Stress-engineered metal films allow the microfabrication of curved beam structures which have been demonstrated for a variety of innovative applications such as interconnect structures, RF coil structures, actuators, and scanning probes. The underlying principle is that a beam structure with a compressive region at the bottom and a tensile region on top is bending away from the substrate surface after a release etching step. Although the beam curvature can be adjusted by the stress gradient within a wide range, the shape of the beam by this approach is basically limited to circular shapes. Therefore, we have developed a double-spring approach with an S-shape side profile. It allows for extreme spring heights and different shapes while still relying on the same stressed-metal base process without additional lithography steps. Using interlocking features and beam overplating, very stiff springs can be fabricated. This presentation introduces the double-spring concept and fabrication. We demonstrate various types of fabricated double-spring structures with and without spring interlocking. The double spring is fabricated using state-of-the-art stressed metal technology whereby the lower beam is pushing upwards and the upper beam is pushing downwards. This results in an S-shape of the upper beam which is mechanically supported by the lower beam. Using this concept, double-spring structures can be created. Our approach allows for nearly vertical spring structures with a spring height nearly equal to the beam length. The two springs can also be interlocked with each other by interlocking features or spring overplating.
Hantschel, T.; Chow, E. S-shaped double-spring structures for high stiffness and spring height. Microelectronic Engineering. 2012 (in press)