RF MEMS Packaging Research Articles

Temperature Effects in Packaged RF MEMS Switches with Optimized Gold Electroplating Process.

Due to its excellent electrical performance, mechanical reliability, and thermal stability, electroplated gold is still the most commonly used material for movable beams in RF MEMS switches. This paper investigates the influence of process conditions on the quality and growth rate of gold electroplating, and the optimized process parameters for the gold electroplating process are obtained. The characterization of the optimized electroplated gold layer shows that it has small surface roughness and excellent thermal stability. With this optimized gold electroplating process, the RF MEMS switches are fabricated and hermetic packaged. In order to obtain the temperature environment adaptability of the packaged switch, the influence of working temperature is studied. The temperature effects on mechanical performance (includes pull-in voltage and lifetime) and RF performance (includes insertion loss and isolation) are revealed.

Design and development of broadband DC-10 GHz packaged RF MEMS switches with on chip CPW-microstrip transitions

Design and development of broadband DC-10 GHz packaged RF MEMS switches with on chip CPW-microstrip transitions

Structured glass substrates in wafer- and panel level packaging: Status and recent achievements.

Abstract Glasses can be used as core substrate for panel- and/or wafer-level packaging to achieve heterogeneous integration of chiplets and integrated passives in increasingly complex packages. Glass has a large number of advantages: The stiffness of glass (i) allows manufacturing of highly accurate buildup layers. These buildup layers can have manufacturing precision of 1μm and below on large dies with sizes of 50mm x 50mm and more, needed for antenna in package (AiP) applications and high performance computing (HPC). Special glasses can be made with adjusted thermal expansion (CTE) (ii), either adjusted to silicon or with larger thermal expansion to allow packages with buildup layers of epoxy molds and metallization that see high thermal loads either during manufacturing or during operation. Glasses can also be optimized with very good dielectric properties (iii) and can be utilized in antenna-in-package applications. But most of all, economic glass structuring techniques (iv) which can provide millions of vias and thousands of cut-outs in a glass panel are important and are being developed. SCHOTTs Structured Glass Portfolio FLEXINITY® and related technologies provide an excellent starting point for highly sophisticated structured glass substrates required for Advanced Packaging. The biggest hurdle for a large-scale commercialization of glass panel packaging is industrial readiness along the whole process chain. This is needed, to bring glass panel packaging in applications like IC-packaging, RF-MEMS packaging and medical diagnostics or, in combination with cutouts for fan-out, embedding of active and passive components. In addition, metallization processes with good adhesion, excellent electrical properties and high geometric accuracy for glasses are an important step. In the current manuscript, we review the status and discuss our contribution towards achieving industrial readiness for glass in panel- and wafer-level packaging.

Investigation of the impact on the radio frequency (RF) behaviour of electrically conductive adhesives (ECAs) used in wafer-level packaging (WLP) of RF-MEMS

In this work, an anisotropic conductive adhesive (ACA), belonging to the family of electrically conductive adhesives (ECAs), exploited for the wafer-level bonding phase in packaging of RF-MEMS (MEMS for radio frequency passives), is investigated in respect to its influence on the RF behaviour of packaged Microsystem structures and CPWs (coplanar waveguides). The explored encapsulation solution is based on the wafer-level packaging (WLP) approach, in which the capping substrate (high-resistivity silicon—HRS) provided of vertical through silicon vias (TSVs) is bonded to an RF-MEMS wafer using ACA as adhesion layer. After bonding of ACA, conductive metal particles within the epoxy-based matrix either contribute to the electrical path formation or, when uncompressed, remain isolated. Influence of such floating particles on the RF performance of adjacent structures is investigated by means of finite element method (FEM) electromagnetic simulations. The results show that the influence of floating ACA conductive particles on the S-parameters of the targeted planar structures is rather limited up to 60 GHz.

