Capacitive RF microswitch for high-power applications

Abstract

The invention relates to a capacitive radiofrequency micro electro-mechanical system comprising a metal membrane suspended above an RF transmission line (110) on the surface of a substrate (100), characterised in that: - the metal membrane comprises: ο a lower element (501) having a first portion located partially above the electrodes (200a, 200b) covered by elements (300a, 300b) made of a first dielectric material (D1) and partially above the RF line covered by an element (111) made of a second dielectric material (D2); ο an upper element (502) located above the first portion of the lower element; - the activation electrodes being arranged on either side of the RF transmission line; - the ground planes being arranged on either side of the RF line and along axes parallel to the first axis X; - the membrane comprising flexible zones (ZF) made up of a portion of the second portions of the lower element and a rigid zone (ZR) made up of the stack of the first portion of the lower element and of the upper element.

Description

DESCRIPTION Title of the invention: Capacitive RF microswitch for high power applications

[0001] The field of the invention is that of high power capacitive RF microswitches (typically greater than 30W) produced using microelectromechanical system or MEMS technology, the acronym MEMS meaning “Micro Electro Mechanical System”.

[0002] The challenge is to design MEMS-RF switches capable of handling high power levels (> 30W) while guaranteeing high levels of reliability (ideally » 10 11cycles). To achieve this, it is therefore necessary to design membranes whose mechanical properties allow them to withstand high powers, while applying relatively moderate actuation voltages (ideally < 50V). It is also necessary to relax the electrical constraints (DC and RF) on the dielectric material of the activation electrodes, so as to significantly increase the reliability of the component. The present invention aims to address these issues by proposing an original architecture.

[0003] Generally speaking, MEMS-RF switches consist of a metal membrane which, under the effect of an external stress (electrical, magnetic, thermal, etc.), will position itself on an RF signal line. Stepan Lucyszun presents the main configurations of MEMS-RF, “Advanced RF MEMS”, Cambridge University Press, 2010.

[0004] There are two configurations of MEMS switches that can be actuated in different ways: - Ohmic MEMS-RF switches where the metal membrane directly contacts the metallic RF line. In this case, there is a Metal-to-Metal contact like the configuration presented by Deepak Bansal et al. “Improved Design of Ohmic RF MEMS Switch for Reduced Fabrication Steps”, IEEE Transactions on Electron Devices, 2019. (Figures (5a) and (5b) of this publication show two configurations of MEMS-RF switches: Ohmic (5a) and capacitive (5b)).

[0005] The performance of this type of component in RF is directly linked to the value of the capacitance when the membrane is in the high state and to the value of the contact resistance when the membrane is in the low state. This configuration has the advantage of being able to address low frequency bands (from DC to a few GHz). However, the powers that these devices can support remain linked to the choice of metallic materials. These must support the currents (from the RF power in the metallizations) without undergoing degradation (microwelding, migration, increase in contact resistance with a degradation of the quality of the ohmic contact, etc.). The power handling of these devices is therefore limited as soon as the operating frequency exceeds a few GHz.

[0006] Concerning capacitive MEMS-RF switches, the metallic movable membrane contacts the metallic RF signal line via a dielectric material that will ensure electrical insulation. This is a capacitive contact whose properties define the RF performance of the component and which is linked to the ratio of the capacitance when the membrane is in the low state to the capacitance when the membrane is in the high state. The higher the ratio of these capacitances, the greater the distinction between the two states will be, thus facilitating the design of these devices for better RF performance as described by Hao Wei et al, “High on / off capacitance ratio RF MEMS capacitive switches”, Journal of Micromechanics and Microengineering, 2017.

