Total volatile organic compounds in cleanroom environments play an increasingly important role in industrial applications. Markus Keller, Fraunhofer IPA, Germany, explains how they are measured.
The outgassing behaviour of materials used in cleanroom environments has gained importance in various industries, particularly in semiconductor, photovoltaic and aerospace cleanroom settings, where a maximum level of volatile organic contamination (VOC), according to ISO 14644-8, is often defined in the planning phase. A standardised method for the determination and classification of cleanroom suitable materials regarding their VOC outgassing properties will be published in the new guideline VDI 2083-17-draft.
A vital tool for the selection of low-VOC-emitting materials is a standardised measuring method resulting in a material classification for comparison and selection of the appropriate cleanroom suitable material (VDI 2083-17-draft).
This paper provides a guide to the easy-to-use calculation model that gives an estimation of the expected total VOC (TVOC) level with regard to the relevant materials. In addition, a comprehensive database on the website www.ipa-csm.com provides cleanroom planners and operators with a good insight into the VOC-emission behaviour of tested materials.
As well as particle generation due to mechanical stress, the outgassing behaviour of materials considered suitable for use in cleanrooms is becoming an increasingly important factor. First, the quantities of substances emitted from the materials deemed suitable may not exceed MAC values – maximum allowable workplace concentrations of VOC emissions from cleanroom suitable materials.
Second, sensitive manufacturing environments require a controlled low level of molecular organic contamination in the ambient air. Regarding the standard ISO 14644-81, airborne molecular contamination (AMC) is defined as the presence in the atmosphere of a cleanroom or controlled environment of molecular (chemical, non-particulate) substances in the gaseous or vapour state that may have a deleterious effect on the product, process or equipment in the cleanroom or controlled environment.
According to ISO 14644-8, AMCs can be categorised into the following substance groups:
- Acids (ac)
- Bases (ba)
- Biotoxin (bt)
- Condensable contaminants (cd)
- Corrosive contaminants (cr)
- Volatile organic compounds (or)
- Dopants (dp)
Organic compounds outgassed from materials (softeners, solvents and other volatile constituents of materials) also play a vital role with regard to the amount of airborne organic contamination contained in a room or in process air.
Various damage scenarios are conceivable: airborne organic contamination (VOC; see ISO 16000-62) could settle on the lenses of lithographic units and significantly impair their performance. In the semiconductor industry, organophosphates or doping substances condensing on wafers could severely damage them. Siloxanes could lead to the complete failure of electric contacts, e.g. when checking wafers.
Some classes of compounds are therefore considered to be particularly critical and may not be contained in the cleanroom atmosphere of a semiconductor factory above the current detection limit. Such compounds include siloxanes, phthalates, amines, organophosphates, doping materials and other dangerous process-specific substances. These substances require particular attention when considering the outgassing behaviour of cleanroom-suitable materials. The material classification presented in this paper enables materials to be selected for use in specific clean production environments.
Figure 4: Schematic of the analytical tools used in determination of the material specific emission rate (SERm)
Determining material-specific VOC-emission rates using an adapted test chamber technique: The aim of this method is to define test criteria that enable materials to be evaluated and classified with regard to their TVOC outgassing properties. The method permits the VOC emissions of different materials to be compared and allows them to be ranked in a list to facilitate the selection and classification of materials.
Figure 5: Material gas chromatography – temperature profile
The quantity of organic compounds outgassed from materials depends upon surface area, outgassing time, age and test temperature. The specific emission rate (SER) is related to such parameters and is expressed as mass per surface area x time (g/m2s) at the corresponding ambient temperature.
To achieve a standardised comparable test procedure, measurements are carried out in an emission chamber in accordance with ISO 16000-93. Outgassing is determined by collecting and concentrating the volatile compounds and subsequently analysing them using thermodesorption with coupled gas chromatography and mass spectrometry (TD-GC/MS).
Sample preparation: Samples have to be typical as far as their geometry and surface characteristics are concerned and need to be tested under the same conditions as those experienced by the material in the cleanroom. In multiple layer applications, layer composition must correspond to the planned usage.
