Harrie Tilmans

Evaluation of platinum as a structural thin film material for RF-MEMS devices

Surface micromachined metal armatures are commonly used for MEMS applications of which RF-MEMS is the most well known. In most cases metals with a high conductivity, such as aluminum or gold, are used. These metals often have a low melting point and therefore have a low thermal stability and show plastic deformation of the structures at relatively low temperatures (<200 °C). High melting point metals, such as platinum, are expected to show plastic deformation only at higher temperatures which makes them interesting for use as a structural layer in RF-MEMS devices. In this paper, we present a technology to realize suspended platinum structures by means of surface micromachining. An improved lift-off process allows patterning 1 µm Pt films on a polyimide sacrificial layer. A comparison of the characteristics and armature resonance frequencies between RF-MEMS switches with Pt armatures and AlCu0.5% alloy armatures reveals an increased thermal stability for the former up to at least 250 °C. This enables zero-level packaging of switches at relative high temperatures without affecting their performances. The lower conductivity of Pt compared to AlCu0.5% does not lead to a significant increase in RF losses. Implementing AlN as a dielectric material, the Pt-based capacitive shunt switches reported in this paper showed lifetimes in excess of 5×107 cycles under standard testing conditions.

Piezoelectric actuation for application in RF-MEMS switches

RF-MEMS switches are commonly electrostatically actuated. This way of actuation has the advantage of technological simplicity. However the actuation voltage is relatively high. Piezoelectric actuation can have significantly lower actuation voltages, depending on the used materials and geometries. Analysis shows that clamped-free beams and clamped-clamped beams can have a reasonable deflection when aluminum nitride is used as the piezoelectric material. A five mask monolithic process has been developed for the realization of piezoelectrically actuated cantilevers and RF-MEMS switches. The complexity of this process is comparable with the complexity of the process for electrostatically actuated switches. Deflections of piezoelectrically actuated cantilever beams have been measured. Due to a high stress gradient in the beams, the assumptions that have been made in the analysis are not valid anymore. Finit element simulations were needed to verify the measurement data. The simulations fit with the measurements when the following values are taken for the properties of the aluminum nitrde film: Youngs modulus Ep = 320 GPa and piezoelectric coefficient d31 = -3.2 pC/N.

Thermomechanical design and modeling of porous alumina-based thin film packages for MEMS

Thin film 0-level or wafer-level MEMS packages exhibit relatively low flexural strength and yet they are required to reliably protect the enclosed MEMS devices under extreme processing and operational conditions. In this paper, we present a thermomechanical study of porous alumina-based thin film MEMS packages by making use of finite element modeling (FEM) techniques. We developed a 2D axisymmetric FEM that includes a porosity-dependent orthotropic representation of the porous alumina layer. The results of the FEM for a typical thin film package show around 15% enhancement over an analytical circular plate model in terms of the accuracy of calculating the maximum cap deflection under 105Pa differential pressure. The simulated package performance illustrates the significance of several parameters such as the package geometry, external hydrostatic pressure, residual stresses in the thin film, and the ambient temperature. Simulations further show that a circular package of 180µm diameter, featuring an 8µm thick cap and a central supporting pillar of 20µm diameter can withstand hydrostatic pressures up to 9MPa, which could occur during the process of plastic packaging with an epoxy molding compound. Furthermore, the thermal expansion mismatch between the different materials composing the thin film package poses a challenge to fulfill the reliability characteristics of these packages. It is however possible, based on FEM simulation results, to achieve reliable operation in the temperature range between −55°C and +125°C for a circular package of 6µm cap thickness and 250µm diameter without a supporting pillar.

Piezoelectric transduction of flexural modes in pre-stressed microbeam resonators

This paper reports on the optimization of the design of piezoelectric transducer elements integrated on doubly-clamped microbeam resonators utilized as (bio)chemical sensors. We report and emphasize the often forgotten influence of membrane stresses on defining the dimensions and optimal position of the piezoelectric transducer elements. The study takes into account stress induced structural changes and provides models for the equivalent motional parameters of resonators with particular shapes of the transducers matching the flexural modes of vibration. The above is analyzed theoretically using numerical models and is confirmed by impedance measurements and optical measurements of fabricated doubly-clamped beam resonators. We propose various transducer designs and highlight the advantages of using higher order vibration modes by implementing specially designed mode matching transducer elements. It is concluded that the paper describes and highlights the importance of accounting for the membrane stresses to optimize the resonator performance and the low power in electronic feedback of resonating sensing systems.

