Systematic material selection for cleanroom applications

Figure 1: View of the open sample chamber of the PET with inserted sample (©: Materiales GmbH)

Table 1: Industry-specific overview of critical contamination types, adapted from [1]

Figure 2: Results of a measurement of the particle contamination of a PEEK sample using the Particle Emission Tribometer (here exemplified by particles with a size > 0.3 μm)

The selection of materials is one of the critical steps in engineering science. In an era where increasingly complex and demanding applications impose higher requirements on the materials used, careful selection becomes a complex challenge. The material selection process includes a systematic analysis, which initially provides a comprehensive overview of the conditions to which the material will be exposed in use. In requirements engineering, besides technical aspects, ecological and economic factors also play a central role. Based on this specifications document, suitable materials are researched and identified that possess both technical suitability and allow for economical manufacturing of the components. From this broad preliminary selection, promising candidates are extracted through selected material tests and ultimately qualified in application-oriented tests or field tests.

While functionality, cost-effectiveness, and sustainability of materials are the decisive factors for traditional engineering applications, material cleanliness is an additional and indispensable prerequisite for contamination-sensitive applications. Therefore, it must be considered in the selection processes.

The ISO 14644 and VDI 2083 standards describe requirements and rules for the operation of cleanrooms. Specifically, sheet 17 of VDI 2083 describes standardized procedures to ensure various aspects of material purity. [1] Parameters to consider include particle emission behavior, outgassing, electrostatic discharge (ESD), chemical & corrosion resistance, and cleanability. Table 1 provides an overview of industry-specific contamination types and their relative importance.

The following sections describe the different types of contamination, methods, and possibilities for their assessment within the framework of material selection.

Particle emission

The air cleanliness according to DIN EN ISO 14644-1 defines cleanroom classes depending on the particle content of the air. Materials can generally harbor particles on their surfaces, originating from manufacturing or the surrounding air. Especially in the case of components where surfaces are in relative contact, new particles are emitted. The extent of particle formation depends on various factors, including the material combination, surface properties, or load, type, and speed of movement.

A method for the relative comparison of particle emission under defined loads and speeds is the particle emission tribometer (PET), developed by Materiales GmbH in collaboration with the Mannheim Tribology Competence Center (see Figure 1). This tribological test stand can be operated in a ball-on-disc or pin-on-disc arrangement. The geometric simplicity of the test specimens allows for a comparison of virtually any material combination. The simple testing principle enables variation of load and speed to simulate real-world conditions as closely as possible. Particles are generated in a sealed environment and directed via compressed air to a particle counter, which records six different particle sizes (>0.3 μm, >0.5 μm, >1.0 μm, >3.0 μm, >5.0 μm, and >10 μm). The classification into a cleanroom class can be made based on the total particles transported within a specific time through a given air volume.

Figure 2 exemplifies measurement results of a sample made of polyetheretherketone (PEEK) for particles with a diameter > 0.3 μm. The reference measurement and the particle formation during the actual measurement are shown, involving a material combination of PEEK and 1.4125 stainless steel at a load of 25 N and a speed of 30 rpm.

Outgassing

In addition to particulate contamination, chemical contamination can also cause a lack of cleanroom suitability of materials. "Outgassing" refers to the release of volatile, often organic substances (VOCs) from materials, such as phthalates, siloxanes, or amines, which are frequently used as additives and plasticizers. Outgassing often affects organic materials like plastics, adhesives, sealants, or coatings. Outgassing can also contaminate technical systems and cause damage. Released substances can deposit on surfaces, impairing the functionality of sensitive equipment, especially optical systems. Outgassing can also cause air pollution indoors or pressure increases in cleanrooms and vacuum environments, negatively impacting product quality and productivity. [2, 3]

Depending on the industry and application, different investigation methods are used. VDA 278 [4], developed by the German Association of the Automotive Industry, is a standard for determining the proportion of condensable and non-condensable volatile substances emitted from non-metallic materials. Originally, it is a standard method for ensuring air quality in vehicle interiors. During testing, a sample is heated in a desorption tube for a defined period. Temperature and duration are specified in VDA 278 but can be adapted to specific requirements. A carrier gas stream transports the volatile substances into a cooling trap, where they are enriched. Subsequently, the cooling trap is heated—substances evaporate and are subjected to gas chromatographic separation coupled with a mass spectrometer (GC-MS). This method allows not only quantification of the emitted substances but also identification of individual chemical compounds. ASTM E595 is a standard from the ASTM Committee on Space Simulation and Space Technology Applications. Similar requirements are found in the European Cooperation for Space Standardization (ECSS) under ECSS-Q-ST-70-02C. The precisely specified measurement apparatus heats the materials under investigation in high vacuum at 125°C for 24 hours and records mass loss. Additionally, the apparatus determines the proportion of condensable and non-condensable species and the water vapor uptake of the tested material.

A third method for evaluating outgassing is residual gas analysis (RGA).

