Quantifying and correcting tilt-related positioning errors in microcantilever-based microelectromechanical systems probes

Abstract

The impact of tilt-related errors on the positioning of microcantilever-based microelectromechanical systems (MEMS) on-wafer electrical probes, having multiple contact pads, is quantified and investigated here. A tilt error associated with probe roll results in the probe contact pads not being parallel to the approaching surface as a downward overtravel is imposed—this leads to one probe pad making contact with the surface before the others. In a MEMS-based probe, the analysis of the impact of roll error angle must consider both the bending and the torsion of the flexible cantilever as the overtravel is increased—something which eventually results in all pads being in contact with the surface, but not with the same contact force. An original mathematical description of the problem is presented. By making some assumptions, the analytical modelling enables the derivation of elegant equations relating the roll error angle and the cantilever deflection to achieve planarity of the cantilever apex with the underlying surface. The modelling predicts probe tip planarity for rectangular and trapezoidal shaped probes. The predictions of the modelling are tested by using macroscopic cantilevers—excellent agreement between modelling and experiment is demonstrated. The macroscopic experimental setup reveals interesting behaviour concerning a bending/twisting, tilted cantilever in contact with—and skating across—an underlying surface. The experimental findings also indicate the pertinence of the modelling for the potential use with understanding the behaviour of microscopic cantilevers—such as MEMS-based probes—similarly in contact with a surface. A flexible microcantilever enables a torsional compensation of the roll error angle. It also enables a protocol where the roll error angle can be corrected. The design geometry of the probe tip will determine which approach is best suited. In principle, the modelling is scalable to MEMS probes composed of silicon-based cantilevers.

Tilt-related alignment issues in miniature micromechanical on-wafer electrical probes composed of multiple flexible microcantilevers

“…The author has previously discussed the relationship between skate and overtravel in rectangular [15], triangular, and trapezoid [16] shaped probes based on flexible microcantilevers. The impact of tilt error on a probe composed of a single flexible microcantilever has also been described by the author [17]. This work revealed interesting aspects such as: probe tip planarity and tangency, the possibility of zero-skate probing, contact force issues, and the interplay between the bending and torsional stiffnesses of single cantilever-based probes.…”

“…At a given value of overtravel, twisting of the cantilever brings the whole of the lower edge of the first cantilever into contact with the surface-figure 2(c). This is the planarity overtravel of a single skating inclined cantilever [17]. Subsequently, further overtravel will now have two consequences: (i) further skating of the lower cantilever (red) along the surface (associated with an increase of its contact force with the surface due to increased bending) and (ii) a repeat of the process described above with the middle cantilever (yellow), i.e.…”

“…Let us consider the probe approaching an underlying surface (blue) by imposing a vertical downward movement (known as probe 'overtravel') to the support chip. In the presence of a tilt roll error j [17], i.e. the cantilever tip edges not being parallel to the approaching underlying surface, the lower bottom corner of the lowest cantilever edge will make contact with the surface first.…”

“…Figure 4 shows a probe contact scenario where tip planarity is never reached due to high torsional stiffness of the cantilevers. This occurs because tip tangency [15] of the first cantilever is achieved before planarity [17]. Again, we consider a probe based on three cantilevers (red, yellow, and green) attached to a rigid support (light blue) being brought into contact with an underlying surface (long horizontal black line) in the presence of a tilt error associated with the roll axis of the probe.…”

“…This is particularly important in terms of the impact of positional errors on optimum electrical contact [6]; something that has been studied in rigid commercial probes [7][8][9][10][11][12][13][14]. In contrast, the micromechanical behaviour (bending and twisting) of miniature microcantilever-based probes must be taken into account for optimum probe contacting [15][16][17]. The author has previously discussed the relationship between skate and overtravel in rectangular [15], triangular, and trapezoid [16] shaped probes based on flexible microcantilevers.…”