off. The subtle pressure changes cause gas to flow which can either transport contaminated gas further into the distribution system, or suppress the spread of contamination by pushing back the contaminated gas which is trying to diffuse into the distribution system.

The portable oxygen analyzer can be used next, to sample from unused tap locations near but downstream of the suspected leak. The closer the sample location is to the leak source, the higher the concentration will be. Therefore, spot-check measurements may be at higher concentration and can be made more quickly.