[0007] Sudhanshu Shekhar (Sudhanshu Shekhar et al., “Thermally robust thin-metal membrane capacitive RF MEMS switch”, Journal of Micro and Smart Systems, 2019) presents an example of a capacitive RF MEMS switch with silicon nitride as dielectric material and whose schematic principle is shown in Figure 1 a (ohmic switch configuration) and Figure 1 b (capacitive switch configuration). The latter type of device is generally more suitable for higher frequency bands (> a few GHz) than ohmic RF MEMS. However, the main advantage of these devices, compared to ohmic RF MEMS, is their better resistance to RF power at higher frequencies thanks to the presence of the dielectric material which will limit the current flow.

[0008] The author LI-YA MA (LI-YA MA et al., “Comprehensive Study on RF-MEMS Switches Used for 5G Scenario”, IEEE Access, 2019) presents a detailed state of the art of MEMS-RF switches whether in “ohmic” configuration or in “capacitive” configuration. This study shows a great diversity of switches from different technologies and architectures for various performances (frequency band, S parameters, switching time, reliability, power handling

[0009] In this context, the applicant proposes a new original architecture capable of handling high RF powers while guaranteeing the reliability of the component.

[0010] The Applicant has already developed a capacitive MEMS-RF switch capable of handling powers greater than 20W and notably described by A. Ziaei et al., “Fast high power capacitive RF-MEMS switch for X-Band applications”, European Solid State Device Research Conference, 2015 (figure 2 of this article).

[0011] The architecture of this component described in this article has limitations, in terms of power handling, defined mainly by the rigidity of the membrane (level of self-activation under the effect of the incident RF power) and by the rigidity of the dielectric material which must withstand the incident power as well as the voltage required to switch the membrane to the low state. The higher the power levels required, the more rigid the metal membrane must be and the higher the voltage required to switch the membrane.

[0012] Figure 2 illustrates a scanning electron microscope (SEM) image of a capacitive MEMS-RF switch, in coplanar architecture, produced by the Applicant for power RF applications.

[0013] Figures 3a and 3b illustrate this type of “parallel” capacitive MEMS-RF switch configuration in coplanar (CPW) technology and relate respectively to a 3D view and a sectional view highlighting the ground planes, the RF line, the air gap between the membrane and the RF line covered with dielectric. More precisely, Figure 3a shows on a substrate 10, the ground planes 13a and 13b, on which a membrane 14 rests, overmetallizations 15a and 15b rest on the RF line. Figure 3b illustrates the RF line 11, covered with a dielectric 12.

[0014] Table 1 below shows a state of the art listing the characteristics of the power MEMS-RF switches developed by the Applicant, the power MEMS-RF switches from academic research and those from component suppliers. These components have power handling capacities less than or equal to 25W for a number of switching operations not exceeding 3.10 9cycles. It should be noted that the number of switching cycles decreases with increasing power level. For capacitive RF MEMS switches, this number of switching cycles decreases drastically with the voltage applied, and seen by the dielectric material, to switch the membrane to the low state. C. Goldsmith (C. Goldsmith et al., “Lifetime characterization of capacitive RF MEMS switches”, IEEE MTT-S International Microwave Symposium, 2001) was able to demonstrate that the number of switching cycles on capacitive RF MEMS decreases by about a decade each time the actuation voltage is increased by 5-7 V: Table 1 https: / / www.analog.com and University of California (Hyun-Ho Yang et al., “A High Power Stress-Gradient Resilient RF MEMS Capacitive Switch”, Journal of Microelectromechanical Systems, 2014).

[0016] Several authors present examples of capacitive MEMS-RF switch architecture with remote activation electrodes (and covered with a dielectric), between the central RF line and the remote ground planes (Hao Wei et al, “High on / off capacitance ratio RF MEMS capacitive switches”, Journal of Micromechanics and Microengineering, 2017, K. Srinivasa Rao et al., “Fabrication and Characterization of Capacitive RF MEMS Perforated Switch”, IEEE Access, 2018). Figure 4a (schematic principle) and Figure 4b (optical microscope image of the component) present for example the architecture proposed by Hao Wei. The purpose of this architecture configuration is to release the constraints on the dielectric of the RF line. To do this, a voltage is applied between the activation electrodes and the membrane connected to the ground planes. The membrane then lowers onto a metallization previously deposited on the dielectric material of the RF transmission line.Thanks to this architecture, the dielectric of the central line is no longer subjected to an activation DC voltage and a simple contact of the membrane on the metal above this same dielectric makes it possible to recover a very good capacitive contact.

