*2.1. Average Film Stress Measurement*

Once released from a substrate, as is common in MEMS processing, films will relax their built-in stress, which can lead to undesired deformation and/or buckling. Thus, knowing the stress in each layer of a MEMS process is vitally important. The average film stress is computed by measuring the wafer's radius of curvature with a profilometer and subsequently using the Stoney equation [3,14,20,26,27]. To separate the various components leading to bending of the substrate, the radius of curvature is measured both before, R0, and after, R, the deposition of each film, allowing the built-in stress of each layer, Tf,BI, [3,14,20,26,27] in a process to be isolated

$$\mathbf{T\_{f,BI}} = \frac{1}{6} \frac{\mathbf{Y\_s}}{(1 - \mathbf{v\_s})} \frac{\mathbf{t\_s^2}}{\mathbf{t\_f}} \left(\frac{1}{\mathbf{R}} - \frac{1}{\mathbf{R\_0}}\right) \tag{1}$$

where Ys, v<sup>s</sup> and t<sup>s</sup> are the Young's Modulus, Poisson's ratio, and thickness of the substrate respectively, and t<sup>f</sup> is the thickness of the film.
