Adv Polym Sci (2004) 168: 51–246 DOI: 10.1007/b12682 Laser Application of Polymers Thomas Lippert Paul Scherrer Institut, 5232 Villigen-PSI, Switzerland E-mail: [email protected] Abstract Laser ablation of polymers has been studied with designed materials to evaluate the mechanism of ablation and the role of photochemically active groups on the ablation process, and to test possible applications of laser ablation and designed polymers. The in- corporation of photochemically active groups lowers the threshold of ablation and allows high-quality structuring without contamination and modification of the remaining surface. The decomposition of the active chromophore takes place during the excitation pulse of the laser and gaseous products are ejected with supersonic velocity. Time-of-flight mass spec- trometry reveals that a metastable species is among the products, suggesting that excited electronic states are involved in the ablation process. Experiments with a reference material, i.e., polyimide, for which a photothermal ablation mechanism has been suggested, exhibit- ed pronounced differences. These results strongly suggest that, in case of designed polymers which contain photochemically active groups, a photochemical part in the ablation mecha- nism cannot be neglected. Various potential applications for laser ablation and the special photopolymers were evaluated and it became clear that the potential of laser ablation and specially designed material is in the field of microstructuring. Laser ablation can be used to fabricate three-dimensional elements, e.g., microoptical elements. Keywords Laser ablation · Ablation mechanism · Photopolymers · Polyimide · Spectroscopy 1 Introduction ................................. 53 1.1 Laser Ablation History .......................... 53 1.2 Laser Ablation of Polymers........................ 56 1.2.1 Mechanisms of Ablation ......................... 58 1.2.2 Why Special Polymers? .......................... 61 2 Ablation Mechanisms of Novel Photopolymers .......... 62 2.1 Synthesis ................................... 62 2.2 Ablation Characteristics–Mechanism of Ablation ......... 65 2.2.1 Etch Rates and Qualities ......................... 65 2.2.1.1 Introduction ................................. 65 2.2.1.2 Results ..................................... 67 2.2.1.3 Discussion .................................. 75 2.2.1.4 Summary ................................... 81 2.2.2 Analysis after Irradiation ......................... 81 2.2.2.1 Results ..................................... 82 2.2.2.2 Discussion .................................. 93 2.2.2.3 Summary ................................... 98 2.2.3 Nanosecond Transmission Studies ................... 98 2.2.3.1 Introduction ................................. 98 Springer-Verlag Berlin Heidelberg 2004 52 Thomas Lippert 2.2.3.2 Results and Discussion .......................... 100 2.2.3.3 Application of the Model ......................... 108 2.2.3.4 Summary ................................... 112 2.2.4 Nanosecond Interferometry ....................... 113 2.2.4.1 Introduction ................................. 113 2.2.4.2 Results and Discussion .......................... 116 2.2.4.3 Summary ................................... 121 2.2.5 Nanosecond Shadowgraphy ....................... 122 2.2.5.1 Introduction ................................. 122 2.2.5.2 Experimental ................................. 125 2.2.5.3 Results ..................................... 126 2.2.5.4 Summary ................................... 130 2.2.6 Time-of-Flight Mass Spectrometry (TOF-MS)............ 131 2.2.6.1 Introduction ................................. 131 2.2.6.2 Experiment and Curve Fitting ...................... 132 2.2.6.3 Results and Discussion .......................... 134 2.2.6.4 Summary ................................... 141 2.2.6.5 Additional Time-of-Flight Mass Spectrometry Experiments. 143 2.2.7 Excimer Lamp Irradiation ........................ 146 2.2.7.1 Introduction ................................. 146 2.2.7.2 Experimental ................................. 146 2.2.8 Picosecond Infrared Spectroscopy ................... 150 2.2.8.1 Introduction ................................. 150 2.2.8.2 Experimental ................................. 151 2.2.8.3 Results and Discussion .......................... 155 2.2.8.4 Summary ................................... 158 2.3 Comparison with a Reference Polymer (Polyimide) ........ 159 2.3.1 DRIFT Analysis of Polyimide After UV Laser Irradiation .... 159 2.3.1.1 Introduction ................................. 159 2.3.1.2 Results ..................................... 164 2.3.1.3 Discussion .................................. 171 2.3.1.4 Summary ................................... 