H.A.C. Tilmans

Analytical Model of the DC Actuation of Electrostatic MEMS Devices With Distributed Dielectric Charging and Nonplanar Electrodes

This paper gives a new insight into the problem of the irreversible stiction of RF microelectromechanical systems (MEMS) attributed to the dielectric charging. We present a model for the electrostatic actuation of MEMS devices taking into account the nonuniform distributions of the air gap and the charges in the dielectric layer. The major result of our study is the impossibility to invoke the sole uniform dielectric charging phenomenon to explain the irreversible stiction of electrostatic MEMS devices. In the absence of other forces, a nonzero variance of the charge distribution is required to explain the stiction of the device. Considering only uniform residual charge densities, previous reported works could only account for a drift of the actuation characteristics as a whole. In case of a uniform air-gap distribution, our analytical model can already account for an increase of the up-capacitance, a shift of the - , its narrowing, and the stiction by a closure of the pull-out window. We further show that the combined nonuniformities of air gaps and charges break the symmetry of the actuation characteristics. The asymmetry can be such that one of the pull-in points disappears, which is replaced by a continuous tuning range while the other pull-in point still exists.

Investigation of the hermeticity of BCB-sealed cavities for housing (RF-)MEMS devices

This paper reports on the hermeticity testing of MEMS cavities using BCB as the sealing and bonding material. Hermeticity has been tested according to the MIL-STD-883D. Gross leak testing based on liquid fluorocarbons revealed that BCB sealed cavities are leak tight, which means that the MEMS devices are well protected during handling and back-end processing (e.g., wafer dicing). Further, it is shown that the He fine leak testing of the MIL-STD is not fully applicable to small volumes (<1000 nl), typically encountered for MEMS. The problem is that the undefined regime normally existing in the MIL-STD is largely extended for small cavity volumes. Microbolometers have been used as test vehicles to confirm this. Large (>10,000 nl) cavities are needed to cover the entire leakage spectrum.

Through-silicon via and die stacking technologies for microsystems-integration

The highest integration density of microsystems can be obtained using a 3D-stacking approach, where each layer of the stack is realized using a different technology, which may include sensors, imagers, rf and MEMS technologies. A key challenge is however to perform such stacking in a cost-effective manner. In this paper, a novel 3D TSV and 3D stacking technologies will be presented. Application examples are MEMS packaging and heterogeneous integration of imaging devices.

Mechanical and electrical characterization of BCB as a bond and seal material for cavities housing (RF-)MEMS devices

This paper reports on the mechanical and electrical characterization of benzo-cyclo-butene (BCB) as a bonding and sealing material for 0-level packages (cavities) housing (RF-)MEMS devices. Shear strength and hermeticity of BCB-sealed cavities are experimentally investigated as functions of the geometrical parameters of the BCB sealing ring and the bonding conditions. The leak rate of BCB-sealed cavities strongly depends on the BCB width, and leak rates as low as 10−11 mbar l s−1 are measured for large BCB widths (>800 µm), dropping to 10−8 mbar l s−1 for BCB widths of around 100 µm. Depending on the bonding conditions, shear strengths as high as 150 MPa are achieved. BCB is also used in 0-level packaging of RF-MEMS devices, such as RF-switches and coplanar waveguides (CPWs). The electrical influence of the 0-level package is studied for different capping materials. It is experimentally shown that a 0-level package using capping chips made of low-loss high-resistivity materials (AF45 glass and high-resistivity silicon) and having a cavity height larger than about 45 µm above RF-MEMS devices, has a negligible impact on the microwave characteristics of an RF-MEMS device. Finally, some reliability testing is performed on BCB-sealed 0-level packages in order to study the influence of temperature and humidity on the mechanical properties of BCB. After testing in relatively harsh conditions, the BCB seal stays gross leak tight and shear strengths as high as 30 MPa are measured. BCB turns out to be a very robust and reliable material to encapsulate MEMS devices.

Wafer-level packaged RF-MEMS switches fabricated in a CMOS fab

Reports on wafer-level packaged RF-MEMS switches fabricated in a commercial CMOS fab. Switch fabrication is based on a metal surface micromachining process. A novel wafer-level packaging scheme is developed, whereby the switches are housed in on-chip sealed cavities using benzocyclobutene (BCB) as the bonding and sealing material. Measurements show that the influence of the wafer-level package on the RF performance can be made very small.

