Elimination of initial stress-induced curvature in a micromachined bi-material composite-layered cantilever

Abstract

Micro-devices with a bi-material-cantilever (BMC) commonly suffer initial curvature due to the mismatch of residual stress. Traditional corrective methods to reduce the residual stress mismatch generally involve the development of different material deposition recipes. In this paper, a new method for reducing residual stress mismatch in a BMC is proposed based on various previously developed deposition recipes. An initial material film is deposited using two or more developed deposition recipes. This first film is designed to introduce a stepped stress gradient, which is then balanced by overlapping a second material film on the first and using appropriate deposition recipes to form a nearly stress-balanced structure. A theoretical model is proposed based on both the moment balance principle and total equal strain at the interface of two adjacent layers. Experimental results and analytical models suggest that the proposed method is effective in producing multi-layer micro cantilevers that display balanced residual stresses. The method provides a generic solution to the problem of mismatched initial stresses which universally exists in micro-electro-mechanical systems (MEMS) devices based on a BMC. Moreover, the method can be incorporated into a MEMS design automation package for efficient design of various multiple material layer devices from MEMS material library and developed deposition recipes.

A fabrication process based on structural layer formation using Au–Au thermocompression bonding for RF MEMS capacitive switches and their performance

“…Figure 5 shows the two-dimensional (2D) profile of a typical MEMS bridge sample. This lateral bending behavior is attributed to the strain mismatch between the individual layers of the MEMS bridge stack as described in [19,20]. 5 that the 1.55-mm-deep recess in the glass wafer agrees with the designed value of 1.50 mm.…”

“…Another immediate observation is that the MEMS bridge exhibits curling along its width dimension, with an approximate radius of curvature of ρ = 450 µm. This lateral bending behavior is attributed to the strain mismatch between the individual layers of the MEMS bridge stack as described in [19, 20]. This side effect was not considered during the design of these first-generation devices; but rather the layer stack arrangement and individual stress levels were optimized from an axial stiffness (hence, pull-in voltage) perspective only.…”

“…To illustrate the procedure, we consider two stacks with different residual stress states, and calculate the lateral deflection as a function of relative layer thicknesses assuming free-free end conditions [19,20]. Elimination of this lateral bending, therefore, will help eliminate most of the shortcomings of this alternative RF MEMS fabrication process.…”

Stress compensation for arbitrary curvature control in vanadium dioxide phase transition actuators

“…Within a certain range, arbitrary curvatures can be achieved by simply adjusting the thickness ratios between the three layers. 26 Based on the Equations (1)-(3), we plot, in Figure 1(e), the calculated curvature of Au/Cr/VO 2 actuators when the VO 2 is in the I-or M-phase, which proves that adjusting the thickness of either or both layers (Cr and Au) results in curvatures with different signs or zero value. Note that the bi-layer cases correspond to the horizontal and vertical axes in Figure 1(e), which means that the limit of curvature adjustment in the tri-layer case is the bi-layer configuration.…”

“…However, such techniques usually involve additional, complex fabrication processes, such as deposition parameter optimization, [21][22][23] thermal annealing, 24 ion implantation, 25 or different deposition methods. 26 Since VO 2 is sensitive to high temperature and oxidation environments, these stress engineering processes not only elevate the fabrication complexity and cost of VO 2 devices, but also may degrade the physical properties of VO 2 films. Moreover, deposition of high-quality VO 2 requires high temperature, [4][5][6][7][8][9][10][11][12][13][14][15][16] which limits the selection of underlayer materials, making it more difficult to find a stress-matched material with VO 2 and further limits the VO 2 actuator fabrication.…”