Polyimides laser ablation

Threshold Behavior. A closer inspection of the Figs. 25 and 26 shows that a different behavior is observed for 248- and 308-nm irradiation. The ablation at 308 nm, where the polymer has a higher absorption coefficient (Fig. 24), shows a very clear and well-defined threshold fluence. This type of sharp threshold was previously found only for irradiation of polyimide with an ArF excimer laser (193 nm). In a study of polyimide laser ablation at various wavelengths, this behavior was assigned to a photochemical process [63]. In this study only single-pulse data were employed and the thresholds of ablation were analyzed by using the QMB technique. 

Figure 5.24. SEM micrograph of a laser-ablated hole (248 nm, 10 pulses at 16 ns and 4.8 J/cm2) in PTFE containing 0.2% polyimide at 170 x (a), the sidewall of the hole at 1500 x (b) and topography of the hole s bottom surface showing periodic structures at 1500 x (c) (from D Couto et al. 7S). (Reduced 35% for reproduction).

Optical properties of the material are less critical for microchips hyphenated with MS than for devices with on-chip optical detection where low background absorption or fluorescence is mandatory. Thus, completely opaque polymers like glassy carbon or polyimide " can be used as microfabrication substrates. Furthermore, polymer microchips are of great interest because their potentially low manufacturing costs may allow them to be disposable. Methods used for the fabrication of plastic chips include laser ablation and molding methods. 

Although Eq. (2) is not adequate to fully describe ablation processes, the absorption coefficient is still a useful parameter as it provides a quantitative evaluation of the level of interaction between a given medium and photons of a specific wavelength and, to a first-order approximation, their propensity to laser ablation. 5 different materials can have different chemical compositions and structural arrangements, it is not surprising that they have different absorption coefficients and thus can exhibit different ablation characteristics. This is indeed found to be the case. For instance, PMMA is readily structured at 193 run ( 193 2 X 10 cm I) although it is essentially transparent and unaffected at 308 nm (a 308 < 1 x 10 cm i). However, for an aromatic polyimide such as the system whose dianhydride/diamine components are pyromelletic dianhy-dride/oxydianiline (PMDA-ODA), ablation occurs readily not only at 193 nm... 

Countercurrent dialysis was achieved through a dialysis membrane sandwiched between a laser-ablated PC sheet for sample flow (0.5 pi. /min) and a polyimide sheet for buffer flow (100 pL/min) (see Figure 5.11). This device produced a desalted sample (horse heart cytochrome c) for subsequent MS analysis (see Figure 5.12) [811],... 

During the past three decades, since the commercialization of Kapton polyimide, an impressive variety of polyimides have been synthesized [258, 259]. Polyimides possess outstanding key properties, such as thermooxidative stability [260], high mechanical strength [261], high modules, excellent electrical [262] and optical properties [263, 264], and superior chemical resistance [265]. Recently polyimides have also been applied as membranes for gas separation [266, 267]. Approximately 15 years ago the direct structuring or laser ablation of polyimides by excimer lasers was first described [73, 130]. 

Fig. 74 SEM images of Siemens stars fabricated by laser ablation. Siemens stars in the triazene polymer (left) and in polyimide (right) using 5 pulses. REPRINTED WITH PERMISSION OF [Ref. 221], COPYRIGHT (2002) Elsevier Science...

The different models include many material parameters and several of these parameters are obtained from fitting of experimental data and have to be adjusted to fit each polymer [9,63]. It is worth mentioning in this context that polyimide is probably the most studied polymer in laser ablation and is also the material for which most ablation models are applied, but great care has to be taken for which type of polyimide the data have been obtained. Polyimide describes a whole group of polymer that can range from soluble polymers to insoluble films and even photosensitive polymers with very different properties [10]. Even products with the same name, such as Kapton , are not one polymer, but there are also many different types of Kapton that are defined with additional letters, for example, HN. 

A third way to build up pFCs based on MEMS-polymers such as poly-dimethylsiloxane (PDMS) or polymethyl methacrylate (PMMA) or PCB-materials such as polyimid (PI) or FR4. These polymers can be micro-machined by molding or by laser ablation. Shah et al. [22,23] have developed a complete PEMFC system consisting of a PDMS substrate with micro-flow channels upon which the MEA was vertically stacked. PDMS micro-reactors were fabricated by employing micro-molding with a dry etched silicon master. The PDMS spin coated on micro-machined Si was then cured and peeled off from the master. The MEA employed consisted in a Nafion - 12 membrane where they have sputtered Pt through a Mylar mask. Despite an interesting method, this FC gave poor results, a power density of 0.8 mW cm was achieved. 

The process time depends on the complexity of the 3D microstructures therefore, it is commonly used for prototyping. An extensive review on stereolithography can be found in Reference 119. The latter method is a subtractive manufacturing process that uses a focused high-intensity laser beam to evaporate the material from the surface. Laser ablation is mostly used to fabricate microchannels in thermosetting polymers such as polyimide due to its physical properties. Microstructures of nanometer scale have been danonstrated, but the surface roughness and properties using laser... 

Fig. 9.6 Laser ablation of polyimide (Kapton " ) at different wavelengths (given in the graph). The ablation rate obtained by single-shot experiments as a function of the...

Fig. 9.7 Laser ablation of polyimide (Kapton ) at 7=193 nm. The longitudinal acoustic signal (20 MHz) received by a piezoelectric transducer as a function of the fluence. The arrow indicates the threshold...

Novel photopolymers have been developed to overcome certain disadvantages, such as debris contamination and insufficient sensitivity, encountered in the application of laser ablation in lithographic techniques. Of note in this context are novel linear polymers containing photochemically active chromophores in the main chain [59]. In relation to the 308 nm laser light generated by XeCl excimer lasers, polymers containing triazene or cinnamylidene malonic acid groups were found to be much more appropriate than a commercial polyimide (see Chart 9.8) ... 

Creasy, W. R. and Breima, J. T., "Large Carbon Cluster Ion Formation by Laser Ablation of Polyimide and Graphite," Chem. Phys., 126, 453-468,1988. 

Interfaces Between Microfluidics and Mass Spectrometry, Fig. 3 Two commercial microfluidics-MS interfaces (a) A polyimide microchaimel device with integrated ESI tip formed by laser ablation (Reprinted with permission from Yin et al. [8]. Copyright 2005... 

Laser Ablation, Fig. 2 KrF laser-drilled ink-jet printer nozzles (30 pm diameter) in polyimide... 

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