The cut edges of solid samples, which should not be considered as being part of the active surface area, need to be covered appropriately (e.g. with an aluminum cutting ring – see figure 2). VOC-free carrier materials (glass dishes, stainless steel) are to be used for free-flowing samples and the active surface of the samples must be quantifiable.
Reactive hardening samples need to be pre-conditioned over a period of 30 days under controlled climatic conditions (room temperature 22°C +/-1°C, relative humidity of 45 % (see also VDI 2083 Part 9.15 and EN ISO 16000-114).
Samples may not become contaminated during storage. This is achieved by storing samples in a VOC-reduced environment (mini-environment with VOC filtration, M+W Products GmbH, Stuttgart, Germany, see figure 1). The TVOC-reduced storage environment has to be at least one class better than the anticipated VOC assessment of the test piece.
Figure 2: Material sample in a Petri dish cast with a sampling chamber and overlying ring for determining the specific emission rate of the material
Figure 3: A micro-chamber/thermal extractor (µCTE) from Markes International, Llantrisant, UK
Taking samples: Following completion of the pre-conditioning period of 30 days, a defined material sample with a surface area of 10 cm2 is placed in a micro-chamber/thermal extractor test chamber (figure 3) with a diameter of d = 45mm and a volume of V = 40ml under atmospheric pressure at a standardised temperature of 22°C +/-1°C (room temp) After an equilibration time of 15 min, the VOC-sampling time is set to one hour.
The flow rate of the rinsing (nitrogen 5.0 or ultrapure VOC-filtered air) gas through the chamber must be adapted to the geometry of the test chamber. A flow rate of 100ml/min is required for the size of micro-chamber mentioned. The rinsing gas transports the VOCs emitted from the material sample to a sorption tube containing as appropriate adsorbent Tenax GR (Markes International, Llantrisant, UK) where they are absorbed (purge and trap method).
The sampling time and flow rate form the basis of the subsequent quantitative classification. Samples can be compared only if sampling temperatures and times are identical. Materials are classified based on tests performed at room temperature.
Analysis: Sorption tubes are analysed by way of thermodesorption (TD) with coupled gas chromatography and mass spectrometry (GC/MS) at 280°C (figure 4). The thermo-desorption process releases the VOCs held by the sorption tubes, enabling them to be subsequently analysed. The analysis was carried out in compliance with VDA 2786 using the parameters shown in Table 1.
| Table 1 | |
| Thermal desorber (TD) | |
| Temperature tube/valve/ transfer-line | |
| Desorption 1 (tube to trap) | |
| Desorption 2 (trap to GC) | |
| Flow desorb | |
| Flow column | |
| Flow outlet | |
| Trap low | |
| Trap high | |
| Trap hold | |
| Heating rate | |
| Gas chromatography (GC) | |
| The used temperature profile of the gas chromatographic system according to VDA 278 is illustrated in figure 4. | |
| Mass spectrometer (MS) | |
| Period of data acquisition | |
| Scan-Mode | |
| Scan-duration | |
| Inter-Scan-Delay | |
Determining the specific emission rate: The emission test initially only gives the mass of volatile organic compounds absorbed by the sorption tube after analysis in accordance with VDA 278. The mass is then related to both the surface area of the sample and the sampling time in order to enable the specific emission rate of the material per surface area to be calculated.
Here: SERm = specific emission rate of material per surface area at a room temperature of 22°C +/-1°C in g/(m2•s); mTVOC = mass of total products outgassed from material in g;
Am = surface area of material m in m2;
t = duration of sampling time in s.
Transferring the specific emission rate of a material to a standardised material classification: The described standardised material classification results in an easy-to-use, comparable and communicable classification number. Due to its logarithmic nature, the whole outgassing spectra from metallic compounds up to high VOC-emitting materials can be expressed. This classification number is later the basis for the calculation of expected TVOC values of real cleanroom environments. The tested material is classified into ISO-AMCm (or) classes in compliance with ISO 14644-8 on the basis of the value TVOCnorm :
Here, Vnorm = normed chamber volume of 1m3; Anorm = normed surface area of material of 1m2; nnorm = normed rate of rinsing gas of 1/s; and TVOCnorm = normed mass of total products outgassed from material in g/m3.
