Photolithography applications such as the fabrication of integrated circuit boards are increasingly using dry film photo-resists, which often form part of laminated stacks with a protective layer such as polypropylene and a polymer substrate such as PET. During use, the protective layer is removed, but the properties of the dry film-protective layer interface influence the efficacy of this process.
The surface analysis of the dry film and protective layers following peeling enables exploration of these properties. This can be effectively performed by making use of the surface sensitivity and the chemical selectivity of X-ray photoelectron spectroscopy (XPS).
Detecting and differentiating slight variations in carbon and oxygen bonding states may be crucial for the comprehensive characterization of polymer surfaces after peeling. Also, neutralizing the electrical charge building up during the X-ray analysis is essential because the polymers are insulators. For this purpose, an XPS tool combining turn-key charge neutralization with high sensitivity and excellent energy resolution is required.
This article discusses the analysis of surfaces created by peeling away the polypropylene protective layer from a dry film, using the Thermo Scientific K-Alpha (Figure 1). The dry film was atop of a PET substrate as illustrated in Figure 2. The K-Alpha was also used to examine the chemistry of the dry film.
Figure 1. The Thermo Scientific K-Alpha
Figure 2. Schematic of dry film photo-resist with protective polypropylene layer and PET substrate
Since the dry film shows photosensitivity, it is necessary to carefully control the XPS analysis in order to obtain accurate chemical information. Since the dry film is exposed after the removal of the protective layer, it is critical to transfer the sample to the analysis chamber as early as possible in order to reduce the chance of the surface to be chemically degraded due to exposure to ambient UV light.
Rapid sample transfer from atmosphere to the analysis chamber is possible with the Thermo Scientific K-Alpha, which can transfer most polymer samples within ten minutes. This, in turn, improves throughput and productivity, while giving protection to the sample.
Dry Film Photo-Resist Surface
XPS analysis of the dry film surface subsequent to the removal of the polypropylene layer (Figure 3) detected the presence of oxygen and carbon, but not other elements. Figure 4 shows a high-energy resolution XPS spectrum of carbon, revealing the chemical bonding states in the surface.
Figure 3. Schematic of dry film surface after polypropylene layer was peeled away
Peak fitting of the raw data in the Avantage datasystem, an integrated software solution from Thermo Scientific for all of its XPS systems reveals that the dry film surface could be composed of partially esterified cellulose (giving rise to the C-C*=O, C-O and C*-C=O components) and aliphatic carbon (C-C).
Figure 4. High energy resolution C1s spectrum acquired from dry film surface after peeling of polypropylene layer
Protective Layer (polypropylene) Surface
A pure polypropylene surface’s carbon spectrum must consist of a single asymmetric peak. It is possible to deconvolute this single asymmetric peak into contributions from CH2 and CH3 groups. Figure 5 presents the protective layer surface that was previously in contact with the dry film, showing a carbon spectrum different from that of pure polypropylene. Figure 6 depicts the carbon spectrum acquired from the peeled polypropylene surface, showing components due to the dry film.
Figure 5. Schematic of polypropylene surface after peeling. The surface in contact with the dry film was analyzed.
Figure 6. High-energy resolution C1s spectrum acquired from peeled polypropylene layer.
The relative proportions of dry film residue and polypropylene were quantified using the Avantage datasystem. The peak fitting procedure for the dry film was applied to the carbon spectrum acquired from the peeled propylene surface. The was followed by the addition of two further components with relative binding energies, peak widths and peak intensities suitable for pure polypropylene.
The Avantage datasystem allowed fitting the XPS spectrum in terms of the total area for the dry film and the total area for pure polypropylene, revealing a surface composition of four parts of polypropylene to one part of dry film.
This article discussed the analysis of surfaces created by peeling away the polypropylene protective layer from a dry film, using the Thermo Scientific K-Alpha. The results showed the transfer of material from the dry film to the polypropylene layer. The XPS analysis results revealed that the dry film was similar to partially esterified cellulose.
This information has been sourced, reviewed and adapted from materials provided by Thermo Scientific – X-Ray Photoelectron Spectroscopy.
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