Interfacial adhesion and friction are important factors in determining the performance and reliability of microelectro- mechanical systems. We demonstrate that the adhesion of micromachined surfaces is in a regime not considered by standard rough surface adhesion models. At small roughness values, our experiments and models show unambiguously that the adhesion is mainly due to van der Waals dispersion forces acting across extensive non-contacting areas and that it is related to 1/Dave2, where Dave is the average surface separation. These contributions must be considered because of the close proximity of the surfaces, which is a result of the planar deposition technology. At large roughness values, van der Waals forces at contacting asperities become the dominating contributor to the adhesion. In this regime our model calculations converge with standard models in which the real contact area determines the adhesion. We further suggest that topographic correlations between the upper and lower surfaces must be considered to understand adhesion completely.

The role of van der Waals forces in adhesion of micromachined surfaces

Reliability issues currently hamper the commercialization of capacitive RF MEMS switches. The most important failure mode is parasitic charging of the dielectric of such devices. In this paper we present an improved analytical model that enables us to calculate and understand the effect of insulator charging on the behavior of capacitive RF MEMS switches, and to describe the way they fail, and their reliability. Emphasis is placed on a shift of the pull-out voltage to predict failures. Tests with capacitive RF MEMS switches have been performed that validate the most important features of the model.

https://iopscience.iop.org/article/10.1088/0960-1317/14/4/011/pdf

A comprehensive model to predict the charging and reliability of capacitive RF MEMS switches

Wireless communication has led to an explosive growth of emerging consumer and military applications of radio frequency (RF), microwave and millimeter wave circuits and systems. Future personal (hand-held) and ground communications systems as well as communications satellites necessitate the use of highly integrated RF front-ends, featuring small size, low weight, high performance and low cost. Continuing chip scaling has contributed to the extent that off-chip, bulky passive RF components, such as high-Q inductors, ceramic and SAW filters, varactor diodes and discrete PIN diode switches, have become limiting. Micro-machining or MEMS technology is now rapidly emerging as an enabling technology to yield a new generation of high-performance RF-MEMS passives to replace these off-chip passives in wireless communication (sub)systems. This paper reviews the progress in RF-MEMS from a device and integration perspective. The worldwide state-of-the-art of RF-MEMS devices including switches, variable capacitors, resonators and filters are described. Next, it is stipulated how integration of RF-MEMS passives with other passives (as inductors, LC filters, SAW devices, couplers and power dividers) and, active circuitry (ASICs, RFICs) can lead to the so-called RF-MEMS system-in-a-package (RF-MEMS-SiP) modules. The evolution of the RF-MEMS-SiP technology is illustrated using IMEC's microwave multi-layer thin-film MCM-D technology which today already serves as a technology platform for RF-SiP.

https://iopscience.iop.org/article/10.1088/0960-1317/13/4/323/pdf

MEMS for wireless communications: 'from RF-MEMS components to RF-MEMS-SiP'

Microelectromechanical systems (MEMS) radio frequency (RF) switches hold great promise in a myriad of commercial, aerospace, and military applications. However, there is little understanding of the factors determining the performance and reliability of these devices. Fundamental studies of hot-switched gold (Au) contacts were conducted using a micro/nanoadhesion apparatus as a switch simulator. Experiments were conducted in a well defined air environment under precisely controlled operating conditions. Fundamental properties were connected to performance with an emphasis on the effects of contact force and electric current on contact resistance (R), microadhesion, and reliability/durability. Electric current had the most profound effect on switch performance. Observations at low current (1–10 μA) include: (1) slightly higher R; (2) asperity creep; (3) high adhesion after rapid switching; (4) switch bouncing; and (5) reasonable durability. Conversely, observations at high current (1–10 mA) include: (1) slightly lower R; (2) melting; (3) no measurable adhesion; (4) less propensity for switch bouncing; (5) necking of contacts; and (6) poor reliability and durability due to switch shorting. Low current behavior was dominated by the propensity to form smooth surface contacts by hammering, which led to high van der Waals force. High current behavior was dominated by the formation of Au nanowires that bridge the contact during separation. Data suggest the presence of an adventitious film containing carbon and oxygen. Aging of the contacts in air was found to reduce adhesion.

Fundamental studies of Au contacts in MEMS RF switches

A new soft-lithographic method for micropatterning polymeric resists, Decal Transfer Microlithography (DTM), is described. This technique is based on the transfer of elastomeric decal patterns via the engineered adhesion and release properties of a compliant poly(dimethylsiloxane) (PDMS) patterning tool. An important feature of the DTM method is the exceptionally broad spectrum of design rules that it embraces. This procedure is capable of transferring micron to submicron-sized features with high fidelity over large substrate areas and potentially simplifies to a significant degree the requirements for effecting multiple levels of registration. The DTM method offers some potential advantages over other soft-lithographic patterning methods in that it is amenable to transferring resist patterns with both open and closed forms, negative and positive image contrasts, and does so for a wide variety of aspect ratios and a significant range of pattern pitches that can be accommodated without degradation due to mechanical distortions of the pattern transfer tool. The most significant advance embodied in the DTM method, however, is that it offers useful new capabilities for the design and fabrication of advanced planar and 3D microfluidic assemblies and microreactors.

Decal transfer microlithography: a new soft-lithographic patterning method.

This paper reviews the recent progress in MEMS for radio frequency (RF) applications from a device perspective. RF MEMS devices reviewed include switches and relays, tunable capacitors, integrated inductors, mechanical resonators and filters, and some representative microwave and millimetre-wave components. Important device parameters are highlighted, as they have significant contributions to the performance of the final products in which the devices are used. The challenges and statuses of these RF MEMS devices are outlined and discussed. The intent of this topical review is to provide perspective to newcomers in the field, and empower potential end-users with an overall device picture, current status, and a vision of their ultimate performance capabilities.

/pdf/rf-mems-from-a-device-perspective-34wvbgeq8p.pdf

J Jason Yao

Papers

RF MEMS from a device perspective