Material emissions – Cleanroom contaminants
Airborne molecular contaminants in cleanroom air
As consumers demand higher performance from electronic goods, greater demand is placed on precision engineering and the quality of sub-components, including silicon-based materials such as hard disk drives.
The performance of such components can be adversely affected by the presence of semi-volatile organic compounds (SVOCs), known in the industry as airborne molecular contaminants (AMCs). Problematic AMCs include plasticisers (such as phthalates), antioxidants, siloxanes, amines and amides, organophosphates and flame retardants.
Outgassing from materials is one of the main sources of AMCs, and these may include:
- Construction products used in the cleanroom, including plastics.
- Computer components.
- Consumables such as gloves, garments, tape and cleaning materials.
- Particulate air filters.
|Airborne molecular contaminants || |
AMCs are classified as acids (e.g. HCl, HF, HNO3, H2SO4), bases (e.g. NH3, amines, N-methylpyrrolidone), condensables (organics with boiling points >150°C) or dopants (phosphorus, boron and arsenic compounds). For more information, see the SEMI F21-95 standard.
Because they are in vapour form, AMCs can pass through most standard particulate air filters, leading to their presence in the cleanroom environment. It is therefore crucial for technology industries – especially semiconductor manufacturers – to monitor cleanroom air for SVOCs, and to minimise emissions from all products and materials used in cleanrooms. As a first line of defence, manufacturers of hard disk drives now pre-screen all cleanroom materials for the outgassing of certain SVOCs (organics with b.p. >150°C, known as ‘condensables’, and amines).
June 2013 saw the publication of a guideline, VDI 2083-17A (Cleanroom technology – Compatibility of materials with the required cleanliness). This defines a standardised method using microchambers for the classification of cleanroom-suitable materials according to their VOC outgassing properties. It also includes a database for material selection and comparison, giving all engineers involved in cleanroom planning and operation a useful tool to estimate the expected level of total volatile organic compounds (TVOC).
Meanwhile, previously used methods including ASTM F1982 and ASTM F1227 have been withdrawn, and the global industry association for the micro- and nano-electronics industries, SEMI International Standards, have taken over the publication of two similar methods:
Direct and indirect outgassing
The two most widely used approaches for assessing outgassing include:
- Direct outgassing, which includes assessment of total outgassing from materials or components under fixed conditions (temperature, time, gas flow etc.). This can be done by measuring weight loss, or – for quantitative and qualitative identification of compounds released – by direct thermal desorption (TD) with GC–MS.
- Indirect outgassing, which involves analysis of ‘witness wafers’ – blank silicon wafers that are used to collect volatiles present in the cleanroom atmosphere.
What Markes can offer
Equipment for monitoring of cleanroom air
The routine monitoring of cleanroom air can be achieved by pumped sampling onto sorbent tubes, followed by analysis by TD–GC–MS.
Equipment for determining material outgassing
The outgassing of materials used in cleanrooms has become increasingly important in a range of industries, with a maximum allowed level of VOC contamination often defined in the planning phase of cleanrooms in the semiconductor, photovoltaic and aerospace industries (in accordance with ISO 14644-8 or VDI 2083-17A).
Two sampling techniques can be used for screening materials used in the final products or in the cleanrooms:
- Exhaustive extraction of small samples by direct desorption is very simple, and involves simply placing a small (milligram) quantity of the sample in an empty TD tube. This tube is then heated and the vapours analysed by TD–GC–MS (for example, using Markes’ UNITY-xr or TD100-xr thermal desorbers.
- Assessing the representative vapour profile of bulk materials uses Markes’ Micro-Chamber/Thermal Extractor. Gram-quantities of a material are placed in one of the microchambers, which are heated while a flow of gas is applied. The vapours released are then collected on a sorbent tube for analysis by TD–GC–MS. Up to six samples can be collected per hour, and the results can be correlated with those of reference methods, making it an ideal tool for rapid emissions screening.
- For examples of the use of thermal desorption to monitor labile and/or high-boiling semi-volatiles such as those found in cleanroom air, see Application Notes 014 and 053.
- For more information on materials emissions testing in the semiconductor and associated industries, see Application Note 062.
- For a description of the Micro-Chamber/Thermal Extractor, and its application (with UNITY-ULTRA) to assess emissions from plastic pellets, wall coverings and polyurethane foams, see: T. Schripp et al., A microscale device for measuring emissions from materials for indoor use, Analytical and Bioanalytical Chemistry, 2007, 387: 1907–1919.
- For a paper describing a standardised fast screening procedure for classifying materials according to their level of VOC outgassing, with the aim of reducing AMC levels, see: M. Keller, U. Gommel and A. Verl, Test procedure to determine material specific VOC emission rates and prediction model of VOC-levels in controlled production environments, Chemical Engineering Transactions, 2012, 30: 301–306.
- For an explanation of how to measure TVOC in a cleanroom environment, see: M. Keller, VOC emissions test method, Cleanroom Technology, January 2011.
- For an example of how the installation of a high-efficiency particulate air filters resulted in unintentional doping of silicon wafers, see: J. A. Lebens, W.C. McColgin, J.B. Russell, E.J. Mori and L.W. Shive, Unintentional doping of wafers due to organophosphates in the clean room ambient, Journal of the Electrochemical Society, 1996, 143: 2906–2909.
- For an example of the use of TD–GC–MS to demonstrate the adsorption of phthalates and amines on silicon wafers, see: M. Tamaoki, K. Nishiki, A. Shimazaki, Y. Sasaki and S. Yanagi, The effect of airborne contaminants in the cleanroom for ULSI manufacturing process, Proceedings of the Advanced Semiconductor Manufacturing Conference and Workshop 1995 (ASMC 95), pp. 322–326.
- For more information on cleanroom technologies (in German), see: Fraunhofer Institute for Manufacturing and Automation (IPA).
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