This measurement method determines the outgassing of a material at room temperature and in high vacuum by ionizing the released substances. The ion detector identifies ions and their fragmentation patterns, enabling the identification of neutral substances. When the composition of the substances is known, calibration coefficients can be used to determine the relative composition. The detection limit is 3 ng/cm³.

Electrostatic discharge (ESD)

Where particulate contamination is an issue, it is essential to understand the electrostatic properties of the materials used. If in an application there is friction between materials or an electric field acts on the materials, electrostatic charging can occur. This can cause particles to adhere to the material and later be emitted uncontrollably during operation. Depending on the application, the material must be classified as antistatic, electrically conductive, electrically dissipative, or electrostatically insulating. Parameters such as surface resistance or volume resistance are measured according to DIN EN 61340-5-1, DIN EN 1081, or SEMI E78-0309.

Chemical & corrosion resistance

In various processes conducted in cleanrooms or for cleaning products or assemblies, materials are exposed to a wide range of media and substances. Therefore, VDI 2083 sheet 17 considers the aspect of chemical resistance and corrosion behavior of materials. For testing, materials are leached in the relevant chemical for a defined period and then examined for changes. The evaluation is usually based on visual characteristics according to DIN EN ISO 2812-1 or DIN EN. Changes in material properties, such as hardness or tensile strength, can also be assessed. The determination of test conditions is not governed by a standard but is set according to individual application requirements.

Cleanability

The procedure described in VDI 2083 sheet 17 for verifying the cleanability of materials relates exclusively to particulate contamination on material surfaces. To evaluate this, a comparison of particle counts before and after cleaning is performed. Particles are counted on the surface in multiple size ranges before and after cleaning. The particle count before cleaning is related to the count after cleaning, allowing for the determination of cleaning efficiency, which can be classified relative to a reference system. Additionally, surface cleanliness classes can be determined according to DIN EN ISO 14644-9 to facilitate classification of materials. The cleaning of components for series production typically occurs in custom cleaning lines, which are unsuitable for testing new materials or evaluating specific contamination risks. However, cleaning processes can also be simulated in scaled-down laboratory environments to reduce contamination risks for dedicated cleaning lines and to save time, resources, and costs.

Additional aspects of material cleanliness

Certain application conditions require consideration beyond the aspects described in VDI 2083 sheet 17. For example, UV light is used for sterilization and contamination visualization in cleanroom applications, and incompatibility, especially of organic materials, with high-energy radiation can lead to degradation processes, increased outgassing, or particle formation. Similar considerations apply to special atmospheres containing reactive species. In space, materials are exposed to reactive oxygen species in Earth-orbiting environments or high doses of energetic radiation such as UV, X-ray, or gamma rays. In some semiconductor manufacturing processes, such as EUV lithography, materials come into contact with reactive hydrogen compounds. All these conditions can lead to contamination in two ways—first, through degradation of the materials, which can also result in property loss; second, reactions with certain components of the base material (e.g., additives or process aids) can produce volatile species.

Chemical trace analysis can play an important role here, for example, in identifying potentially critical material components or degradation products. Depending on the specific application, a variety of methods are available. Some examples include mass spectrometry (MS), X-ray fluorescence analysis (XRF), infrared spectroscopy (IR), or energy-dispersive X-ray spectroscopy (EDX). The choice of the most suitable method is crucial, and an experienced specialist can assist in this process.

Materiales considers performance and cleanliness together

Material selection processes for use in cleanrooms pose a complex challenge for companies. The experience of Materiales in high-performance industries such as semiconductor technology or aerospace has shown that both performance and cleanliness must be considered jointly and holistically. This includes the legal frameworks and regulations that are of significant importance in various cleanroom industries. It is also essential to thoroughly understand the properties of the materials used to achieve optimal functionality and avoid costly material failures. The specialists at Materiales are experienced in contamination-sensitive applications and possess the necessary know-how and specialized testing equipment to support material selection and other material-specific questions. Furthermore, the experts at Materiales can assist with legal and formal issues related to chemical safety. This ensures that all aspects of the materials and processes intended for cleanroom use are thoroughly examined and evaluated.

[1] D. Verein Deutscher Ingenieure e.V., "Cleanroom Technology - Cleanliness Suitability of Materials," VDI 2083 Sheet 17, 2013.

[2] C. Yu and D. Crump, "A Review of the Emission of VOCs from Polymeric Materials used in Buildings," Building and Environment, Vol. 33, No. 6, pp. 357-347, 1998.

[3] L. Zhu, D. Shen, and K. H. Luo, "A critical review on VOCs adsorption by different porous materials: Species, mechanisms, and modification methods," Journal of Hazardous Materials, Vol. 389, No. 122102, 2020.

[4] V. d. A. e.V., VDA 278: Thermodesorption Analysis of Organic Emissions for the Characterization of Non-metallic Automotive Components, 2016.