In the semiconductor, photovoltaic and LED sectors, airborne contamination is frequently an issue, but measurement and monitoring techniques often lag behind product development. An ongoing EU research project aims to remedy this situation
View into the cleanroom facility at the Physikalisch-Technische Bundesanstalt (PTB) – one of the MetAMC project partners
Technological progress in several high tech industries is enabled, if not driven by the ability to operate at ever smaller scale. This introduces new challenges in the metrological realm. Airborne molecular contamination (AMC) is chemical contamination, in the form of vapours or aerosols, that has adverse effects on products, processes or instruments. Sectors for which the control of AMC is crucial include semiconductor, nanotechnology, photovoltaic and high brightness and organic LED. In 2013, a new European project was set up to enhance the measurement capabilities for airborne molecular contamination.
The European semiconductor industry was ranked as the most R&D intensive industry sector by the European Commission in 2011.1 This sector supports around 110,000 jobs directly and up to 500,000 jobs indirectly in Europe, operating in a worldwide market valued in excess of €215bn in 2011.2 In such a high value business, where product yield directly affects the profitability of the industry, even a small change in the yield can lead to multi-million euro savings.
Since AMC is one of the major components that affect product yield, and will do so even more in the future, the demand for practical AMC monitoring devices will be high. A more effective implementation of AMC monitoring by European industry would provide a competitive advantage.
The formation and behaviour of airborne molecular contaminants in production environments is largely unknown due to a lack of sensitive on-line and on-site measurement methods for airborne chemical contaminants. Important sources for AMC include process chemicals, filter breakthrough, building and cleanroom construction materials and operating personnel.
The primary sources of AMC in manufacturing environments are chemicals used during production. These molecules are often reactive and adsorb easily to surfaces, making them difficult to avoid and to measure while being disruptive to the microfabrication process. Examples of possible adverse effects include the corrosion of metal surfaces on the wafer, and the formation of contamination layers on surfaces such as optics and wafers after reaction/condensation.
AMC monitoring is typically based on mass spectrometry and environmental reactivity coupons, in spite of their limitations
The relevant chemicals are diverse in nature and include acids, bases, condensables, dopants and metals and lists of the critical ones are available on the ITRS website.3 The relevant levels of these molecules are typically at (sub) parts per billion (ppb) level, making their detection extremely challenging. Regulations, knowhow and analytical capabilities in this field are less well developed than in the field of contamination by particles.4
AMC monitoring is typically based on mass spectrometry and environmental reactivity coupons (ERCs), in spite of their limitations. In many cases AMC monitoring is completely neglected. Current commercially available gas analysers utilising optical measurement methods are usually based on photo-acoustic spectroscopy (PAS) or cavity enhanced spectroscopy (CES), although they are generally designed for different applications. From a scientific point of view, these are the most promising optical measurement techniques for real-time detection of extraordinary diluted gases. The typical sensitivity of CES-based commercial gas analysers lies in the ppb range with a time resolution of some minutes depending on the analyte.
An AMC monitor that provides analysis and feedback within approximately one minute would allow for a more timely detection of higher than acceptable contamination; determination of the contamination source; and initiation of corrective actions before valuable data is lost and/or products are affected.
For many other AMCs such as semi-volatile organic compounds reference materials are completely lacking
Currently available instrumentation is often not fit for purpose due to high costs, large size, limited reliability or difficult handling. Furthermore, a need from industry exists for reference materials for AMCs at appropriate amount of substance levels. For an important AMC, such as ammonia, the lowest amount of substance level in reference gases currently provided by national metrology institutes (NMIs) is 1ppm, far above the limit set by the industry.
For many other AMCs such as semi-volatile organic compounds (sVOCs), the situation is markedly worse as reference materials are completely lacking. Current practice is to set up ad hoc systems for gas mixture generation using, in particular, permeation or diffusion of unknown performance. Hence generated concentrations are not traceable to the SI units. Alternatively, gas standards in cylinders, of often unknown or even known poor quality, are used and diluted using mass-flow controllers.
The new EMRP-project MetAMC brings together experts with years of experience in sensitive measurement techniques and reference materials, to provide a comprehensive study of how to monitor relevant chemical contaminants now and in the future. The limitations of PAS and CES will be investigated with respect to temporal resolution and detection limits – both of which are crucial for potential applications of these techniques in cleanroom monitoring.
In a second step, the uncertainty and traceability of the measurement techniques are evaluated to ensure the high quality measurement standards needed in the semiconductor industry are met. Based on this data, the usability for AMC monitoring will be evaluated. Both measurement techniques will be tested in a cleanroom environment to evaluate their practical applicability.
On-line/real-time measurement techniques for AMC monitoring are one of the key needs of the industry
According to the International Technology Roadmap for Semiconductors5 on-line/real-time measurements techniques for AMC monitoring are one of the key needs of the industry. If the project succeeds in showing the usability of at least one of the proposed measurement techniques, this technique would be an innovative answer to the urgent demand of semiconductor industry to improve cleanroom AMC monitoring.