[0017] However, this approach has some limitations. First of all, in coplanar waveguide (CPW) configuration, for adaptation reasons, the width of the RF line and the distance between this same line and the ground planes is a function of the substrate properties (thickness, tan 5, £ r) on which the component is made. This means that for a given substrate, these dimensions are fixed in order to be adapted to 50Q and therefore the space (between the center line and the ground planes) to make activation electrodes is restricted. This becomes more problematic when trying to address high RF power levels, while maintaining moderate switching voltages. For high power levels, it is necessary to increase the air gap and / or the membrane stiffness in order to push back the level of self-activation of the component under the effect of the electromagnetic field. In the case of the architecture presented by Hao Wei, it would be necessary to increase the size of the electrodes, significantly, so that the electrostatic force applied to the membrane is greater, while maintaining moderate activation voltages.To this must be added the influence of the activation lines that pass between the RF line and the ground planes (figure 4b). These metallic lines cause signal losses (by electromagnetic coupling). with a degradation of insertion losses (S21). The results of S parameters, on this architecture, show insulations in agreement with the simulations while a difference is observed on the insertion losses between the measurement and the simulations. In addition, reliability issues, linked to the injection of charges into the dielectric, are reported at the level of the activation electrodes without improving the reliability of the component.

[0018] The use of remote electrodes for capacitive RF MEMS is a proven methodology for reducing stress on the dielectric material. However, for power handling purposes, this solution requires having enough space between the RF line and the ground planes to insert electrodes large enough to apply an electrostatic force that can compensate for the membrane stiffness. As a reminder, this stiffness, along with the air gap, defines the power handling of the component. The higher the power levels, the larger the electrodes must be to operate the device with reasonable voltages.

[0019] In this context, the Applicant proposes a solution comprising the use of a membrane, composed of a rigid part and a flexible part, allowing the activation electrodes to be moved to the ground planes, thus providing flexibility on the size of the activation electrodes. In addition, they are no longer located between the RF line and the ground plane, thus avoiding electromagnetic coupling which can cause RF losses. This approach makes it possible to keep components compact, even for high power levels, while limiting the impact on the overall architecture. The flexible part as well as the size of the activation electrodes with the part of the rigid membrane opposite can be adjusted to the required power level while ensuring moderate actuation voltages.

[0020] More specifically, the present invention relates to a capacitive radiofrequency electromechanical microsystem comprising a metal membrane suspended above an RF transmission line arranged along a first axis X on the surface of a substrate, activation electrodes and ground planes on which said membrane rests, characterized in that: - said metal membrane comprises: o a lower element having a first portion located partially above said electrodes covered by elements comprising at least a first dielectric material and partially above said RF line covered by an element made of a second material, said lower element having second portions located partially in contact with said ground planes; o an upper element located above said first portion of the lower element; - the activation electrodes being arranged on either side of the RF transmission line; - said ground planes being arranged on either side of the RF line and along axes parallel to said first axis X; - said membrane comprising flexible zones constituted by a part of the second parts of the lower element and a rigid zone constituted by the stacking of the first part of the lower element and the upper element.

[0021] According to variants of the invention, said elements comprising at least one first dielectric material are structured and have structuring patterns, with dimensions between 50 nanometers and 5 micrometers.

[0022] It should be noted that existing solutions on remote electrodes only postpone the failure of capacitive RF-MEMS from the dielectric of the central line to the dielectric on the activation electrodes. The introduction of micro / nanostructuring of the dielectric on the activation electrodes makes it possible to limit the injection of charge into this material by drastically minimizing the surface in contact between this dielectric and the membrane when the latter is in the low state. The limitation of actuation voltages, thanks to this type of architecture, and the structuring of the dielectric of the activation electrodes thus makes it possible to improve the reliability of this type of component even for high power levels (>30W).