A modelling and simulation approach for radio frequency (RF) parasitic effects reduction in wafer-level packaging (WLP) of RF-MEMS passive components

RF-MEMS, i.e. micro electro mechanical-systems (MEMS) for radio frequency (RF) passive components, started to make their way into mass-market applications in the telecommunication segment. One critical aspect to ensure easy employment of such devices within systems and sub-systems is that of packaging and encapsulation. In packaging of RF-MEMS, next to the protection of the fragile movable structures and membranes, optimisation of the package electrical performance plays a very important role. In this work, a wafer-level packaging process is investigated and optimised in order to minimise its electrical parasitic effects. The capping silicon substrate resistivity, the substrate thickness and the geometry of through-silicon vias are optimised relying on finite element method electromagnetic simulations. Moreover, a preliminary analysis on the electromagnetic effects of the wafer-to-wafer bonding techniques (i.e. solder bump reflow and Isotropic or anisotropic conductive adhesive – ICA/ACA) is presented.

Miniaturized Folded Ridged Quarter-Mode Substrate Integrated Waveguide RF MEMS Tunable Bandpass Filter

This paper presents a miniaturized RF MEMS tunable bandpass filter design developed by application of folded ridged quarter-mode substrate integrated waveguide cavity. Using packaged RF MEMS SP4T chips, a switchable reactive loading is applied to the folded ridged quarter-mode substrate integrated waveguide cavity, tuning the resonance frequency. Overall miniaturizations of 70.3% and 78.8% are achieved for inductively- and capacitively-loaded filters compared to an RF MEMS tunable filter using half-mode substrate integrated waveguide. Here, an analysis of inductive or capacitive loading along with their advantages and disadvantages are discussed. Two different two-pole folded ridged quarter-mode substrate integrated RF MEMS tunable bandpass filters are designed and measured. The first prototype employing inductive loading achieves 4.4% tuning range at a center frequency of 1.713 GHz, insertion loss of 3.10-3.92 dB, and return loss greater than 15 dB for all tuning modes. The second prototype employing capacitive loading is measured with a 35.2% tuning range at a center frequency of 865 MHz, insertion loss of 1.6-4.3 dB, and return loss greater than 10 dB for all tuning modes. The reported performance and extreme miniaturization of folded ridged quarter-mode substrate integrated waveguide cavities shows great promise for their application in tunable filter design.

RF MEMS packaged switch evaluation tests up to 25 GHz

This article presents the results of the series of experimental tests of a packaged RF MEMS switch manufactured as a chip on a silicon substrate in the Center for Materials and Microsystems of Fondazione Bruno Kessler. Experiments have been performed up to 25 GHz and included S-parameters check in different operation and environmental conditions, including variation of input power, ambient temperature and number of switching cycles. Presented RF MEMS SPST switch is a basic element of more complex reconfigurable networks such as SPxT switches, phase shifters, power attenuators etc.

Wafer-Level Packaging Method for RF MEMS Applications Using Pre-Patterned BCB Polymer.

A radio-frequency micro-electro-mechanical system (RF MEMS) wafer-level packaging (WLP) method using pre-patterned benzo-cyclo-butene (BCB) polymers with a high-resistivity silicon cap is proposed to achieve high bonding quality and excellent RF performance. In this process, the BCB polymer was pre-defined to form the sealing ring and bonding layer by the spin-coating and patterning of photosensitive BCB before the cavity formation. During anisotropic wet etching of the silicon wafer to generate the housing cavity, the BCB sealing ring was protected by a sputtered Cr/Au (chromium/gold) layer. The average measured thickness of the BCB layer was 5.9 μm. In contrast to the conventional methods of spin-coating BCB after fabricating cavities, the pre-patterned BCB method presented BCB bonding layers with better quality on severe topography surfaces in terms of increased uniformity of thickness and better surface flatness. The observation of the bonded layer showed that no void or gap formed on the protruding coplanar waveguide (CPW) lines. A shear strength test was experimentally implemented as a function of the BCB widths in the range of 100–400 μm. The average shear strength of the packaged device was higher than 21.58 MPa. A RF MEMS switch was successfully packaged using this process with a negligible impact on the microwave characteristics and a significant improvement in the lifetime from below 10 million to over 1 billion. The measured insertion loss of the packaged RF MEMS switch was 0.779 dB and the insertion loss deterioration caused by the package structure was less than 0.2 dB at 30 GHz.