This project will also provide guidance in spectroscopy techniques other than PAS and CES (which consists of cavity ring-down spectroscopy and cavity-enhanced absorption spectroscopy) applied to AMC monitoring. In particular, the practical applicability of an ultrasensitive noise-immune cavity-enhanced optical-heterodyne molecular spectroscopy (NICE-OHMS) technique will be evaluated in this project, which will be crucial when evaluating which techniques would allow the detection of extremely small amounts of AMC.
|The European Metrology Research Project: Metrology for Airborne Molecular Contamination – MetAMC|
|Project volume||€2.9 million|
|Objectives||Check the principle and practical usability of PAS and CES for AMC online detection|
|Develop an advanced NICE-OHMS with improved sensitivity for AMC detection|
|Improve the applicability of GC to AMC monitoring|
|Develop dynamic generation methods for trace level AMCs|
|Project partners||Mittatekniikan Keskus (MIKES), Finland|
|Česk. metrologick. institut Brno (CMI), Czech Republic|
|Istituto Nazionale di Ricerca Metrologica (INRIM), Italy|
|Physikalisch-Technische Bundesanstalt (PTB) Germany|
|VSL B.V. (VSL), Netherlands|
|NPL Management Limited (NPL), UK|
|HC Photonics Corporation (HCP), Taiwan|
|Politecnico di Torino (POLITO), Italy|
Non-spectroscopic techniques are also integrated in this project. A case study related to off-line AMC monitoring using novel techniques compatible with classical gas chromatography is included to study how widely employed laboratory equipment could be better used for AMC monitoring.
Proper reference materials are essential for calibrating an instrument and to validate periodically measurement data. Trace gas analysers are commonly calibrated using reference mixtures in cylinders. These mixtures are prepared using the gravimetric method which consists of weighing high-purity gaseous or liquid components into cylinders. This provides traceability for the measurement of gases with relative uncertainties down to 0.02%.6 For reactive gases, however, the situation is dramatically different. The usual precision of the gravimetric preparation method is lost with such compounds. This is exemplified by the recent key comparison CCQM-K46 on 30ppm ammonia in nitrogen, which is one of the most important AMCs for the semiconductor industry.
Proper reference materials are essential for calibrating an instrument and to validate periodically measurement data
Although the amount of substance level is >3 orders of magnitude higher than is found in a typical cleanroom, this comparison showed a wide dispersion in the results. At lower concentrations the situation is expected to be markedly worse.7 This situation is particularly alarming as the demand for such mixtures is growing in the industry. Many key AMCs belong to this class of highly reactive molecules such as hydrogen chloride and hydrogen fluoride.
For other AMCs, including the important group of sVOCs, accurate reference materials in cylinders are also lacking as their preparation is complicated due to their high boiling point and thus low vapour pressure. One viable alternative is dynamic preparation of reference gas mixtures for AMCs, i.e. by mixing of known (mass) flows of calibration gas compound(s) and complementary gas, usually nitrogen or air, according to ISO 6145.8 In particular, diffusion-based methods are most promising in this respect as the chemical is contained in a glass tube which is very inert. However, diffusion systems currently operated at NMIs are not transportable outside the metrological laboratory for on-site instrument calibration. To perform in situ calibration, the focus of this project will be on the development of innovative compact and miniaturised dynamic standards.
1. R&D ranking of the top 1000 non-EU companies by industrial sector, in ‘The 2011 EU Industrial R&D Investment Scoreboard’. iri.jrc.ec.europa.eu/research/docs/2011/vol_II_3.pdf
2. Latest Semiconductor Sales Data, Brussels, 2 Aug 2012, https://www.eeca.eu/data/File/ESIA_WSTS_PR0612m.pdf
3. AMC definitions, www.itrs.net/Links/2011ITRS/Chapter%20Links/Yield/AMC_ITRS_Definitions_090724_AN.xls
4. ISO 14644-8 Classification of airborne molecular contamination
5. The International Technology Roadmap for Semiconductors, 2011 Edition, www.itrs.net/Links/2011ITRS/Home2011.htm
6. MJT Milton, GM Vargha and AS Brown; Metrologia 48 R1–R9 (2011)
7. AMH van der Veen et al 2010. International comparison CCQM-K46: Ammonia in nitrogen. Metrologia 47 08023 (2010)
8. ISO 6145: Gas analysis – Preparation of calibration gas mixtures using dynamic volumetric methods www.iso.org
Tuomas Hieta, Mittatekniikan Keskus (MIKES), Tekniikantie 1, P.O. Box 9, FI-02151 Espoo, Finland
Anne Rausch, Olav Werhahn and Volker Ebert, Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany Above: View into the cleanroom facility at the Physikalisch-Technische Bundesanstalt (PTB) – one of the MetAMC project partners