Abstract

Low volume fabrication of MEMS structures by using conventional photolithography is expensive, tedious and time consuming. In initial design and prototyping phase of MEMS devices, laser micromachining provides a much faster, flexible and easier alternative that has a significantly low operating cost. Laser ablation may not only be used to cut through thick support layers (e.g. silicon nitride with thickness of the order of micro-meters) to shape MEMS structures, but also to selectively remove thin metal films (with thickness of the order of nano-meters) from these structures for formation of electrodes.

This talk will focus on exploitation of laser micromachining for selective removal of Titanium-Tungsten (TiW) thin film from Silicon Nitride (SiN) support layer for fabrication of MEMS thermal flow sensors.  Micromachining performance was evaluated in terms of patterning quality and the ability to remove TiW with minimal damage to an underlying SiN layer.  Two types of samples were micromachined using single shots of UV (355nm) and green (532nm) radiation of a Nd:YAG laser.  One type of samples had only a 3 µm SiN layer on Si substrate while the others had another layer of 500 nm DC magnetron sputtered TiW on top of SiN.  The effect of laser fluence and number of shots on the laser-machined features was investigated using surface profilometry, 3D optical interferometry and scanning electron microscopy.  The average and maximum removal depths of both TiW and SiN layers at various numbers of laser pulses and fluence levels were characterized to find an optimum laser fluence range and laser fluence value for selective removal of TiW layer without damaging the underlying SiN layer.  To determine the point of complete selective removal of TiW film from SiN, in-situ electric resistance was measured during single shot laser ablation of TiW film on dog-bone structures.  Meander shape micro-patterns on Ti-W film on free standing SiN micro-bridges have been successfully fabricated using this technique. These structures have their application as micro-heaters and MEMS thermal flow sensors.