Robust Pop-Up Shape Encapsulation Using Dual-Sealing

This letter presents pop-up-shaped (having a spherical top shape) thin film encapsulation (TFE) using two-step sealing method. The first sealing step seals the etch holes in the cap layer, whereas the second step provides the shape. A pop-up shape with 2.4-μm upward deformation is demonstrated using a negative pressure difference between the two sealing layers-0.5-μm SiN at 1100 mTorr followed by 3-μm SiO² at 390 mTorr on an aluminum nitride cap encapsulating 800-μm × 800-μm size cavities. Besides a deeper cavity, the pop-up shape has smaller downward deformation of the cap layer due to its sturdy design. The increase in the robustness of TFE with pop-up radius is verified through finite element method simulations and theoretical calculation.

I. Introduction

Packaging of microelectromechanical system (MEMS) devices such as RF-MEMS switch [1], MEMS resonator [2], MEMS based filter and oscillator [3] is essential to protect their fragile structure from the harsh environment and provide ambience for their reliable operation [4]. Thin Film Encapsulation (TFE) is an attractive alternate technique to the wafer bonding due to possible reduction of overall thickness of the final MEMS device [4], [5], reduction of collective area of the packaged device and low cost from elimination of the cap wafer [6]–[8]. However, TFE has a drawback of downward deformation of thin cap layer due to its lower strength against external pressure after the sealing process. Various approaches have been reported to improve the robustness of the TFE against external pressure in the literature. Mele et al. [7] proposed strengthening of cap layer by creating columns to support it and improve its spring constant to reduce its downward deformation. However, this arrangement cannot be universally applied for all kinds of MEMS devices, particularly, for bigger MEMS devices. Shimooka et al. [8] used additional thick polymer sealing layer to improve spring constant of the cap layer and reduce downward deformation of the cap layer. However, the additional polymer layer increases unnecessary topography of the final device which may increase residual stress. Also proper handling of polymer is needed. In this letter, pop-up shape TFE formation was proposed to avoid the limitation on MEMS device design and unnecessary topography of final encapsulation. This letter presents formation of pop-up shape TFE, means change in shape of TFE top from flat (fig 1(a)) to spherical shape (fig 1(d)), by simple dual sealing process. This enhances robustness of TFE. Fig. 1.

Schematic diagram of pop-up shape TFE using double sealing process. (a) TFE after 1st thin sealing at relatively high pressure. (b) Enlarged view of TFE near etch channel. (c) Pop-up deformation of TFE during low pressure deposition. (d) Final pop-up shape TFE after 2nd sealing layer deposition.