Advanced Wafer Level Packaging of RF-MEMS with RDL Inductor

Abstract The market for portable and mobile data access devices that are wirelessly connected to the cloud anytime and anywhere is exploding. The trend to access any network from anywhere is driving increased functional convergence in the radio, which translates into increased packaging complexity and sophistication. This is creating unprecedented demand for RF components providing more integration- in smaller package sizes. There are exciting interconnect technologies in wafer level packaging such as wafer level chip scale packaging (WLCSP) or fan-out wafer level packaging (FO-WLP) solutions such as embedded Wafer Level Ball Grid Array (eWLB) to meet these needs. One of the most promising solutions to enable the required RF performance levels in mobile and wearable devices is the use of RF MEMS Tuners. Mobile original equipment manufacturers (OEMs) are rapidly adopting antenna tuning solutions to be able to provide the required signal strength across the large number of LTE spectrum bands used globally. With RF MEMS technology now maturing, the biggest challenge to address the fast growing opportunity was to find a suitable packaging technology that can deliver RF MEMS tuners in the smallest possible form factor, while maintaining the excellent performance characteristics of the RF MEMS technology. After careful analysis, an eWLB/FO-WLP package was adopted and released to volume production in 2015. The commercial eWLB/FO-WLP RF MEMS tuners outperform traditional RF silicon-on-insulator (SOI) switch-based antenna tuning solutions, resulting in much higher data rates (up to 2×) and improved battery life (up to 40%). Redistribution layers (RDL) in eWLB are utilized for higher electrical performance and complex routing to meet electrical requirements. The ability to utilize embedded passives in a multi-layer eWLB structure provides a number of advantages including cost reduction, footprint reduction and increased reliability. Inductors in eWLB offer significantly better performance compared to inductors in standard on-chip technologies. In this paper, we examine the WLCSP and eWLB packaging assembly flow, solutions to RF design challenges as well as characterization of RF performance. Further improvement of the quality factor of the integrated inductor and capacitors by using low-loss thin-film dielectrics and molding compound in eWLB will be reported as well. Package level reliability test results will also be presented in this paper.

K-band MEMS-based frequency adjustable waveguide filters for mobile back-hauling

This paper presents the first experimental results of a novel MEMS-based waveguide filter for mobile back-hauling at K-band with adjustable center frequency. The tuning concept employs commercial packaged RF MEMS switches, wire-bonded within a rectangular cavity. The switches are used to perturb the current distribution of the TE101 mode so as to tune the resonant frequency of the cavity. A tuning range up to 2% with unloaded Qs of the order of 1000 in K-band can be obtained. A second-order filter prototype employing commercially available MEMS on silicon substrate has been tested. Measurements show a 160 MHz frequency shift (0.7%) and Qs up to 1000 using only one MEMS per resonator. New higher-order prototypes using low-loss substrate MEMS (e.g. quartz) will be fabricated in order to get higher Qs. The concept here present leads to a very simple structure. The number of MEMS per resonator is, in fact, minimized (we use only one MEMS) and the design is simple. On the other hand, the achievable tuning range is lower (<2%) than the other concepts. Higher tuning ranges are achievable using more MEMS, but at the expense of lower Qs of the resonators.

A 1.7–2.7-GHz 4-bit Phase shifter based on packaged RF MEMS switches

This study presents a 4-bit phase shifter with a maximum phase shift degree of 75° based on packaged metal-contact single-pole double-throw (SPDT) radio frequency microelectromechanical system (RF MEMS) switches mounted on a 20 mil RO4350 substrate and assembled with its driver module on a 4-layer printed circuit board (PCB) board. Electric length of 10° and characteristic impedance of 61.7 Ω are selected for T-stub matching. Standard PCB processing makes it easy to develop and suitable for taking place of mechanical tilt employed in recent base station. Measurements indicate a maximum insertion loss of 1.37 dB at 2.7 GHz and a minimum return loss of 11.71 dB. The phase accuracy is within 1.75° at 2.7 GHz.