The value TVOCnorm is logarithmised in a decadal way to obtain the material-specific normed ISO-AMCm class (or). The term “or” defines the volatile organic compounds as a contaminant group.
This is expressed as follows: ISO-AMCm Class N (or) with a value lying between 0 and 12. Interim classification values may also be stated. In the process, 0.1 is the lowest permissible increment of N. The classification procedure can be carried out for all other AMC contaminant groups as well, as long as the contaminant group is marked according to ISO 14644-8.
The following section looks at converting a ISO-AMCm class to the ISO-AMCCR class of a real cleanroom environment.
Calculation of the ISO-AMCCR class: Already, during the planning and construction phase of a cleanroom environment, the derived TVOC-level is of concern. The described calculation model enables a first estimation of the TVOC charge of the cleanroom environment.
Regarding all the relevant VOC-emitting materials in a cleanroom (CR), the resulting ISO-AMCCR-class (or) in the cavity can be calculated using Equation 4.
Where TVOCCR = calculated total mass of products emitted from material in cleanroom in g/m3; ACR = surface area of material in cleanroom in m2; VCR = cleanroom volume in m3; nCR = rate of fresh air introduced into cleanroom in 1/s.
This equation takes into consideration the factors of fresh air rate introduced into the cleanroom environment (nCR), the surface area of material in the cleanroom and the volume of the cleanroom influencing the TVOC concentration. The rate of fresh air is calculated as follows:
Here, nCR = rate of fresh air introduced into cleanroom; AERCR = air exchange rate in the cleanroom; fCR = fraction of fresh air rate into the cleanroom.
When calculating the fresh air rate nCR, it is assumed that VOC-free fresh air is introduced into the cleanroom. The decadal logarithm of TVOCCR gives the ISO-AMC class of the cleanroom and is expressed in the following way: ISO-AMCCR Class N (or).
The ISO-AMCCR classification is made in accordance with DIN EN ISO 14644-8 (see figure 6). Regarding the relevant materials for the ISO-AMCCR estimation, the proportionate material surface area and materials with a high TVOC emission play the major relevance with regard to the SERm-values. Those most relevant are for large surface areas, such as flooring and wall systems, ceiling (filter systems) and air conditioning technology. Potentially high TVOC-sources can be sealants, adhesives and lubricants.
Theoretical background
Mass flow equilibrium at the stationary phase: Regarding the air flow design of a cleanroom, the following simplification can be achieved. The recirculated air from the air-conditioning system can be regarded as part of the cleanroom and has not to be considered for the calculation of the mass flows. Only the incoming fresh air with a specific mass concentration of the regarded contamination (csupply) and the exhaust air with the mass concentration of the cleanroom (cCR), both with the same volume flow F (in m3/s; calculates from VCR • nCR), have to be regarded for the calculation of the stationary phase of the mass flow equilibrium (see figure 7).
The stationary phase in a laminar cleanroom with an appropriate air exchange rate will settle in a very short period of time when all summed up relevant mass flows do not change any more. For the determination of the total mass flow of the sources in the cleanroom [∑(SERm • ACR)], the single mass flows from all relevant materials SERm1 • ACR1; SERm2 • ACR2;… have to be summed up. The mass flow equilibrium and, therefore, the concentration of the regarded contaminant in the cleanroom is calculated as follows:
Practical example: The following parameters were used in the test and subsequent calculation of the ISO-AMCm class (or) of a flooring material according to VDI 2083-17-draft:
- Volume of chamber Vchamber = 40 ml
- Active surface area of material Am = 10 cm2
- Sampling time t = 1 h = 3600 s
- Flow rate of rinsing gas F = 6000 ml/h = 1.66 ml/s
- Sample temperature: T = 22 +/-1°C
A total TVOC mass of 750ng was detected in the gas chromatography analysis. Using Equation 1 results in a specific emission rate of material per surface area SERm of 2.1• 10-7 g/(m2s). Using SERm in Equation 2, TVOCnorm is calculated to be 0.75 • 10-3 g/m3. The numerical value of TVOCnorm is then logarithmised in a decadal manner to obtain the material-specific normed ISO-AMCm class 3.1 (or). This enables the conditions to be calculated for a cleanroom with an active surface area of flooring material of ACR = 140m2, a cleanroom volume of VCR = 560m3 and a fresh air rate nCR = 0.0078/s.
