RF-MEMS and RF-MEMS packaging
RF MEMS is a disruptive technology, commonly regarded as key for future telecommunication systems as it potentially alleviates several of the natural trade-offs presented by current wireless communication systems. The RF-MEMS technology that is under development at IMEC, is based on IC-fabrication technology that yields small, low-weight and high-performance RF components, including switches, varicaps, resonator and (tunable) filters. Two main research directions are currently explored by IMEC:
1. Metal RF-MEMS passive technology platform for building switches, varicaps together with regular passive components like transmission lines, inductors and capacitors.
2. Special type high-Q resonators and filters, including micromechanical resonators, piezoelectric acoustic resonators (FBAR), contour mode resonator (CMR) and cavity resonators.
Metal RF-MEMS passive technology platform
In 2007, IMEC further developed its metal RF-MEMS passive technology platform along two main axes: On the one hand, the mainstream thin-film RF-MEMS technology was optimized for its stability and reliability. As an example, the RF-MEMS capacitive shunt switch from figure 1 showed remarkable reliability (see section MEMS reliability) and repeatability as depicted in figure 2.
Figure 1: Top view microphotograph of a capacitive shunt switch fabricated in the mainstream RF-MEMS technology platform from IMEC.
Figure 2: Measured RF characteristics from ten devices of the type shown in figure 1.
On the other hand, a novel and extremely low complexity RF-MEMS technology was developed. This technology is a simple 2-mask process involving only one sacrificial layer and one metallization as it relies only on EFFA's (electrostatic fringing-field actuators) for its actuation. By its simplicity, this technology allowed researchers at IMEC to access a novel insight in the reliability of electrostatic MEMS devices and especially in the problems linked to dielectric and substrate charging (see section MEMS reliability).
The basic electrostatic fringing field actuator (EFFA), shown in figure 3, consists of 2 sets of free-standing interdigitated cantilever beams processed on an insulating substrate. There is no electrode underneath these fingers. In idle state the beams curl out-of-plane as shown in figure 4 due to their residual stress gradient, lowering the rest-capacitance between the fingers. In actuated state, the fingers contact the substrate with εr>1 what increases the interdigitated capacitance P14399.
Figure 3: Top view microphotograph of a typical series EFFA RF capacitor composed of 6/7 Al fingers, 1µm thick, 300µm long, 20µm wide and 5µm spaced.
Figure 4: Coventorware simulation of the curvature of the fingers in idle state due to a built-in stress gradient in the metal film.
The electrostatic actuation of the EFFA's is simple to understand considering the electrostatic force between 2 metal electrodes tends to increase the capacitance these define. Applying a DC-voltage between the 2 electrodes from figure 3 and figure 4, the interdigitated fingers first tend to align to each other and then to smash on the dielectric substrate with εr>1 to maximize their capacitance.
In the basic 2-masks EFFA technology, we demonstrated series and shunt switchable capacitors as well as λ/8 phase-shifters at 5.25GHz with insertion loss better than 0.6dB as shown in figure 5 and figure 6.
Figure 5: Microphotograph of a phase shifter implementing 2 shunt EFFA capacitors combined using a low-loss high-impedance line.
Figure 6: Measured and designed S-parameters of the phase-shifter presented in figure 5.
With an additional metallization, in a 3-masks EFFA process, we demonstrated EFFA-based series tunable LC-tanks, relay-actuated parallel-plate capacitors with increased capacitance ratio as well as relay-actuated RF-switches. At 9GHz, for example, the device from figure 7 presents at 9GHz insertion loss and isolation respectively better than 0.6dB and 20dB.
Figure 7: Shunt capacitive relay implementing 4 EFFA's as side-actuators (a) top view - (b) measured S21 parameter.
