Fabrication of beam structures with micro-scale cross-sections and meso-scale spans

To allow testing of micro-scale aerodynamics, a process was created to manufacture beam structures that combine spans of 1 cm with a cross-section of 5 µm by 100 µm. The structural considerations limiting the fabrication of a structure combining macro-scale spans with a micro-scale cross-section are analyzed. Limiting considerations include forces during operation, fluid forces during release, vibrational limitations and beam buckling. Based on these results, a fabrication process for creating a beam structure for large spans without support structures is devised, incorporating the use of back-side etches and extra handling wafers to avoid stiction. This process is used to successfully fabricate the desired structure.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/58143/2/jmm7_12_018.pd

Fully Integrated Electrothermal Multi-Domain Modeling of RF MEMS Switches

RF MEMS switches have demonstrated excellent performance. However, before such switches can be fully implemented, they must demonstrate high reliability and robust power-handling capability. Numerical simulation is a vital part of design to meet these goals. This paper demonstrates a fully integrated electrothermal model of an RF MEMS switch which solves for RF current and switch temperature. The results show that the beam temperature increases with either higher input power or increased frequency. The simulation data are used to predict switch failure due to temperature-related creep and self pull-in over a wide range of operating frequency (0.1-40 GHz) and power input (0-10 W). Self pull-in is found to be the dominant failure mechanism for an example geometry.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87264/4/Saitou33.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

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

Analysis of RF-MEMS Switches Using Finite Element-Boundary Integration with Moment Method

This paper presents a new hybrid methodology for modeling RF-MEMS switches. This method combines the usual finite element-boundary integration (FE-BI) method for the fixed section of the switch, and the method of moments for the movable beam. This approach is intended to address the large 100:1 scale variation within a single computational domain, which also spans a very small fraction of a wavelength.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87249/4/Saitou98.pd

Force Dependence of RF MEMS Switch Contact Heating

Contact-type RF MEMS switches have demonstrated low on-state resistance, high off-state impedance, and very large bandwidth; however, their power handling capability is low due to failure caused by contact heating. This paper examines contact heating by measuring V-I curves for contacts in gold switches. Multiphysics modeling allows extraction of contact temperature. Contacts are found to soften and self-anneal at a temperature of about 100¡C, corresponding to a contact voltage of about 80 mV. Larger contact force induces a larger decrease in contact resistance during softening, suppressing contact heating. The data provide a better understanding of micro-scale contact physics, leading to design for switches for improved power-handling capability.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87263/4/Saitou88.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

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

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