The TVOCCR value is calculated using Equation 4, with the result TVOCCR = 6.7 •106g/m3. The actual ISO-AMC (or) class of the cleanroom is obtained from the decadal logarithm of TVOCCR: ISO-AMCCR class -5.2 (or).
The calculated value ISO-AMCCR class (or) reflects the situation 30 days after introducing the material into the cleanroom environment.
As VOC outgassing from materials reduces over time, the TVOC-level in the cleanroom will lower accordingly. Therefore, this represents a good estimation of the ISO-AMC class (or) and gives a not very exact TVOC-value of the cleanroom environment since only major VOC emitting materials are considered relevant for calculation, not all utilised materials. But, together with the described material classification, this calculation tool enables the user to make fast and effective material selection according to the needs of the application.
The actual precision of the calculation tool for real cleanroom environments will be assessed in the future by Fraunhofer IPA.
Figure 6: ISO-AMC air cleanliness classes in accordance with DIN EN ISO 14644-8
In conclusion, the use of a suitable test chamber method and corresponding analytics enable the specific emission rate of VOCs to be determined from a material sample. Through standardisation, a material class can be calculated based on this value. The standardised material classification permits a direct comparison of the tested materials to be made with regard to the emission of VOCs.
Figure 7: Simplification of the relevant air flows for setting up the equitation for the mass flow equilibrium in a cleanroom
The procedure presented in this article is implemented by the Fraunhofer IPA industrial alliance for cleanroom suitable materials (CSM), the VDI guideline 2083-17-draft7 and was put forward recently as a new work item proposal at the ISO Technical committee TC 209, in Tokyo, Japan. The tested and classified materials are awarded a corresponding CSM seal and are entered into the database belonging to the industrial alliance (see www.ipa-csm.com).
The database not only enables materials to be compared directly as far as outgassing behaviour is concerned but also with regard to other parameters such as resistance to chemicals, particulate abrasion and bio-resistance. This makes it a useful tool for selecting cleanroom-suitable materials for semiconductor and life-science applications.
References
1. ISO 14644-8: Cleanrooms and associated controlled environments - Part 8: Classification of airborne molecular contamination, Beuth Verlag, Berlin, 2006.
2. ISO 16000-6: Determination of volatile organic compounds in indoor and test chamber air by active sampling on Tenax TA® sorbent, thermal desorption and gas chromatography using MS/FID, Beuth Verlag, Berlin, 2004.
3. ISO 16000-9: Determination of the emission of volatile organic compounds from building products and furnishing - Emission test chamber method, Beuth Verlag, Berlin, 2006.
4. ISO 16000-1: Indoor air - Part 11: Determination of the emission of VOC emissions cleanroom suitable materials volatile organic compounds from building products and furnishing -Sampling, storage of samples and preparation of test specimens, Beuth Verlag, Berlin, 2006.
5. VDI 2083 9.1: Clean room technology - Compatibility with required cleanliness and surface cleanliness, Beuth Verlag, Berlin, 2006.
6. VDA 278: Thermo desorption analysis of VOC emissions for the characterization of non-metallic materials for automotive industries, 2002. VDA-QMC, Berlin, 2002.
7. VDI 2083-17-draft: Cleanroom technology – Compatibility with required cleanliness class and surface cleanliness, Beuth Verlag, Berlin, 2010.