RF MEMS BASED HALF MODE BOWTIE SHAPED SUBSTRATE INTEGRATED WAVEGUIDE TUNABLE BANDPASS FILTER

A tunable planar bandpass filter based on a technique that utilizes a half mode substrate integrated waveguide (HMSIW) and novel inter-resonator coupling is presented. The tunable HMSIW based bandpass filter is implemented using two half triangle shaped cavities coupled together through inter-resonator coupling forming half mode bowtie shaped structure. The bowtie-shaped filter exhibits similar performance as found in rectangle- and circle-shaped SIW based bandpass filters. This concept reduces the circuit foot print, and miniaturization high quality factor is maintained by the structure. The tunable filter utilizes packaged RF MEMS switches; switching between different configurations of switches achieves four distinctive frequency states between 4.8-5.3 GHz. The filter maintains a constant absolute 1 dB bandwidth of 100 ± 10 MHz for all frequency states.

RF-MEMS packaging by using quartz caps and epoxy polymers

This work reports on RF MEMS chip capping to protect sensitive devices by quartz caps having a cavity to enclose MEMS devices and an epoxy film polymer as a sealing ring. Full hermeticity is not possible due to the permeability of polymer but the goal to protect the mobile parts of the devices during dicing and assembling was achieved. Good RF MEMS devices are produced by die-to-die bonding and a reliable fabrication process is defined while wafer-to-wafer process still needs improvement to increase the fabrication yield. In this paper the fabrication process for both die-to-die and wafer-to-wafer 0-level capping is presented. Good adhesion of caps to the substrate is demonstrated by shear tests, while RF measurements on CPW lines indicate a negligible insertion loss increase. Capping of both capacitive and ohmic contact switches is reported showing no loss of functionality but a modification of actuation voltage induced by thermal treatment.

Low-Loss Broadband Package Platform With Surface Passivation and TSV for Wafer-Level Packaging of RF-MEMS Devices

Packaging of radio frequency (RF) microelectromechanical system (MEMS) devices requires a good electrical performance, and thus requires the parasitic effects of packaging to be minimal. This paper presents the design, fabrication, and characterization of improved low-loss wafer-level packaging (WLP) platform with through-silicon-via (TSV) interposer for RF-MEMS packaging. The insertion loss because of parasitic effects is reduced using optimized grounding configuration and surface passivation. The coplanar waveguide (CPW) test vehicle, high-frequency antenna, RF tuner, and film bulk acoustic resonator (FBAR) filter are fabricated and characterized using the developed WLP platform with TSV. To determine the RF performance of the package, the CPW transmission lines are fabricated on high-resistivity Si substrates, and the grounding configuration is optimized. The fabrication of the RF-MEMS WLP involves the process of a TSV cap wafer and CPW transmission lines or RF MEMS on the substrate wafer. TSV cap process includes TSV etching, void-free TSV plating, and redistribution layer postprocess on thin TSV wafer. The electrical characterization of the fabricated devices is performed. The optimized model has a wide bandwidth of 26.5 GHz and a packaging loss of 0.1 dB at 10 GHz. This implies that the effect of packaging on the performance of RF device is expected to be minor. The developed WLP platform is used for the 94-GHz antenna, RF tuner, and FBAR filter packaging and the characterization of RF-MEMS devices is presented.

Through-glass copper via using the glass reflow and seedless electroplating processes for wafer-level RF MEMS packaging

We present a novel method for the fabrication of void-free copper-filled through-glass-vias (TGVs), and their application to the wafer-level radio frequency microelectromechanical systems (RF MEMS) packaging scheme. By using the glass reflow process with a patterned silicon mold, a vertical TGV with smooth sidewall and fine pitch could be achieved. Bottom-up void-free filling of the TGV is successfully demonstrated through the seedless copper electroplating process. In addition, the proposed process allows wafer-level packaging with glass cap encapsulation using the anodic bonding process, since the reflowed glass interposer is only formed in the device area surrounded with silicon substrate. A simple coplanar waveguide (CPW) line was employed as the packaged device to evaluate the electrical characteristics and thermo-mechanical reliability of the proposed packaging structure. The fabricated packaging structure showed a low insertion loss of 0.116 dB and a high return loss of 35.537 dB at 20 GHz, which were measured through the whole electrical path, including the CPW line, TGVs and contact pads. An insertion loss lower than 0.1 dB and a return loss higher than 30 dB could be achieved at frequencies of up to 15 GHz, and the resistance of the single copper via was measured to be 36 mΩ. Furthermore, the thermo-mechanical reliability of the proposed packaging structure was also verified through thermal shock and pressure cooker test.