173 2.3.2 DRIFT of Kapton After Thermal Decomposition .......... 174 2.3.2.1 Introduction ................................. 174 2.3.2.2 Experimental ................................. 175 2.3.2.3 Results and Discussion .......................... 176 2.3.2.4 Summary ................................... 188 2.4 Discussion of the Ablation Mechanisms ............... 189 2.4.1 Designed Polymers ............................. 189 2.4.1.1 Polyimide ................................... 191 2.4.1.2 Additional Experiments .......................... 192 2.4.1.3 Summary ................................... 199 3 Applications ................................. 199 3.1 Laser Ablation Resists (Dry Etching) ................. 199 3.1.1 Introduction ................................. 199 3.1.2 Results ..................................... 201 Laser Application of Polymers 53 3.1.3 Summary ................................... 202 3.2 New Stable Resists–Combined Positive-Negative Resists ..... 203 3.2.1 Introduction ................................. 203 3.2.2 Results and Discussion .......................... 203 3.2.3 Summary ................................... 207 3.3 Laser Ablation–Implantation ...................... 208 3.3.1 Introduction ................................. 208 3.3.2 Experimental ................................. 209 3.3.3 Results and Discussion .......................... 209 3.3.4 Conclusion .................................. 214 3.4 From Micro- to Nanostructuring .................... 214 3.4.1 Introduction ................................. 214 3.4.1.1 Microtechnology .............................. 214 3.4.1.2 Nanotechnology ............................... 215 3.4.2 Interference Gratings ........................... 217 3.4.2.1 Introduction ................................. 217 3.4.2.2 Results and Discussion .......................... 218 3.4.2.3 Conclusions ................................. 222 3.4.3 Phase Masks for the Fabrication of Microstructures ....... 223 3.4.3.1 Introduction ................................. 223 3.4.3.2 Conclusion .................................. 226 3.4.4 Structuring with AFM and SNOM ................... 226 3.4.4.1 Introduction ................................. 226 3.4.4.2 AFM Structuring .............................. 227 3.4.4.3 SNOM Structuring ............................. 228 3.4.5 Conclusion .................................. 229 3.5 As Fuel for Micro/Nanosatellites Using Laser Plasma Thrusters 229 3.5.1 Introduction ................................. 229 3.5.2 Results ..................................... 230 3.5.3 Conclusion .................................. 233 3.6 Discussion of Possible Applications .................. 233 4 Summary ................................... 236 References ........................................ 238 1 Introduction 1.1 Laser Ablation History At the ripe age of 40 years old (the first laser, i.e., ruby laser, was reported in 1960 by T.H. Maiman [1]; in 1954 there were masers [2], the abbreviation also being used for “means of attaining support for expensive research” [3]), the laser has become a mature technological device with many applications. 54 Thomas Lippert This was not always true, of course. For many years, the laser was viewed as “an answer in search of a question”. That is, it was seen as an elegant device, but one with no real useful application outside of fundamental scientific re- search. In the last two to three decades however, numerous laser applications have moved from the laboratory to the industrial workplace or the commer- cial market. Lasers are unique energy sources characterized by their spectral purity, spatial and temporal coherence, and high average peak intensity. Each of these characteristics has led to applications that take advantage of these qualities: – Spatial coherence: e.g., remote sensing, range finding and many holograph- ic techniques. – Spectral purity: e.g., atmospheric monitoring based on high-resolution spectroscopies. – and of course the many other applications in communication and storage, e.g., CDs. All of these high-tech applications have come to define everyday life in the late twentieth century. One property of lasers, however, that of high intensi- ty, did not immediately lead to “delicate” applications but rather to those re- quiring “brute force”. That is, the laser was used in applications for remov- ing material or heating. The first realistic applications involved cutting, drilling, and welding, and the laser was little more advanced than a saw, a drill, or a torch. In a humorous vein A.L. Schawlow proposed and demon- strated the first “laser eraser” in 1965 [4], using the different absorptivities of paper and ink to remove the ink without damaging the underlying paper. Other early applications [5] used a laser to generate plasma