National Physical Laboratory

Gas sensing in clean environments

Semiconductors found in the circuitry of everyday electronic devices need to be manufactured in ultra-clean environments, as airborne contaminants such as dust particles, vapours and aerosols can adversely affect the finished products.

A micro-scale diffusion device developed at NPL for creating dynamic reference standards
A micro-scale diffusion device developed at
NPL for creating dynamic reference standards

In the semiconductor manufacturing process, highly toxic gases are used in doping, etching and cleaning, etc. Even though every precaution is taken to maintain safe working environments, when gas escapes occur, gas monitoring equipment is the last line of defence in protecting people.

Honeywell Analytics manufactures and sells advanced gas detection devices for ultra-low level gas detection and asked the National Physical Laboratory (NPL) to investigate the best methods of producing reference standards for a series of problematic, reactive substances: hydrogen chloride, hydrogen fluoride, chlorine, diborane and arsine. The availability of reliable and repeatable reference standards is essential to correctly maintain and calibrate equipment.

An NPL Technology Innovation Fund project was set up to carry out the work and, after conducting a thorough investigation, NPL scientists were able to recommend the best methods for generating standards for each compound.

There are two main ways of producing gas standards. The static method is relatively simple and inexpensive and it involves mixing a known amount of gas with a known volume of dilutent (such as air), inside a sealed container. However, such containers are susceptible to leaks and the static technique is limited when dealing with the reactive substances involved in this project.

Dynamic methods are more elaborate and overcome some of the problems encountered with the static technique. Essentially, a pure gas is continuously introduced into a known flow rate of dilutent within a flowing system. This way, standards are generated continuously, reducing the effect of absorption and condensation on the walls of the container. Higher volumes of the standard can also be produced and the amount can be varied in real-time, adding flexibility to the system.

There are several ways of achieving dynamic reference standards. Seven well-established techniques were analysed in the project for their suitability and recommendations were made for each of the substances under test.

The project also investigated the feasibility of using surrogate molecules to calibrate the sensors (with similar chemical and physical properties to the target compound), for which reference standards were already available. The investigation found that the measurement uncertainties would be too great for this approach to work reliably and accurately.

The development of standards for trace level gases such as those studied in this project will help accelerate the uptake of a new generation of ultra-sensitive sensors. These will improve our understanding and hence the protection of people working in semiconductor manufacturing ultra-clean environments.

Steve Forrest, Strategic Marketing Leader EMEAI for Honeywell Analytics, said:

"We take the protection of people very seriously and understand that small variations in reference standards can have big consequences in the reliability, accuracy and overall performance of our customers' safety equipment. The highest performance and repeatability is essential to us as a business and we wanted to remove the potential for minor variances in the gas mix constituents of problematic gases like Arsine. The work done by NPL will assist us in our continued efforts to ensure the highest provision of safety in environments where gas leaks may occur."

More on NPL's work in Chemical Metrology

More on NPL's Technology Innovation Fund

For more information, please contact Paul Brewer

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