Photo-lithography applications, such as the production of integrated circuit boards, are increasingly using dry film photo-resists. The photo-resist is often used as a component of a laminated stack, with a polymer substrate like PET, and a protective layer like polypropylene. The protective layer is removed during use, but the efficiency of this process relies on the features of the dry film-protective layer interface. It is possible to explore these features by examining the dry film surfaces and the protective layers after they are peeled.
With superior chemical selectivity and surface sensitivity, X-ray photoelectron spectroscopy (XPS) is considered to be the most suitable method for such analysis. To carry out a complete characterization of the polymer surfaces after peeling, it may be essential to identify and determine the very small differences in oxygen and carbon bonding states. As the polymers are insulators, it is important to neutralize the electrical charge developed during the X-ray analysis. This process needs an XPS tool that has a unique combination of turn-key charge neutralization, superior energy resolution, and high sensitivity.
In this experiment a Thermo Scientific K-Alpha X-ray Photoelectron Spectrometer (Figure 1) was used to examine the surfaces developed after removing the polypropylene protective layer from a dry film placed on a PET substrate (Figure 2). The K-Alpha system was also used to analyze the chemistry of the dry film.
Figure 1. The Thermo Scientific K-Alpha system
Figure 1. Schematic of dry film photo-resist with protective polypropylene layer and PET substrate
Due to the dry film’s photosensitivity, care must be taken to control the XPS analysis in order to obtain precise chemical information. After the removal of the protective layer and the exposure of the dry film, the sample must be sent to the analysis chamber as soon as possible. This will help to reduce the chances of chemical degradation in the surface, by exposure to ambient UV light. The K-Alpha spectrometer allows the samples to be rapidly transported from the atmosphere to the analysis chamber. An increasing number of polymer samples are transferred within just 10 minutes, which increases throughput and productivity and protects the sample.
Dry Film Photo-Resist Surface
The results of the XPS analysis after the polypropylene layer was removed from the dry film surface highlighted that it contained oxygen and carbon with no traces of other elements (Figure 3).
Figure 2. Schematic of dry film surface after polypropylene layer was peeled away
A high energy resolution XPS spectrum of carbon (Figure 4) showed the states of chemical bonding in the surface. Peak fitting of the raw data in the Avantage data system, which is an integrated software solution of Thermo Scientific for all of its XPS systems, highlighted the possibility of the dry film surface as a combination of partially esterified cellulose (giving rise to the C-C*=O, C-O and C*-C=O components) and aliphatic carbon (C-C).
Figure 3. High energy resolution C1s spectrum acquired from dry film surface after peeling of polypropylene layer
Protective Layer (polypropylene) Surface
Only one asymmetric peak should be contained by the carbon spectrum of a pure polypropylene surface. This peak can be deconvoluted into contributions from CH3 and CH2 groups. Analysis was carried out for the surface of the protective layer, which was previously associated with the dry film (Figure 5). It was identified that the carbon spectrum was not that of pure polypropylene.
Figure 4. Schematic of polypropylene surface after peeling. The surface in contact with the dry film was analyzed.
The carbon spectrum obtained from the peeled polypropylene surface contained components as a result of the dry film (Figure 6). The Avantage data system was used to measure the relative proportions of polypropylene and dry film residue.
Figure 5. High energy resolution C1s spectrum acquired from peeled polypropylene layer
The dry film’s peak fitting protocol was applied to the carbon spectrum obtained from the peeled propylene surface. Two additional components were added, with peak widths, relative binding energies, and peak intensities suitable for pure polypropylene. The Avantage data system enabled fitting the XPS spectrum in relation to the total area for the dry film, and the total area for pure polypropylene. The composition of the polypropylene surface was four parts polypropylene to one part dry film.
The Thermo Scientific K-Alpha spectrometer was used to examine the surfaces formed as a result of peeling away the protective polypropylene layer from a dry film. The transfer of materials from the dry film to the polypropylene layer was detected. XPS analysis carried out for the dry film highlighted that it was very much like partially esterified cellulose.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – X-Ray Photoelectron Spectroscopy (XPS).
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