Achieving sub-ppb impurity detection to ensure the purity of electronic specialty gases

What types of gases are used in semiconductor manufacture?

Electronic specialty gases include both high purity gases and gas mixtures, which are specially configured for these specific applications.

Semiconductor manufacture may use up to 30 different gases in all the various processes. These include gases such as helium, nitrogen and argon which are used either to provide an inert atmosphere or to flush chambers at the end of a process or before cleaning.

Other gases are used in carefully controlled amounts to ensure that reactions do take place. These gases are used in the etching and deposition processes to create the layers and tracks of the semiconductor chips. These gases include the familiar (hydrogen, ammonia and chlorine) as well as more unusual compounds such as hexafluoroethane (C2F6), octafluorocyclobutane (C4F8) and germane (GeH4).

Because of the precision involved in semiconductor manufacture, very high purity gases are needed. Typically this is between 99.998% to 99.99994% purity, but varies between the gases supplied and the application.

Common contaminants in electronic specialty gases and how to detect them

The most common contaminants in electronics and semiconductor gases are moisture and oxygen. Controlling the levels of these is important for both gas manufacturers and users. We cover this in detail in our previous post Oxygen and moisture: a costly combination.

Because many ESGs are complex, there are a wide range of other trace gases that are potential contaminants, and because of the purity requirements, instruments with a very low detection limit are required.

While dedicated moisture and oxygen analyzers are highly effective at detecting trace levels of these impurities, for traces of other gases, analyzers that are capable of measuring multiple trace gases at very low levels are necessary.

Gas chromatographs are an ideal solution. They can be configured to detect various gas profiles and there are models available that detect multiple gas impurities simultaneously. This saves both capital expenditure and space. Having an instrument with a small footprint and designed for an industrial setting is also important.

Sample gas handling ensures high accuracy

When detecting impurities at sub-ppb levels, achieving the optimum flow rate of sample gas through the analyzer is essential to ensure the sensitivity of the analyzer. At the same time, analyzers with online capabilities provide an instant alert if gas quality is reduced suddenly (perhaps with a new bottle being attached) or if a leak occurs in the system.

Online analyzer systems need to account for these factors and use the right combination of sample extraction, carrier gas purification, stream selection and flow rate to ensure sensitivity.

Application examples of trace impurity measurements in semiconductor gases

Because this is such a complex area, we will look at some specific cases. Detailed application notes, including chromatographs can be found on our semiconductor industry page.

Detecting parts per trillion impurities in UHP gases

Measuring traces of permanent gases down to parts per trillion levels in ultra-high purity gases such as helium, argon, oxygen, nitrogen and hydrogen is a very common requirement in semiconductor manufacture.

The success of these measurements depends on a combination of detector, suitable gas chromatograph, sample dilution and gas stream selection.

LDetek has developed a system that successfully measures trace impurities down to less than 100 ppt with the combination of:

• PlasmaDetek2 Plasma Emissions Detector
• MultiDetek3 Gas Chromatograph
• LDGDSA automatic dilution system
• LDGSS sample stream selector
• LDP1000 gas purifier to produce grade 10.0 helium for use as a carrier gas

The table below shows the specific lower detection limits for each impurity in samples of He, Ar, O2, H2 and N2.

Download the complete application note which contains detailed information on the application and includes chromatographs.

Analysing UHP Octaflurocyclobutane (C4F8) semiconductor speciality gas

Octafluorocyclobutane, or perfluorocyclobutane, C4F8, is a compound of carbon and fluorine used in the production and processing of semiconductor materials and devices, for example as a deposition gas and etchant.

It is a complex gas to produce and there are a number chloroflurocarbon contaminants present, as well as carbon dioxide.

This gas is often required at ULSI 5N grade (99.999% purity) and this requires a highly sensitive quality control analyzer, which is capable of detecting all these gas impurities down to parts per billion levels.

The table below shows the sample composition of C4F8:

The LDetek MultiDetek3 gas chromatograph with the PlasmaDetek2 detector offers the requirements for such type of specialty gas application. The configuration with purged diaphragm valves combined with coated inert gas flow path and columns makes the system perfectly adapted for such specialty and complex gases. The system is rackmount and compact offering a full remote control. The industrial communication protocols are all built in and must simply be selected specifically for your requirements.

Example chromatographs are available in the full application note from ProcessSensing.com: MultiDetek3 gas chromatograph with PlasmaDetek2 detector uses for the analysis of semiconductor specialty gases as UHP Octafluorocyclobutane (C4F8)

Measurement of trace impurities in Germane (GeH4)

Germane is a highly flammable gas and any sample handling requires a high level of safety. As with all semiconductor gases it requires a high level of purity – at least 99.999% - and has a wide range of impurities to be detected.

The PlasmaDetek2 and the MultiDetek3 combined with the highly safe continuous monitoring sampling system allows the measurement of GeH4 purity with reduced risk. The N2 monitoring of the purged box is essential to ensure there is no ignition risk inside the MultiDetek3. The trace impurities measurement can be realized with a relatively short analysis time and can offer very low limit of detection of the measured impurities what is required for GeH4 purity analysis.

Download the complete application note here.

Discover the full range of Process Sensing applications for the electronics and semiconductor industry

Our semiconductor industry page contains application notes and product information specifically for the electronics and semiconductor industries.

You can contact us to talk about your specific application or to find out more about our range of industrial gas chromatographs and plasma emission detectors.