Depending on requirements and the technical equipment implemented, with the appropriate cleanroom environment a controlled level of particulate cleanliness can be achieved for the ambient air inside a cleanroom. According to their size distribution, the residual concentration of particles after filtration is expressed as an ISO Class in compliance with ISO 14644-1.
A further form of contamination, which is becoming increasingly relevant as far as clean manufacturing environments are concerned, is that of chemical compounds contained in the air (ACC — airborne contamination by chemicals). These substances can precipitate onto the surface of products. In compliance with ISO 14644-8, the ACC concentration is expressed as an ISO-ACC Class (x) according to the values in Table 1.
Chemical compounds can actually affect processes because they condense on the product surface and cause surface contamination. This type of contamination is known as SCC (surface contamination by chemicals). In compliance with ISO 14644-10, its concentration is expressed as an ISO-SCC Class (x) according to the values in Table 1.
Table 1: Definition of ISO-ACC classes according to ISO 14644-8 and ISO-SCC classes according to ISO 14644-10
|
ISO-ACC Class (x) 1) |
ACC concentration |
ISO-SCC Class (x) 1) |
SCC concentration |
|
0 |
100 |
0 |
100 |
|
-1 |
10-1 |
-1 |
10-1 |
|
-2 |
10-2 |
-2 |
10-2 |
|
-3 |
10-3 |
-3 |
10-3 |
|
-4 |
10-4 |
-4 |
10-4 |
|
-5 |
10-5 |
-5 |
10-5 |
|
-6 |
10-6 |
-6 |
10-6 |
|
-7 |
10-7 |
-7 |
10-7 |
|
-8 |
10-8 |
-8 |
10-8 |
|
-9 |
10-9 |
-9 |
10-9 |
|
-10 |
10-10 |
-10 |
10-10 |
|
-11 |
10-11 |
-11 |
10-11 |
|
-12 |
10-12 |
-12 |
10-12 |
|
1) The descriptor x describes the contaminant group considered, e.g. “cd” for “condensables” (compounds that condense under normal conditions) |
|||
The descriptor required for the classification describes the class of chemical compounds under consideration, for example “VOC” for volatile organic compounds, “dp” for dopants, “ac” for acids in a gaseous state, such as hydrogen fluoride and hydrogen chloride, and “ba” for bases in a gaseous state, such as ammoniac and volatile amines. In the guideline SEMI F21-1102 for the semiconductor industry, airborne chemical compounds (ACC) are divided into groups according to their chemical or physical effects, as shown in Fig. 1. This method of grouping was adopted by the guideline ISO 14644-8, which has since been extended appropriately. In the aerospace industry, especially where contamination-sensitive satellites are built, the term “MOC” is generally used to abbreviate “molecular organic contamination” on surfaces. In principle, MOC is synonymous with SCC (or).
Bases (ba) Acids (ac) Dopants (dp) Condensables (cd)
Risk potentials
If an airborne base compound such as ammoniac comes into contact with photoresist on a wafer, the photoresist can be irreversibly damaged. Acids coming into contact with materials may cause corrosion. Dopants may dope sensitive wafer surfaces in an uncontrollable manner. Condensables, however, may condense on material surfaces without causing any direct chemical damage. These are generally low-volatile organic compounds, so-called SVOC, such as organophosphates from flame retardants, siloxanes from silicone coatings and sealants or high-molecular hydrocarbons, e.g. softeners and paraffin compounds. Nevertheless, sensitive lenses may become lastingly damaged due to condensables causing modified transmission properties, undesired scattering effects or the absorption of certain bands of electromagnetic waves. In the semiconductor industry, scattering effects from molecular organic contamination on a lithography lens inevitably cause image defects during the exposure process, thus resulting in defective components. Paraffin compounds, siloxanes or softeners may condense on the highly-sensitive infrared lens of an Earth observation satellite during its assembly. This would lead to specific wavelengths in the infrared range being absorbed when the satellite is in operation and could significantly reduce the capacity of the complete imaging system. Even monomolecular films of siloxanes and phthalates on material surfaces drastically alter surface wettability, therefore causing defects in photoresist coatings on wafers or impairing other coating processes.
Determination of MOC condensation rates
Is there a way to assess contamination risks due to organic compounds condensing on surfaces? Can a value be stated for a cleanroom environment that indicates the mass of organic compounds condensing on a surface in dependence on exposure time?
General description of the method
To address these questions, condensation samples (so-called witness samples) are used, which are placed in the test cleanroom for a defined period of time. On completion of the exposure time, the organic compounds that have condensed on the witness samples are then quantified directly by performing an infrared spectroscopic transmission test.
In a transmission test, the principle of Lambert-Beer’s Law is applied. The law relates the attenuation of light to the properties of the material through which the light is transmitted (spectral absorption coefficient). There is a direct linear dependence between the concentration of substances absorbed by the substrate and the layer thickness of the substrate through which the light passes. This method is described in detail in the new guideline DIN EN 16602-70-05 and is one of the services offered by the Department of Ultraclean Technology and Micromanufacturing at the Fraunhofer Institute for Manufacturing Engineering and Automation IPA in Stuttgart, Germany.
