Silicon Microfabrication: Laser Ablation vs. Inductively Coupled Plasma (ICP) Etch
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Silicon Microfabrication: Laser Ablation vs. Inductively Coupled Plasma (ICP) Etch Megan M. Owens, Joseph W. Soucy, Thomas F. Marinis Electronics Packaging and Prototyping Division, The Charles Stark Draper Laboratory, Inc., Cambridge, MA 02139, U.S.A. ABSTRACT Mechanical prototypes of a silicon interposer device, having high-aspect ratio straight walls, were required for experiments to qualify various packaging assembly options. The ideal solution would yield expendable, quickly made, and relatively inexpensive replica parts. This need led to the investigation of micromachining of silicon via laser ablation. We sought to optimize the results by varying several parameters, the most influential of which was shown to be laser wavelength. Silicon wafers of thickness 500 µm were machined using laser wavelengths of 10,600 nm, 1064 nm, and 355 nm. As was predicted according to the theoretical silicon absorption profile, the best results were obtained using 355 nm wavelength light from an ultraviolet laser source. Several post-machining cleaning routines, to remove melted silicon debris, were evaluated as well. Pictorial results of both laser processing and cleaning are shown and discussed. ICP was concurrently investigated as a viable alternative to laser machining. These cuts were markedly cleaner and straighter than those induced by laser; additionally, cut features were more well defined. Pictorial results are compared and contrasted with those of laser ablation. Cost and time are traded for quality when choosing between laser ablation and ICP etch. Therefore, process choice should be application-dependent. For wafers with moderately complex cutout features, laser micromachining is significantly lower in cost and manufacturing time. However, the laser process does produce residual thermal stress surrounding the cut region, machined edges are relatively rough, and fine features are more difficult to produce. Laser ablation is an attractive manufacturing option for quick-turnaround prototyping as well as high-volume production; ICP is better suited to producing high-definition parts without potential for thermal stress damage. INTRODUCTION Many micro-electromechanical systems (MEMS) devices perform best when isolated from stresses in their operating environment; for example, vibration and temperature changes are often factors to be avoided. To this end, an interposer may be used as a physical bridge between the MEMS device and its external packaging, dampening the impact of temperature changes as well as unexpected mechanical forces. Silicon is a highly desirable interposer material, as MEMS devices are typically silicon-based structures; both the interposer and the device would share a similar coefficient of thermal expansion (CTE). This match would reduce concerns regarding potential thermal variation impacts to the system. B8.4.1
We sought to fabricate just such an interposer. The piece was designed of single crystal silicon with a cutout width-to-depth aspect ratio of 4:1, the cut width being 125 µm and the depth 5
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