[0023] According to variants of the invention, the first dielectric material has a rigidity greater than or equal to 5MV / cm, and is for example made of SiO2 or SiN or AI2O3.

[0024] According to variants of the invention, the element made of second dielectric material is covered with an upper metallic element.

[0025] According to variants of the invention, the total thickness of an activation electrode and of an element made of first dielectric material is equal to the total thickness of the thickness of the part of said RF line under the membrane and of an element made of second dielectric material.

[0026] According to variants of the invention, the thickness of the lower element of the membrane is between 500 nanometers and 2 micrometers.

[0027] According to variants of the invention, the thickness of the upper element of said membrane is greater than 2 micrometers.

[0028] According to variants of the invention, the upper element of said membrane is structured, said structure being for example in lattice form and having patterns of dimensions greater than 1 micrometer.

[0029] According to variants of the invention, the second dielectric material has a dielectric constant EQ greater than that of silicon oxide, said second dielectric material being for example HfO2 or ZrO2 or Y2O3 or AI2O3 or SiN.

[0030] According to variants of the invention, the lower element of the membrane is made of multilayers based on refractory material(s) for example W, Mo, Ta, TaN, TiN, TiW and conductive material(s) for example Au, Al, Cu, the upper element of the membrane being made of conductive material for example Au or Cu.

[0031] The invention also relates to a method for manufacturing a capacitive radiofrequency electromechanical microsystem according to the invention, comprising: - a step of producing activation electrodes on the surface of a substrate; - a step of producing elements in at least one first dielectric material on the surface of a portion of said activation electrodes; - a step of producing the RF line located between said activation electrodes; - a step of producing an element in second dielectric material above a part of the RF line; - a step of producing said ground planes on either side of said RF line; - a step of producing the lower element of said membrane; - a step of producing the upper element of said membrane.

[0032] According to method variants of the invention, the step of producing the activation electrodes on the surface of a substrate is carried out by cathode sputtering or by evaporation.

[0033] According to method variants of the invention, the lower element of the membrane is produced by multilayer depositions comprising at least one metallic layer of Au or Al or Cu and at least one other metallic layer, for example of refractory material, carried out by cathodic sputtering and / or evaporation.

[0034] According to method variants of the invention, the upper element of said membrane is produced by electrolytic deposition.

[0035] According to method variants of the invention, the element made of second dielectric material on the surface of said RF line is produced by physical vapor deposition (PVD) or by chemical vapor deposition (CVD) or by deposition of atomic thin layers (ALD) with thicknesses for example between 20 nanometers and 300 nanometers.

[0036] According to method variants of the invention, said method comprises carrying out a step of partial thickening of said RF line in regions located at the periphery of said membrane.

[0037] According to method variants of the invention, the step of partial thickening of said RF line and the step of producing said ground planes are carried out during the same metal deposition step.

[0038] According to method variants of the invention, the production of the ground planes or the production of the ground planes and the step of partial thickening of said RF line are carried out by electrolytic growth.

[0039] According to method variants of the invention, the structuring patterns of said elements made of at least a first dielectric material and present above said electrodes are produced by etching a first material dielectric or by localized deposition of a dielectric material on a first dielectric material.