Witness samples
Polished zinc sulfide crystal disks are used as a substrate for the witness samples. These are mounted into suitable optical tubes that are fixed in place in a transport box by a clamp. The transport box is made entirely of untreated aluminum, which is closed and then wrapped in aluminum foil. This largely excludes nearly influences during transport of the witness samples. Despite this, a field blank value is also analyzed, which undergoes the same transport and handling processes as the actual witness samples but is not exposed to the atmosphere in the test cleanroom. At the start of the exposure period, the aluminum packaging is removed and the lid of the transport box opened. On completion of the specified exposure time (usually several weeks), the transport box is closed again, wrapped in aluminum foil and sent back to the laboratory. In the lab, the tube containing the crystal disk is removed from the mounting device in the transport box and placed in another suitable holder located directly in the measuring chamber of a Fourier transformation infrared spectrometer.
IR spetroscopy
In compliance with DIN EN 16602-70-05, the measuring device used is a Fourier transformation infrared spectrometer (FTIR spectrometer) with a scanning range of 4000 — 600 cm-1, a resolution of 4 cm-1 and an absorption rate of 0.0001 as a detection limit for transmission measurements. To calibrate the device each time, separate calibration standards are prepared containing a defined concentration of paraffin in cyclohexane (for the compound class of hydrocarbons, peak maximum at 2920 cm–), Hexamoll DINCH (for the compound class of esters, peak maximum at 1735 cm-1) and polydimethyl siloxane (for the compound class of siloxanes, peak maximum at 1260 cm–). After determining the background signal in each case, the individual calibration samples are generated. This is done by applying the calibration standards to the crystal disks, then evaporating the solvent under a chemical extraction hood and finally measuring them with the FTIR spectrometer.
For each class of compound, the area of a typical peak is integrated and subsequently correlated with the absolute masses applied and the area of the IR beam. This results in a linear calibration line over a broad concentration range in the form of the integrated area in relation to the surface concentration analyzed. The lower detection limit for compound classes of hydrocarbons, siloxanes and esters is currently 12.5 ng/cm2.
Assessment
For example, based on a measured surface concentration of siloxane of 50 ng/cm2 and an exposure time of 20 days, the condensation rate of siloxane is calculated to be 2.5 ng/cm2. This condensation rate is a coefficient that describes the potential of exposed surfaces in the manufacturing environment to be contaminated by siloxanes.
The value can be reduced by using an appropriate molecular filtration system for incoming and circulating air, by using materials with extremely low outgassing rates, or by strictly avoiding all materials containing critical compounds. An alternative method would be to heat the exposed material surface. By raising the temperature of the substrate, the thermodynamic balance between condensation and desorption of a chemical compound shifts towards desorption.
Depending on the material (hydrophobic, hydrophilic, surface energy, etc.) and its surface characteristics (texture, roughness, etc.), the value specific to each material may vary slightly. Despite this, provided the same method and an identical substrate is used to assess different manufacturing environments, this value is highly suitable for making a comparative assessment of the various measuring sites. However, if the absolute value of a contamination rate needs to be determined for a specific material surface, a sample must be prepared from exactly this material. Provided it is not an IR-inactive transparent material, the organic compounds which have condensed on it are eluted from the surface using one or more suitable solvents (cyclohexane, chloroform, methanol, acetone or other solvent). The eluate is then applied to a zinc sulfide crystal and measured via transmission FTIR once the solvent has evaporated. 
The condensation rate of organic compounds is a further important parameter for evaluating manufacturing environments. It can be included directly in an assessment of the contamination of a product surface by organic compounds in dependence on exposure time. Up till now, the air quality of a cleanroom with regard to organic molecules has been defined by an ISO-ACC class (or) to achieve the necessary process reliability for a product. The concentration of organic contaminants in the air is therefore an indirect measure for assessing the risk of a product surface becoming contaminated. The analysis method applied is generally sample-taking using a suitable adsorbent followed by thermal desorption and analysis by gas chromatography. This method can only be used to detect organic compounds with a boiling point no higher than the maximum operating temperature of the gas chromatography device, especially volatile and semi-volatile organic compounds (VOC and SVOC). However, from the result of the assessment of air quality with regard to VOCs and SVOCs, no direct relationship can be established with the condensation rate of organic compounds on a surface. Product damage only occurs if the critical form of contamination actually comes into contact with the product surface. The method presented in this paper and in the guideline DIN EN 16602-70-05 is the method of choice for making a direct assessment of risks due to condensing organic compounds. This is because surface contamination (SCC) is regarded in a comparative manner without the need to establish a correlation between air cleanliness (ACC) and surface cleanliness (SCC).
Markus Keller and Stefanie Weisser are with Fraunhofer IPA of Stuttgart, Germany. markus.keller@ipa.fraunhofer.de
This article appeared in the November/December 2015 issue of Controlled Environments.