Simple and Robust Air Gap-Based MEMS Switch Technology for RF-Applications

This paper presents a simple and robust process for fabrication of functional electrostatic RF-MEMS switching devices with lifetimes easily exceeding 108 cycles with unipolar actuation at 100Hz. The device implements a switchable air gap capacitor and is therefore not limited in lifetime by dielectric charging as opposed to contact-type capacitive switches implementing high-k dielectrics. It is shown how these switched capacitors, even though having a capacitance ratio of only 2.8, can still form adequate switching devices and RF-circuits by proper design and combining of these devices with high-Q inductors and transmission lines. The novelty of the proposed process is that it combines a sacrificial layer consisting of a single layer with a single dry etching step for the dimples which define the air gap in the down-state. This airgap is switched by electrostatic actuation of a thick electroplated Nickel bridge-structure. The device is realized in a 4-lithographic steps process with low complexity and high robustness.

A surface micromachined tunable film bulk acoustic resonator

This paper reports on the design, modeling, fabrication and measurement of a novel-structured film bulk acoustic resonator (FBAR) that allows frequency tuning by MEMS actuation. FBARs are micromachined frequency controlling devices working in RF regime. For many applications, a small range of tuning is desired to cope with drifts from different origins. To realize this functionality, it has been suggested to place tunable elements such as variable capacitors or inductors in the circuit. A conventional approach, in which an external element is used, would introduce parasitics and might seriously degrade the quality factor of the system. In contrast, our work integrates the piezoelectric resonating film and the tuning element to build a compact structure. By reducing possible parasitics and electrical resistance, this structure enables frequency tuning while maintaining a high quality factor.

A MEMS autonomous switched oscillator

This work presents the model, design and characterization of an autonomous electrostatic MEMS oscillator for sensing application. The proposed system, consisting of a parallel-plate electrostatic MEMS device, a DC voltage source and a displacement-dependent resistive circuit, is capable of sustaining oscillations autonomously. It can exhibit periodic or aperiodic behavior, impacted by its working point and by environmental parameters, e.g. pressure, temperature. Electronic and non-electronic information can be submitted or retrieved from the oscillator system that present a very rich behavior sensitive to external stimuli what makes it a good candidate for threshold sensing.

CMOS integrated poly-sigemems accelerometer above 0.18 µm technology

The paper demonstrates the very first CMOS integrated monolithic MEMS (Micro Electro Mechanical System) accelerometer with SiGeMEMS technology on top of TSMC 0.18 μm CMOS technology. The developed SiGeMEMS technology shows the ability for integration above any standard foundry process. This has allowed us to build a surface micromachined accelerometer with a very small form factor. The accelerometer dimension, is one of the smallest of its kind for consumer application.. The total area of the accelerometer including the MEMS structure and the CMOS readout is 1.35 mm × 1.35 mm and the total thickness of the active device including MEMS and CMOS (excluding substrate ) is ~ 25 μm. Full functionality of the device is demonstrated with a sensitivity of ~0.14 volts/g This paper doesnt emphasize on the performance of an micro-accelerometer (merely serves as a test vehicle) but rather on technology development (SiGeMEMS above foundry CMOS).

Micromechanical ring resonators with a 2D phononic crystal support for mechanical robustness and providing mask misalignment tolerance

This paper reports on the design of ring-type electrostatically transduced bulk acoustic wave resonators designed for increased shock and vibration resistance. This was achieved through a 2D Phononic Crystal (PnC) support. The PnC is designed to operate in its bandgap so that it acts as a non-propagating medium, hereby achieving simultaneously a mechanically strong and acoustically well-confined support. We manufactured SiGe-resonators at 137.8MHz with a Q-factor of around 17,000. Another feature of this design is the process tolerance of the Q-factor (within 5%) and the resonance frequency towards mask misalignment (<;7μm) for the center support.

RF Technologies and Systems

This chapter gives an overview of assembly and packaging technologies that allow heterogeneous system integration for RF applications