Models for laser ablation of polymers.

UV laser ablation of polymers was discovered 20 years ago.1,2 This effect has many fascinating applications in lithography, micromechanics, and medicine. For the last 20 years, more than 103 papers describing this phenomenon were published (see, e.g., comprehensive reviews, refs 3-7). Despite the high research activity, the nature of UV laser ablation of polymers is still far from being fully understood, e.g., one can find contradictory interpretations of the same results in different papers. Originally, UV laser ablation of polymers was believed to be a pure photochemical effect, resulting from the direct bond breaking by UV photons.8-10 Gradually, investigators obtained evidence that laser heating of materials is significant and a pure thermal nature of laser ablation was considered.11-13 Here we use the term “photothermal”7 for the process resulting from laser heating of material. Polymers are complex materials; therefore, laser ablation of polymers is also a complicated phenomenon. In this paper, we will focus on the models of laser ablation. In addition, we inevitably have to answer the question of what are the specific features of laser ablation of polymers that distinguish them from the laser ablation of other materials (metals, semiconductors, inorganic dielectrics, molecular solids). There are several approaches to modeling of polymer ablation. The authors of this paper represent one of the “schools”; thus, our view of the problem is somewhat subjective. Despite the complex nature of laser ablation, we prefer to develop simplified models (e.g., “pure” photochemical or “pure” photothermal), and analyze them in detail to be able to assign specific features, which are experimentally observed for a particular ablation mechanism. We start with the photochemical model. In fact, the first consideration of laser ablation10 employed a very simple but constructive idea that takes into account the specific feature of polymer materials. This idea is that polymers consist of long molecular chains with strong, covalent, bonds inside. At the same time, molecules belonging to different chains interact weakly. Therefore, polymer material becomes simple molecular solid (i.e., easily removed material), if we break the long polymer chains into small pieces by direct photochemical effect (UV photons are considered to have energy pω exceeding the energy of * Corresponding author, e-mail: [email protected]. † Institute of Applied Physics Russian Academy of Sciences. ‡ Data Storage Institute. 519 Chem. Rev. 2003, 103, 519−552