Proceedings of the 2012 24th International Symposium on Power Semiconductor Devices and ICs 3-7 June 2012 - Bruges, Belgium
Performance Limits of MEMS Switches for Power Electronics Peter G. Steeneken and Olaf Wunnicke NXP Semiconductors HTC32, 5656 AE Eindhoven, The Netherlands
[email protected] transistors (sect. IV and V). The model will also be compared to experimental data on MEMS switches, like the NXP switch shown in Fig. 1. The MEMS switch consists of a circular SiN membrane, with a central gold contact that is surrounded by semicircular actuation electrodes. For more details on the manufacturing and performance of this MEMS switch we refer the reader to [3]. We conclude by discussing other parameters of MEMS switches (sect. VI and VII) including safe operating area, switching speed and integration with CMOS.
Abstract— Advances in semiconductor technology have brought the performance of power transistors near the physical limit. Substantial performance enhancement of power switches will therefore require either new materials, or new devices that obey fundamentally different limits. One of the new power devices that might offer an alternative to the transistor is the microelectromechanical (MEMS) switch. Here we analyze the potential of metal-contact MEMS switches for power electronics by exploring their physical performance limits and by benchmarking them against transistors. Based on a semiempirical model we show that MEMS switches could outperform Si transistors for actuation voltages Vact>30 V and could even beat GaN for Vact>1000 V. Therefore we conclude that MEMS switch technology potentially offers an interesting alternative route towards high performance power devices, although switching time and safe operating area remain points of concern.
I.
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INTRODUCTION
The development of microelectromechanical (MEMS) switches has mainly been driven by their superior RF performance compared to high electron mobility transistors [1]. However, the potential of MEMS switches in DC and low-frequency power applications has been much less explored, despite attractive advantages like low on-resistance Ron, high breakdown voltage BV, small area A. In order to estimate the performance of MEMS switches in power electronic applications, models and experiments are needed to compare their performance to current transistor technology. The most common metric for the comparison of power devices is the relation between specific on-resistance RonA and breakdown voltage BV [2]. In field effect transistors this relation is fundamentally limited by material properties of the semiconductor material like its electron ionization rate, mobility and dielectric constant. The physics that determines these figures of merit in MEMS switches is entirely different: • In the on-state the resistance of the MEMS switch Ron is mainly limited by the resistance between the metallic contacts, which strongly depends on contact force, Young’s modulus, roughness and cleanliness. • In the off-state the breakdown voltage BV depends strongly on the contact gap distance gcont and is determined by fieldemission and ionic gas discharge between the contacts. In this paper we derive a model for on-resistance Ron (sect. II) and breakdown voltage BV (sect. III). This model is used to benchmark the performance of MEMS switches to that of
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Figure 1.
Microscope image of NXP’s MEMS switch [3].
II.
ON-RESISTANCE RON
The on-resistance Ron of MEMS switches is theoretically limited [4] by the constriction resistance Ron=ρ/(2reff) between the contact metals, where ρ is the resistivity of the metal and reff is the radius of the circular contact area. For a spherical elastic contact with radius of curvature rc, the contact radius reff depends on the total contact force Fc according to reff=(4Fcrc/3E)1/3 ,where E is the Young’s modulus of the contact metal [5]. Combining these equations one finds: Ron= ρ[6E/(32 Fcrc)]1/3
(1)
From this equation it is found that a single gold contact with a radius of curvature rc=1 μm has a contact resistance Ron=0.1 Ω for a force of about 150 μΝ. In practical MEMS switches this low contact resistance is often not reached for two reasons: the gold atoms on both sides of the contact are not as closely
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packed as in bulk gold such that additional interface resistance is present and contaminating atoms can be present on the surface that decrease the effective surface area and increase contact resistance. In Fig. 2 we show a 4-point measurement of the contact resistance Ron of one of the best samples of the NXP MEMS switch after application of 100,000 cold switching cycles and 60 seconds at 40 mA DC current. In Fig. 2 the contact force Fc is estimated from the actuation voltage Vact and the geometry of the switch. Comparison with the dashed curve shows that the measured contact resistance is indeed higher than the theoretical estimate from (1). Especially below 50 μN the resistance is a lot higher. This corresponds with earlier studies, which report that a force of at least 100 μN is needed to form a reliable gold contact [1]. Possibly a minimal force of 100 μN is needed to form an intimate contact between the gold atoms and to break through contaminating surface layers. Based on these observations we employ the following simplified description of the MEMS switch contact: Ron,r=0.3 Ω, Fc,r=100 μN
(2)
Where Fc,r is the minimum contact force required for reliable operation of the switch during its lifetime and Ron,r is the contact resistance at this force. There is no need for these equations to capture the complete force dependent resistance of equation (1), since for a total available force Ftot the design can be optimized by placing N MEMS switches in parallel. In that case the force per contact is Fc=Ftot/N=Fc,r and the total on-resistance equals Ron=Ron,r/N=Ron,rFc,r/Ftot. This design yields an Ron that is, according to (1), a factor N2/3 lower than the Ron obtained by concentrating all available force on a single contact. Therefore the optimum switch operates at the minimum value of Fc=Fc,r that allows reliable operation. The performance limits of MEMS switches can be further extended if contact materials are found with lower products Ron,rFc,r
