Packaging

MEMS packaging and reliability: An undividable couple

“…For the 0-level packaging, two general approaches have been taken [4][5][6] here referred to as ''thin-film capping'' and ''chip-capping'', illustrated in Fig. 2 and Fig.…”

“…Chip capping as opposed to thin-film capping has the advantage of a large flexibility in processing temperatures and control of the cavity ambient (pressure and gas composition), but on the other hand leads to a rather large form factor and profile. The capping is either done as a die-to-wafer (D2W) capping [12], or as a wafer-to-wafer (W2W) capping [13,5]. The end result is in essence the same, but from the view of process complexity and cost, W2W is the preferred route for small caps (<2 mm 2 ), whereas D2W is preferred for larger cap sizes.…”

“…Wafer (or die) bonding technologies include direct bonding (glass-to-Si anodic and Si-to-Si fusion bonding) and bonding using intermediate layers [1,2,5,6]. To the latter class belong solder bonding (e.g., Au/Sn at around 280°C), gold-gold thermo-compression bonding, eutectic bonding (e.g., Au-Si eutectic at 363°C) and, bonding using low temperature glasses (''frit'' seals at around 400°C), epoxy seals or polymer (e.g., BCB at around 250°C) adhesives.…”

“…The bond is characterized by its strength (shear and pull or tensile strength) and the hermeticity of the seal. More details of the different bonding techniques used can be found in [2,5,6].…”

“…The hydrostatic strength or perhaps better, the flexural stiffness of a membrane or (composite) capping layer is determined by a complex interplay between the geometry and thickness of the membrane, the residual stress in the membrane layer(s), the Young's modulus of the membrane material(s), and the clamping conditions of the membrane. Finite element modeling (FEM [5,21]) is often needed to adequately design the package. From a simple model, which excludes residual stress in the capping layer, it is found that a square silicon-based thin film cap (with thickness typically less than 10 lm), the lateral dimensions are limited to below 500 lm in order to keep the center deflection below 0.5 lm when the cap is subject to 10 5 Pa differential pressure.…”