Microsystems Encapsulation using Nanoporous Alumina (Inkapselen van microsystemen met behulp van nanoporeuze alumina)
Microsystems Encapsulation using Nanoporous Alumina
Zekry, Joseph Eid Estafanous; S0199508
Packaging of traditional integrated circuits (ICs) has been reliant for several decades on the techniques of metal bonding and plastic overmolding. However, such conventional packaging techniques are failing to cope with the rapidly shrinking IC dimensions and the growing variety of new microsystems (like micro-electro-mechanical systems, or MEMS) used in modern appliances including biomedical implants and smartphones. In this context rises the need for this research to set a step forward in the direction of package miniaturization, improved reliability and increased functionality of state-of-the-art microsystems.This thesis deals with the technological challenges as well as with the design and performance aspects of new micropackages created using thin membranes of nanoporous alumina. The new micropackages are intended to encapsulate microsystemsand MEMS in particularat wafer-level in a controlled environment. Such a micropackage can accommodate one or more microsystems in a planar microcavity of 1 to 10 µm height with lateral dimensions between 0.1 and 1.0 mm. The on-wafer microcavities are formed by etching a sacrificial layer underneath nanoporous alumina membranes of 1 to 3 µm thickness. These membranes feature a large density of cylindrical nanopores with diameters between 10 and 20 nm and height equal to the thickness of the membrane; facilitating the sacrificial layer etching process. A novel wafer-level anodization process performed at a temperature close to 30 °C is developed to produce fully perforated nanoporous alumina membranes within an Al thin film in a single fabrication step. A specially designed photoresist mask is used to define the lateral shapes of the alumina membranes with high precision, while maintaining a low-resistance path for the anodization current across the large area of a 200 mm wafer. A controlled environment inside the microcavities is achieved during a process of depositing an impermeable sealing layer on top of the nanoporous alumina membranes. The resulting micropackages typically feature dielectric (and optically transparent) caps with a thickness between 4 and 9 µm. The caps are normally anchored around the microcavities using an Al-based sealing ring of 10 to 50 µm width. Empty micropackages of different shapes and configurations as well as encapsulated RF transmission lines and other microsystems (like Ni-based MEMS) have been produced on 200 mm Si wafers. Moreover, micropackages with sufficient robustness to undergo a plastic overmolding processperformed at high pressure of 30 bar and a high temperature of 175 °Chave been designed, fabricated and tested.Analytical and finite element models have been developed to analyze the thermomechanical and electromagnetic characteristics of the new micropackages and the embedded microsystems. These models provide much insight into the strength, reliability and compatibility of the micropackages with different applications. Furthermore, experimental studies of the hermeticity and reliability of the new micropackages are presented. An extensive hermeticity analysis is carried out based on optical monitoring of the cap deformation under different environmental conditions. This includes short-term (less than 10 days) exposure to helium at 3 bar pressure and long-term (up to 14 months) exposure to air under atmospheric pressure. In this experiment, the significant impact of the sealing ring configuration on the package hermeticity is demonstrated. Moreover, other methods for hermeticity evaluation, including the use of an embedded microresonator or micro-Pirani gauge, are discussed. Additionally, the outcome of a comprehensive set of reliability tests is presented; including the impact of repeated exposure to mechanical shocks and extreme temperatures, in addition to exposure to high humidity levels (at high temperatures). Finally, the compatibility of the new micropackages with radio frequency (RF) microsystems is evaluated. Special Al-based feedthroughsthat can transmit high frequency signals through the package boundaries with minimal added losseshave been designed, implemented and tested.