Modeling and design of high force RFMEMS contact switches

This thesis describes the modeling and design issues of high force electrostatically actuated RFMEMS contact switches. Various reliability issues and challenges are associated with the design of RFMEMS contact switches. The reliability of microcontacts is of major importance in increasing the lifetime of such devices. A semi-analytical model based on the beam deflection theory is developed to calculate the contact force delivered by parallel-plate electrostatically actuated switch structures. The model is capable of fast and accurate static solution for complicated switch structures which are normally analyzed using finite element analysis techniques. High force designs are presented together with finite element simulations confirming the model results and reflecting on the reliability of these switch structures. The RF performance is expected to be comparable to recently announced commercial counterpart specifications. A simple all-metal surface micromachining technology providing thick electroplated nickel beams will be used to implement high force switch structures

Microsystems Encapsulation using Nanoporous Alumina (Inkapselen van microsystemen met behulp van nanoporeuze alumina)

Packaging of traditional integrated circuits (IC s) 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 thiscontext 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 microsystems and MEMS in particular at 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 shapesof 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 process performed at high pressure of 30 bar and a high temperature of 175 °C have 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 comprehensiveset of reliability tests is presented; including the impact of repeatedexposure 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 feedthroughs that can transmit high frequency signals through the package boundaries with minimal added losses have been designed, implemented and tested.nrpages: 180status: publishe

Wafer-level thin film vacuum packages for MEMS using nanoporous anodic alumina membranes

This paper reports on a wafer-level thin film vacuum packaging technology for MEMS. It is based on the fabrication of freestanding porous anodic alumina membranes of a typical thickness of 2 to 3 μm, featuring cylindrical nanopores that are a mere 15-20 nm in diameter (aspect ratio >100). The fabrication process involves in situ perforation of the thin AlOₓ barrier layer present at the bottom of the nanoporous membranes. For the present paper, a silicon oxide sacrificial layer and a vapor-phase HF release etch through the pores are utilized. The thin film packages are next sealed with a 4 μm-thick PECVD nitride layer. Strong and “air-tight” thin film packages are obtained this way. Negligible impact on the RF transmission losses (up to 67 GHz) is observed. A basic assessment of the package hermeticity based on the cap deflection method is also presented.status: publishe

Wafer scale fabrication of miniature vacuum packages for MEMS/NEMS using thin membranes of nanoporous alumina

poster + abstractstatus: publishe

Built-in self-limitation of masked aluminum anodization using photoresist

This paper reports on a simple method for wafer-scale production of precisely-shaped 2D patterns of porous anodic alumina (PAA) by masked anodization of aluminum films. The produced dielectric PAA structures, featuring cylindrical nano-pores of very high aspect ratio (>100), are useful for several applications including the production of nanowires and miniature MEMS packages. The fabrication process utilizes a photoresist mask; but to overcome the instability of the photoresist during the anodization process, special borderlines are included in the mask design. These borderlines act as self-synchronized switches for the anodization current, preventing the undesired photoresist delamination and lateral extension of the PAA structures. Employing such borderlines resulted in a reduction of the lateral extension of masked Al anodization on 200mm wafers from more than 300 μm to approximately 6 μm.status: publishe

Built-in Self-Limitation of Masked Aluminum Anodization using Photoresist

AbstractThis paper reports on a simple method for wafer-scale production of precisely-shaped 2D patterns of porous anodic alumina (PAA) by masked anodization of aluminum films. The produced dielectric PAA structures, featuring cylindrical nano-pores of very high aspect ratio (>100), are useful for several applications including the production of nanowires and miniature MEMS packages. The fabrication process utilizes a photoresist mask; but to overcome the instability of the photoresist during the anodization process, special borderlines are included in the mask design. These borderlines act as self-synchronized switches for the anodization current, preventing the undesired photoresist delamination and lateral extension of the PAA structures. Employing such borderlines resulted in a reduction of the lateral extension of masked Al anodization on 200mm wafers from more than 300μm to approximately 6μm

Design, fabrication and testing of wafer-level thin film vacuum packages for MEMS based on nanoporous alumina membranes

This paper reports on the mechanical design, the fabrication technology and the key performance and reliability aspects of novel 0-level thin film vacuum packages for (RF-)MEMS. The packages typically feature a dielectric cap composed of nanoporous alumina and PECVD silicon nitride with a total thickness between 6 and 8.3 µm. The surface micromachining fabrication process is based on a relatively simple method for the wafer-level formation of freestanding nanoporous alumina membranes, featuring extremely narrow cylindrical nanopores with diameters in the range of 10–20 nm and aspect ratio exceeding 100. The package impact on an encapsulated coplanar waveguide (CPW) is minimized (up to 67 GHz) by locally narrowing the RF feedthroughs underneath the package anchor. Furthermore, the hermeticity of various package configurations is evaluated by means of optical monitoring of the cap deflection under different environmental conditions, including short-term exposure to helium and long-term (up to 14 months) exposure to air. Finally, a comprehensive investigation of the reliability of the thin film packages and their compatibility with high-pressure epoxy overmolding 1-level packaging is discussed.status: publishe

MEMS packaging and reliability: An undividable couple

This paper reviews various approaches to package MEMS, illustrated mainly with examples from imec. Wafer-level or 0-level packaging is mostly dealt with. The role the package plays in achieving the required performance and reliability characteristics is elucidated. Package requirements, such as hermeticity and strength, are named, discussed and illustrated with examples. Considerations of reliability testing are presented. It is made conceivable that vacuum maintenance of tiny MEMS packages is a dominant reliability issue, something not at all obvious to achieve. © 2012 Elsevier Ltd. All rights reserved.status: publishe