GR1010742B - Micro-electro-mechanical switch with floating electrodes on structure minimizing field emission currents and related phenomena - Google Patents

Abstract

The invention relates to a new micro-electro-mechanical switch arrangement (Micro-Electro-Mechanical-System switch, MEMS) that simultaneously combines the advantages of conventional structures and of structures provided with a floating electrode. Said arrangement aims at minimizing the phenomena arising from the presence of emission currents due to fields (field emission currents) without the use of side actuation pads/electrodes. In the proposed arrangement, on top of the dielectric film and on either side of the area where the moving metal bridge will land -when activated-there are two floating electrodes which are non-electrically interconnected in such a way that no part of the floating electrodes is under the bridge. Activation of these floating electrodes occurs when an additional metal part of a material different from that of the moving bridge, this material being in electrical contact but perpendicular and above the moving bridge (therefore not in contact with the dielectric during activation), creates conductive contact with the floating electrodes. This part is introduced in a way that selectively affects the mechanical and electrical properties of the moving part; the material is chosen to lead to a minimization of field emission currents and of related phenomena, when approaching the said floating electrodes.

Description

DESCRIPTION

Microelectromechanical switch with floating electrodes in a structure that minimizes emission effects due to electric field

The invention falls within the broader category of electronic devices and specifically concerns a new arrangement for a Micro-Electro-Mechanical-System switch (MEMS). This new arrangement aims to simultaneously combine the advantages of conventional and floating electrode structures, aiming to minimize the presence of field emission currents and related phenomena without the use of side actuation pads/electrodes.

Microelectromechanical switches for microwave applications (Radio Frequency Micro-electro-mechanical System, RF MEMS) exhibit a number of properties that make them particularly attractive compared to the usual semiconductor devices that are mainly used for the same functions. They exhibit excellent response over a wide frequency range while at the same time having almost zero consumption and therefore attract great scientific and technological interest [G.M. Rebeiz, RF MEMS: Theory, Design and Technology, John Willey & Sons, 2003).

A very common arrangement of a capacitive microelectromechanical switch (J.B. Muldavin, G.M. Rebeiz "High isolation CPW MEMS shunt switches -Part 1:Modeling" IEEE Trans. Microwave Theory & Techniques vol.48, pp. 1045-1052, 2000) involves a metal bridge that is electrically connected to the two ground lines of a coplanar waveguide and is initially suspended in an air gap above its central electrode (Signal line). By applying a suitable potential difference between the central electrode and the ground lines of the waveguide, the metal bridge collapses onto the central electrode of the waveguide. At the point where this contact will occur, under the bridge and on the central electrode of the waveguide, there is a thin dielectric film (thickness d) to prevent the conductive connection of the two metals that would lead to a short circuit. This dielectric film can extend beyond this area. By zeroing or removing the potential difference, the metal bridge returns to its original position. The operation of the switch is determined by the capacitances created by two positions in which the moving bridge can be found, suspended at a distance above the dielectric (Cup-state) or on the dielectric film (Cdown-state). In many cases, the ratio of these two capacitances (Cdown-state/cup-state) is considered important, but what is always and undoubtedly of exceptional importance is the value of the capacitance at the position where the bridge has collapsed and is on the dielectric film (Cdown-state). At this position the value of the capacitance with a good approximation takes the value given by the relation.

Cdown-state = ε0εrA/d

Where ε0 is the value of the dielectric constant of the vacuum, εr is the relative dielectric constant of the dielectric film, A is the contact area and d is the thickness of the dielectric film.