RF MEMS for ubiquitous wireless connectivity. Part II. Application

This paper reports on RF-MEMS switchable capacitors, varactors, and ohmic switch technology attribute for both base stations and handsets in realizing frequency-agile RF/wireless systems capable of serving multiple frequency bands. For the handset, this leads to a smaller footprint combined with low power consumption of the RF radio. For the base station the benefit lies in ability for reconfiguration of the air interface, which leads to high logistical savings for infrastructure vendors through a reduction in the number of product variants. Reconfigurable frequency-agile radios are a perfect addition to reconfigurable baseband processing. Both together form the basis of a realistic and reasonable approach to realize software radios.

Microelectromechanical tunable capacitors for reconfigurable RF architectures

This paper reports on metal-based MEMS tunable capacitors, fabricated in a thin-film process on high-ohmic silicon. Continuous and reversible tuning has been demonstrated with an average tuning ratio of 4.5. A quality factor between 100 and 300 has been obtained in a frequency range of 0.5 to 4 GHz. The combination of a high quality factor and large tuning range makes these tunable capacitors very suitable as building blocks in many radio-frequency (RF) applications. The tuning speed, temperature stability and RF power handling have been studied in terms of self-actuation. Finally, the need for a hermetic package as well as a packaging concept which can potentially provide this has been demonstrated. After packaging, the devices can be handled as standard silicon dies, making them fit very well with a system-in-package approach.

Optimization of 0-level packaging for RF-MEMS devices

This paper reports on the optimization of the 0-level package for RF-MEMS devices like switches and tunable capacitors. The 0-level package consists of an on-chip cavity obtained by flip-chip mounting a capping chip over the RF-MEMS device, using BCB as the bonding and sealing material. A process for realizing low-profile packages, with caps less than 100 /spl mu/m thick, is described. Coplanar RF feedthroughs are implemented using BCB as the dielectric. It is experimentally shown that a 0-level package using capping chips made of low-loss high-resistivity materials and having a cavity height larger than about 45 /spl mu/m, has a negligible impact on the microwave characteristics of an RF-MEMS device, built on a 50 /spl Omega/ CPW line with ground-to-ground spacing of 150 /spl mu/m.

An electrostatic fringing-field actuator (EFFA): application towards a low-complexity thin-film RF-MEMS technology

This paper presents a novel electrostatic actuator using fringing fields as the actuation mechanism, i.e. an electrostatic fringing-field actuator or EFFA. The novel device is produced on an insulating substrate in a simple two-mask process involving only one sacrificial layer and one metallization. To demonstrate the EFFA capabilities, we produced and characterized EFFAs in various technological implementations of remarkable simplicity. This simplicity allows a vast flexibility for the processing and as a result strongly eases the integration of the EFFAs into existing technologies. For the basic device, we report a non-de-embedded measured capacitance ratio of 1:3 and a lifetime of more than 107 cycles with 40 V bipolar actuation at 100 Hz in N2 atmosphere. Both the capacitance ratio and the C–V profile were tuned by modifying the technology, e.g. coating the substrate before processing the EFFAs and the design, e.g. switching from clamped-free to clamped–clamped devices. We finally report a complete RF-MEMS technology using the EFFAs as single actuators. Switchable and tunable LC tanks, phase shifters, series and shunt parallel-plate capacitors actuated as relays and capacitive relays are presented to demonstrate the possibilities of this technology.

From zero- to second-level packaging of RF-MEMS devices

This paper reports the full packaging and assembly of RF-MEMS devices for operation below 6GHz, comprising 0-level, 1-level and 2-level packaging. The fabrication process for the RF-MEMS devices is based on the Philips PASSI/spl trade/ process, which is adapted to accommodate the packaging. The 0-level assembly is based on solder bonding, and is realized on a flip-chip aligner/bonder under atmospheric pressure, at 240/spl deg/C. The 1-level package comprises a chip-scale-package (CSP) in which the 0-level packaged device is directly equipped with solder balls for the interconnect. The individual 1-level assemblies are then flip-chip mounted on a RF-board at 240/spl deg/C, completing the 2-level packaging. RF measurements of a 2nd-level packaged series switch showed an increase of the insertion loss of about 0.1 dB after 0-level packaging, increasing to about 0.5dB after 2nd-level packaging. Preliminary evaluation of the 0-level package hermeticity using switches configured as mechanical resonators indicates an hermetic seal.