Transition from multiple to single microcontact conduction during hot switching of microelectromechanical switches with ball-shaped dimples

Previous studies of electron transport within direct contact microelectromechanical switches have found that conduction occurs via nanoscale contact asperities. It has been claimed that reduced contact resistance can be achieved by using multiple contact switches; however, the ability of these switches to enhance power handling or lifetime remains a question. To study the contact mechanism, single-input-multiple-output switches with ball-shaped dimples were specially designed and tested. At all voltage levels of hot-switching operation, uneven current sharing among the outputs was observed. Furthermore, at softening voltage, an irreversible multiple to single conduction transition occurs and is found to alternate among different outputs.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87795/2/133501_1.pd

Low-force contact heating and softening using micromechanical switches in diffusive-ballistic electron-transport transition

We demonstrate softening of the gold-to-gold contact in surface micromachined microelectromechanical switches under electrostatic force near 30 μN30μN, which results from the heating of contact asperities sustaining electron transport. A bias potential that causes the switch contacts to soften is measured for initial contact resistance varying between 0.5 and 300 Ω300Ω. The asperity sizes in this range are comparable to the electron mean-free path at room temperature. We show that contact spots smaller than the mean-free path require larger bias for softening. Our results can be explained using a model accounting for ballistic electron transport in the contact.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87858/2/023507_1.pd

Lifetime Extension of RF MEMS Direct Contact Switches in Hot-Switching Operations by Ball-Grid-Array (BGA) Dimple Design

Direct contact RF microelectromechanical systems switches have demonstrated excellent ultrawideband performance from dc to 100 GHz. However, they are prone to failures due to contact adhesion and arcing, particularly for pure-gold/pure-gold contacts. In this letter, we present a new contact design employing ball grid array (BGA) dimples that limit the effective contact area to a few tens of nanometers in diameter. We experimentally show the performance of the BGA dimple with pure-gold/pure-gold contacts and demonstrate RF power handling greater than 1 W during hot switching in excess of 100 million cycles.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87268/4/Saitou10.pd

Adhesion effects on contact opening dynamics in micromachined switches

We propose a technique to measure the opening time for micromachined switches and present substantial experimental data for switches with gold–gold contacts. The data demonstrate that contact opening time increases dramatically as apparent contact area increases or as pull-apart force or contact resistance decreases. A model of opening time is also presented with model parameters that fit the experimental data. Moreover, we show that transient mechanical vibrations can play an important role in reducing switch opening time.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87500/2/103535_1.pd

Contact Physics Modeling and Optimization Design of RF-MEMS Cantilever Switches

RF MEMS direct-contact switches exhibit many advantages over the conventional semiconductor switches; however, existing drawbacks such as low power handling, high pull-in voltage and long switch opening time are most critical. This paper presents an optimization design for an RF-MEMS cantilever direct-contact switch to achieve maximum power handling capability, minimum pull-in voltage and switch opening time simultaneously. A 2-step optimization technique is proposed to achieve the optimal design to allow for a power handling capability of 130 mW, a pull-in voltage of 52 V, and a switch opening time 4.4 _s presented. The optimization results show that substantial room exists for improving the current designs of RF MEMS direct-contact switches.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87255/4/Saitou72.pd

Asperity heating for repair of metal contact RF-MEMS switches

We have experimentally observed the failure of metal contact RF MEMS switches due to a rapid rise in contact resistance during switching. We were able to repair the failed switches through heating the contact asperities by applying sufficient contact voltage. The data suggest the hypothesis that increasing contact resistance is caused by strain hardening of the contact surface. With this understanding, appropriate corrective measures can be taken to overcome failure, and suggestions for doing so are given in the paper.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87250/4/Saitou87.pd

Effect of Nanoscale Heating on Electrical Transport in RF MEMS Switch Contacts

This paper explores contact heating in microelectromechanical systems (MEMS) switches with contact spot sizes less than 100 nm in diameter. Experiments are conducted to demonstrate that contact heating causes a drop in contact resistance. However, existing theory is shown to over-predict heating for MEMS switch contacts because it does not consider ballistic transport of electrons in the contact. Therefore, we extend the theory and develop a predictive model that shows excellent agreement with the experimental results. It is also observed that mechanical cycling causes an increase in contact resistance. We identify this effect as related to the build-up of an insulating film and demonstrate operational conditions to prevent an increase in contact resistance. The improved understanding of contact behavior gained through our modeling and experiments allows switch performance to be improved.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87261/4/Saitou21.pd

A Preconditioner for Hybrid Matrices Arising in RF MEMS Switch Analysis

Despite the excellent characteristics of RF MEMS switches, they generally suffer from low power-handling capability. This limitation is due to the complex interactions among electromagnetic losses, heat transfer, and mechanical deformations associated with the switches. To understand these failure mechanisms, we proposed a multiphysics model (Jensen, B.D. et al., IEEE Microwave and Wireless Components Letters, vol.13, no.9, p.364-66, 2003). This model is based on an extended finite element-boundary integral (EFE-BI) model that allows efficient modeling of the boundary (MEMS beam for our case) exterior to the volumetric region modeled by the standard FE-BI method. The condition number of the resulting EFE-BI matrix system increases rapidly as the frequency decreases. The matrix condition number required at 2 GHz warrants computational accuracy beyond the capability of normal CPUs. For this reason, our EFE-BI analysis and validation of the code were limited to high frequency cases. We propose a preconditioning approach that lowers the condition number of the system. The proposed approach allows for fast, efficient analysis of RF MEMS switches at practical RF frequencies as low as 500 MHz, which will enable the desired multiphysics modeling.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87247/4/Saitou83.pd

Skin Effect Aggregated Heating in RF MEMS Suspended Structures

This paper presents experimental data together with 2 modeling approaches to demonstrate the increased heating of MEMS suspended structures at radio frequencies due to skin effects. Distinguishable average temperature rises are measured at 2, 13.5, and 18 GHz in a 616 _m _ 20 _m _ 2.7 _m suspended coplanar waveguide using 4-wire measurement configuration. Our measurements compare well with: (1) previous electromagnetic simulations and (2) a newly introduced analytical thermal model incorporating only skin effects. Buckling and plastic yielding have been observed during and after measurement. This study provides a simple and quantitative approach for the design of suspended structures such as low loss transmission lines, filters and switches with high power handling capability.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87275/4/Saitou79.pd

Skin-Effect Self-Heating in Air-Suspended RF MEMS Transmission-Line Structures

Air-suspension of transmission-line structures using microelectromechanical systems (MEMS) technology provides the effective means to suppress substrate losses for radio-frequency (RF) signals. However, heating of these lines augmented by skin effects can be a major concern for RF MEMS reliability. To understand this phenomenon, a thermal energy transport model is developed in a simple analytical form. The model accounts for skin effects that cause Joule heating to be localized near the surface of the RF transmission line. Here, the model is validated through experimental data by measuring the temperature rise in an air-suspended MEMS coplanar waveguide (CPW). For this measurement, a new experimental methodology is also developed allowing direct current (dc) electrical resistance thermometry to be adopted in an RF setup. The modeling and experimental work presented in this paper allow us to provide design rules for preventing thermal and structural failures unique to the RF operation of suspended MEMS transmission-line components. For example, increasing the thickness from 1 to 3 mum for a typical transmission line design enhances power handling from 5 to 125 W at 20 GHz, 3.3 to 80 W at 50 GHz, and 2.3 to 56 W at 100 GHz (a 25-fold increase in RF power handling)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87277/4/Saitou15.pd