In practice, the value of the capacitance is significantly less than that calculated by the above formula. This is because the actual/effective contact area of the bridge with the dielectric film (A*) is less than the value A due to the deformed shape of the bridge when it is in the lower position but also due to the existence of interfacial roughness on the free surface of the dielectric. This uncontrolled reduction of the capacitance in relation to its theoretical calculation was the reason for the introduction of a category of devices usually characterized by the term floating electrode. ;;The main difference in floating electrode devices is the existence of a thin metal film (electrode) directly on the dielectric film. This electrode or part of it is located under the bridge and part of it can extend outside this area since the same happens with the dielectric film. This electrode has no conductive connection to any other part of the switch and is therefore characterized as floating. Its presence ensures that when the moving bridge collapses on it, even any minimal contact between them activates, due to conductive connection, the entire surface of the floating electrode, resulting in maximizing the capacitance (C down-state) to the value predicted by theoretical calculations. In the case of the floating electrode, the moving bridge is usually activated by applying a potential difference to the side actuation pads/electrodes and not directly through the transmission line (Signal line) (X. Rottenber et al. "Novel RF-MEMS capacitive switching structures," 2002 32nd European Microwave Conference, Milan, Italy, 2002, pp. 1-4, X. Rottenberg et al. "Boosted RF-MEMS capacitive shunt switches." Proceedings of Workshop on Semiconductor Sensor and Actuator (SeSens, Veldhoven, The Netherlands, 2003, F. Giacomozzi et al., "Technological and Design Improvements for RF MEMS Shunt Switches," 2007 International Semiconductor Conference, Sinaia, Romania, 2007, pp. 263-266, F. Giaccomozzi et al. "Development of High ConCoffration RF MEMS shunt switches" Rom. J. of Infor. Scien. Technol. vol. 11, pp. 143-151 2008, EP1398811B1). The actuation of floating electrode devices through the transmission line, like conventional devices, has been shown to be possible as well. In this case the operation of the device is determined by the presence of filed emission currents which are due precisely to the high value of the electric field intensity that develops as the moving metal bridge approaches the floating electrode (G. Papaioannou et al "Floating electrode microelectromechanical system capacitive switches: A different actuation mechanism" Appl. Phys. Lett. 99, 0735501, 2011, L. Michalas etal., "Electrical characteristics of floating electrode MEMS capacitive switches, " 2012 7th European Microwave Integrated Circuit Conference, Amsterdam, Netherlands, 2012, pp. 36-39). The existence of these currents (field emission currents) makes the actuation of floating electrode devices through the transmission line extremely complex and a not always controllable process due to the factors (e.g. surface roughness) that enter into the relevant process. ;;Taking into account the level of existing knowledge, the present invention relates to a new arrangement of a micro-electromechanical-system switch (MEMS) such that it simultaneously combines the advantages of conventional structures and those with a floating electrode (Floating Electrode) aiming at minimizing the phenomena arising from the presence of field emission currents (field emission currents) without the use of side actuation electrodes (side actuation pads/electrodes). ;The proposed arrangement and simple/direct variations thereof are illustrated in plan view in Figure 1 (a-f) in front view in Figure 2 (a,b) and in side view in Figure 3 (a,b). Specifically, it includes the transmission line consisting of three metal films (2,3 in Figure 1a) at appropriate distances between them on an insulating substrate (1, in Figure 1a) which can be glass, ceramic or semiconductor on which a dielectric layer has been grown. Part of the central metal film is covered by a dielectric film (4 in Figure 1a) the thickness of which is decided by the relationship 1. Alternatively, it is possible for more than one film to be deposited sequentially one on top of the other (4 in Figure 2b and Figure 3b). Then, with an appropriate process that includes the deposition, patterning and removal of a suitable sacrificial material, a metal bridge (5 in Figure 1) is created by deposition. The bases of the bridge are located in conductive connection with the two metal films located at the ends of the transmission line (2 in Figure 1a) (not the central one - 3 in Figure 1a) and suspended above the central metal film of the transmission line which is covered by the dielectric film. The movable metal bridge can have a solid shape or have holes (8 in Figure 1b) or even arms (5 in Figure 1c). Also alternatively the arrangement can be based on another form of transmission line or general electrode in which the lateral conductors (2, in Figure 1a) are absent. In this case there is only the central electrode (Figure 1d). In the proposed arrangement the dielectric film (4 in Figure 1a) (or the successive dielectric films 4 in Figure 2b and Figure 2c) covering the central metal film of the transmission line must extend over an area larger than the area defined by the projection of the moving bridge on the plane of the coplanar waveguide. ;This must be the case because in the proposed arrangement there are two non-electrically connected floating electrodes (6 in Figure 1a), on the upper part of the dielectric film and on either side of the area in which the moving metal bridge will land upon activation, in such a way that no part of them is located under the bridge, i.e. in the aforementioned area defined by the projection of its shape on the plane of the transmission line. The proposed arrangement can also exist in a variant with a single floating electrode (Figure 1e). ;In addition, the structure includes an additional metal part (7 in Figure 1a) or more than one (7 in Figure 1f) of a different material (relative to that of the moving bridge) which is located in an electrical contact but perpendicularly and in a plane above the moving bridge (and therefore does not come into contact with the dielectric during activation). This part is inserted in a way that selectively affects the properties (mechanical, electrical) of the moving part and the material is chosen to lead to a minimization of field emission currents and related phenomena. ;The two floating electrodes (6 in Figure 1a) may consist of a metal film or of multiple metal layers (multilayer stack) (6 in Figure 2b and 6 in Figure 3b), so as to ensure good adhesion with the dielectric film, appropriate thickness (spacer) and/or in combination with the material to be deposited by the moving bridge to lead to a minimization of field emission currents and related phenomena. ;The activation of these floating electrodes is realized when, during the activation of the bridge, the additional metal section (7 in Figure 1a or 7 in Figure 2b and Figure 3b) of a different material (in relation to that of the moving bridge) which is in electrical contact but perpendicular and level above the moving bridge (and therefore does not come into contact with the dielectric during activation) creates conductive contact with the floating electrodes (6 in Figure 1a). The thicknesses of the metal bridge and the floating electrodes are chosen appropriately so that this happens. ;With the proposed arrangement, the conventional activation of the switch (via the transmission line) is achieved, free from the peculiarities created by the presence of floating electrodes under the bridge and at the same time the effect of these phenomena (related to the presence of emission currents due to the electric field) is minimized without the need for additional activation electrodes (side actuation pads/electrodes), thus providing great flexibility in the design of the devices. ;In the case of the proposed switch the final value of the capacitance during activation (C down-state) is given by the relationship. ;; ; Where A1 and A2 are the surfaces defined by the floating electrodes and A* is the active surface of the moving bridge with the dielectric film.

Claims (7)

1. A microelectromechanical switch consisting of the transmission line on a suitable substrate, a dielectric film covering part of the central electrode of the transmission line, a moving metal bridge, two floating electrodes on the dielectric film in an area not below the moving metal bridge and an additional metal part on the moving bridge in conductive contact with it and in a plane above it and in a perpendicular orientation to it so that during activation, which is carried out by the transmission line and without activation electrodes (side actuation pads/electrodes), it touches the floating electrodes without however coming into contact with the dielectric. 2. A variation of this structure that includes one instead of two floating electrodes 3. A variant that includes more than one additional metal section on the moving bridge in a geometry that controls the mechanical properties of the bridge. 4. A variant where the transmission line is not a planar waveguide but another form of waveguide or simple electrode. 5. A variation where the dielectric film may consist of more than one film, sequentially one on top of the other. 6. A variant in which the floating electrode(s) consist of multiple metal layers (multilayer stack). 7. A variation of this structure may include a movable bridge with holes and/or arms.