An Electronically-Scanned 1.8–2.1 GHz Base-Station Antenna Using Packaged High-Reliability RF MEMS Phase Shifters

This paper presents the first 4-element base-station antenna that is passively scanned using low-loss RF MEMS phase shifters. The design is based on packaged RF MEMS SPDT switches, which are capable of handling Watts of RF power with high reliability. The results agree very well with simulations and show pattern scanning to 9° with a measured gain of 8.6-8.3 dB at 2 GHz. The array is capable of handling 5-10 W of power with no distortion.

Circular/Linear Polarization Reconfigurable Antenna on Simplified RF-MEMS Packaging Platform in K-Band

A linear-polarization (LP)/circular-polarization (CP) switchable reconfigurable antenna using a radio frequency micro-electro-mechanical-system (RF-MEMS) switch is proposed and a novel package platform is introduced. Since the package substrate of the novel package platform is used as a radiation aperture substrate of the antenna structure, the fabrication process can be simplified, and degradation of radiation performance can be prevented. As the radiation aperture exists on the top side of the package substrate, an aperture-coupling feed structure is employed. To implement the LP-CP-switchable antenna based on the package platform, a stub, acting as the other feed, is added to a slot ring and is connected to the RF-MEMS switch with an on/off state. The proposed antenna is simulated using the Finite-Element Method, after which it is fabricated and measured. The measured 10 dB impedance bandwidths (BWs) are 22.90% (LP state) and 28.43% (CP state). The measured 3 dB axial ratio BW is 13.07% in the CP state. The measured gains at 21 GHz are 2.63 dBi (LP state) and 3.90 dBi (CP state).

Lifetime limitations of ohmic, contacting RF MEMS switches with Au, Pt and Ir contact materials due to accumulation of ‘friction polymer’ on the contacts

We present lifetime limitations and failure analysis of many packaged RF MEMS ohmic contacting switches with Au–Au, Au–Ir, and Au–Pt contact materials operating with 100 µN of contact force per contact in hermetically sealed glass wall packages. All metals were tested using the same switch design in a controlled environment to provide a comparison between the performance of the different materials and their corresponding failure mechanisms. The switch lifetimes of the different contact materials varied from several hundred cycles to 200 million cycles with different mechanisms causing failures for different contact materials. Switches with Au–Au contacts failed due to adhesion when thoroughly cleaned while switches with dissimilar metal contacts (Au–Ir and Au–Pt) operated without adhesion failures but failed due to carbon accumulation on the contacts even in a clean, packaged environment as a result of the catalytic behavior of the contact materials. Switch lifetimes correlated inversely with catalytic behavior of the contact metals. The data suggests the path to increase switch lifetime is to use favorable catalytic materials as contacts, design switches with higher contact forces to break through any residual contamination, and use cleaner, probably smaller, packages.

Fabrication of void-free copper filled through-glass-via for wafer-level RF MEMS packaging

A novel fabrication method of copper filled through-glass-via is presented for wafer-level RF MEMS packaging. By using glass reflow and seedless electroplating process, a void-free copper via with a smooth side wall can be obtained. This enables a vertical interconnection between the electrical pads through the glass substrate. Furthermore, the proposed through-glass-via was applied to the packaging of a CPW transmission line to investigate its effects on the RF performances. For the encapsulation of the packaged device, which is the CPW line in this case, the glass cap with the cavity is anodically bonded to the substrate. The measured insertion loss of the whole packaging including the copper vias and the CPW line covered with the glass cap was 0.197 dB and the return loss was 20.032 dB at 20 GHz.

Pattern reconfiguration of microstrip antenna using flip‐chip mounted packaged MEMS

AbstractThis work describes the design of a radiation pattern reconfigurable microstrip antenna electrically controlled by packaged Radio frequency micro‐electro‐mechanical system (RF MEMS). The antenna operates at 12.24 GHz with good return loss characteristics and a ∼7.5 dBi gain at ±30° scan angles from broadside direction in the H‐plane. The integration of packaged RF MEMS and associated bias networks is also explained. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 574–577, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24978