Special type high-Q resonators and filters
In 2007, IMEC further developed special type high-Q filters along three main axes:
a) Micromechanical (MEM) resonators for operation below 1GHz
b) Piezoelectric acoustic resonators for operation in the range 1-20GHz
c) Fixed and (tunable) cavity resonators for operation above 20GHz
Micromechanical (MEM) resonators for operation below 1GHz
The technology for fabricating MEM resonators is being realized as part of the IMEC Silicon Germanium program, leveraging the expertise present for SiGe within IMEC while trying to solve some unique processing requirements for MEM resonators. A distinctive design constraint of electrostatic MEM-resonators is the narrow gap between the resonating structure and the electrostatic transducers, which is in the order of 50-100nm. Two different technology modules are under investigation to achieve these stringent constraints. The first approach employs a sacrificial sidewall spacer. This layer is deposited over the structural layer forming a 100nm barrier with the electrode that is grown over this stack. During the release the sacrificial sidewall spacer is removed leaving the desired gap between electrode and resonator. During the past year advances were made in optimizing the individual steps of this processing module.
A second approach is the trench narrowing technology; here resonator and electrode are realized in the same layer with a standard 0.5μm process. The transduction gaps are then narrowed down using a low-pressure chemical vapor deposition (LPCVD) to achieve the desired trench width. Several approaches have been under investigation. In one approach, we started with a non-selective deposition, which deposits on the bottom of the trench short circuiting the resonator and electrode. A clearing step is performed with a DRIE etch tool to remove this short-circuit. A recipe for both the deposition and removal was successfully developed achieving a final trench width of 200nm, as verified by SEM measurements. Some results of this technique, illustrated for a disk resonator, are shown in figure 8.
Figure 8: A Silicon Germanium disk resonator from which the gaps are narrowed down using the LPCVD trench narrowing technology. The trenches are narrowed down from an original width of 0.5μm to 200nm.
Piezoelectric acoustic resonators for operation in the range 1-20GHz
IMEC is working towards the realization of single-chip switched filter banks with filters operating at different frequency bands. The first step consists of the realization of novel switchable film bulk acoustic resonators (FBARs), taking the function of a series connection of an RF-switch and an FBAR as depicted in figure 9. Previously demonstrated frequency tunable FBARs C15876 form the building block of our approach. They are fabricated with surface micromachining techniques such that a piezoelectric membrane is suspended above an actuation electrode, thus defining an acoustic resonator at fixed frequency in series with a variable capacitance using electrostatic actuation. This design can also be operated in a `switch' mode. In order to improve the isolation characteristics of the switch, the actuation and RF characteristics must be decoupled. For this a new actuation mechanism is implemented in the design of the switchable FBAR, namely piezoelectric actuation. Compared to electrostatic actuation, this actuation is not limited by the pull-in restriction, is bi-directional which can be used to prevent stiction, has more stability and more importantly the actuation voltage is not determined by the area of the air gap capacitor, which provides the aforementioned decoupling. A possible design of a switchable FBAR with piezoelectric actuation is shown in figure 9. In this design, the two series capacitors are balanced with the acoustic resonators to create a symmetric device in which the FBAR is actually formed by two FBARs placed in series to facilitate the fabrication process. The piezoelectric layer is used at the same time for the actuation mechanism and for the resonator, thus limiting the number of masks needed for fabrication.
Figure 9: Switchable FBAR. (a) Functionality of a switchable FBAR. (b) Schematic built-up (top view and cross section). The piezoelectric layer serves as resonating layer and as actuation layer.
Fixed and tunable cavity resonators for operation above 20GHz
Cavity resonators (and dielectric resonators) are based on conventional wave guide technology or micromachined. These components are generally operated at mm-wave frequencies if higher Q-factors (>500) are required. IMEC has been investigating the realization of waveguides or cavities that are micromachined from silicon and which can next be readily integrated with MCM-D in a hybrid fashion. To allow frequency tuning of the cavity resonator a novel method using MEMS actuators inside the cavity is implemented. The frequency tuning relies on the fact that any disturbance, e.g. an enclosed volume, inside the cavity will change the electrical dimensions of the cavity and thus the resonant frequency of the cavity. A variable volume can be attained by MEMS actuators placed in an array type configuration, e.g., the EFFA array of figure 4 leading to the situation shown in figure 10. A simulation of the tuning characteristics of the cavity of figure 10 is shown in figure 11.
Figure 10: Cross-sectional view of a MEMS tunable Si etched cavity resonator.
Figure 11: Tuning characteristics of a 60GHz cavity resonator of the type shown in figure 10. Tuning is based on volume tuning.