This paper describes the development of the lithography process over a large topographic step (up to 6μm), an essential part for the fabrication of micromachined thermopiles. Micromachined thermopiles are considered as a cost-effective breakthrough solution for energy scavengers working at low thermal gradients and weak heat flows, typical for e.g. human body and some types of machinerelated waste heat. The thermoelectric generators will be used, in the first instance, for autonomous wireless sensor nodes in a Body Area Network.

In this paper we report the use of projection optical lithography to pattern micron-sized features on 100 mu m thick PEN foils. A foil-on-carrier lamination process was developed to ensure a good dimensional stability during the lithographic processing and imaging of the foil. A stepper based leveling metrology was used in characterizing the surface flatness of the foil-on-carriers. A lithography process was developed to image micron and submicron patterns on foil substrates. The process window for 1-10 mu m features was determined from focus and exposure energy experiments. The lithographic study indicated a reproducible and excellent imaging accuracy for patterning micron-sized features on flexible substrates. This makes the technology very suitable for the manufacturing of electronic devices with critical dimensions in the micron and submicron range. In addition, we made transistors-on-foil demonstrators with the developed foil-on-carrier lamination and imaging technology.

Manufacturing transistors on thin flexible polymer foils is challenging and differs from standard Si processing due to the dimensional instability of the substrate influenced by moisture uptake, temperature and handling. A thorough analysis of material properties of the tested foil was performed to understand its behavior during lithography and subsequently to improve the processing. Imaging experiments on 100 µm polyethylene naphthalate (PEN) foils were performed with a PAS 5500/100D ASML step and repeat I-line (365 nm) system equipped with reticles having features of several microns and also sub-micrometer dimensions. A foil lamination process was developed to improve the dimensional stability during processing and to achieve a good surface flatness crucial for sub-micrometer imaging. The optimum process window for sub-micrometer critical dimensions was determined by performing a Focus Exposure Matrix (FEM) experiment in which the energy and focus were increased stepwise. The optimum imaging conditions were derived from SEM analysis. The results indicated a reproducible and good patterning accuracy for making patterns below 1µm size.