[0040] The invention will be better understood and other advantages will appear on reading the description which follows, given without limitation and thanks to the appended figures among which:

[0041] [Fig 1 a] illustrates the principle of an ohmic MEMS-RF switch;

[0042] [Fig 1 b] illustrates the principle of a capacitive MEMS-RF switch;

[0043] [Fig 2] illustrates a scanning electron microscope (SEM) image of a capacitive MEMS-F switch, in coplanar architecture produced by the Applicant for power RF applications;

[0044] [Fig 3a] illustrates a 3D view of a capacitive MEMS-RF switch in “parallel” configuration according to the known art;

[0045] [FIG.3b] illustrates a sectional view with a metal membrane (connected to the ground plane) above (separated by an air gap) an RF transmission line covered by a dielectric according to the known art;

[0046] [Fig 4a] illustrates the schematic principle of a capacitive RF MEMS-RF switch architecture in “CPW” architecture with remote activation electrodes according to the known art;

[0047] [Fig 4b] illustrates a microscope image of the component shown in Figure 4a;

[0048] [Fig 5a] illustrates a 3D view of a component according to the invention;

[0049] [Fig 5b] illustrates a top view of a component of the invention;

[0050] [Fig 6] illustrates the evolution of stiffness as a function of dielectric constant for different materials;

[0051] [Fig 7a] illustrates the accumulation of charges in a planar dielectric material when the membrane is in the low state in a component of the invention;

[0052] [Fig 7b] illustrates the accumulation of charges in a structured dielectric material when the membrane is in the low state in a component of the invention;

[0053] [Fig 8a] illustrates a configuration according to the invention, without metal on the dielectric covering the RF line, with application of electrostatic fields E ch on the activation electrodes and E' C h on the RF line, in the high state, in a component of the invention;

[0054] [Fig 8b] illustrates a configuration according to the invention, without metal on the dielectric covering the RF line, with application of electrostatic fields E ch on the activation electrodes and E' C h on the RF line, in the low state, in a component of the invention;

[0055] [Fig 8c] illustrates a configuration according to the invention, with metal on the dielectric covering the RF line, with application of electrostatic fields E ch on the activation electrodes and E' C h on the RF line, in the high state, in a component of the invention;

[0056] [Fig 8d] illustrates a configuration according to the invention, with metal on the dielectric covering the RF line, with application of electrostatic fields E ch on the activation electrodes and E' C h on the RF line, in the low state, in a component of the invention;

[0057] [Fig 9] illustrates an example of a growth of electrolytic gold of a few micrometers, in a lattice, to reinforce the rigidity of the membrane in a configuration of the invention;

[0058] [Fig 10a] illustrates the flexible areas of the membrane of a component of the invention;

[0059] [Fig 10b] illustrates the rigid zone of the membrane of a component of the invention;

[0060] [Fig 11] illustrates the geometric parameters of the MEMS-RF component architecture according to the invention;

[0061] [Fig 12a] illustrates an optical microscope image of a component according to the invention;

[0062] [Fig 12b] illustrates a 3D view using an optical profilometer of a component according to the invention;

[0063] [Fig 13a] illustrates an optical profilometer acquisition of a first MEMS-RF switch fabrication according to the invention with a first flexible zone length;

[0064] [Fig 13b] illustrates an optical profilometer acquisition of a first MEMS-RF switch fabrication according to the invention with a second flexible zone length;

[0065] [Fig 13c] illustrates an optical profilometer acquisition of a first MEMS-RF switch fabrication according to the invention with a third flexible zone length;

[0066] [Fig 14a] illustrates an acquisition with an optical profilometer in the high state of a MEMS-RF switch according to the invention;

[0067] [Fig 14b] illustrates an acquisition with an optical profilometer in the low state of a MEMS-RF switch according to the invention;

[0068] [Fig 15] illustrates the S parameters of a component according to the invention and the comparison with those of an architecture of the known art;

[0069] [Fig 16] illustrates a top view of a component according to the invention and of the cutting axes A and B;

[0070] [Fig 17a] illustrates a view along section A in the high position of the component illustrated in figure 16;

[0071] [Fig 17b] illustrates a view along section A in the low position of the component illustrated in figure 16;

[0072] [Fig 18a] illustrates a view along section B in the high position of the component illustrated in figure 16;

[0073] [Fig 18b] illustrates a view along section B in the low position of the component illustrated in figure 16;

[0074] [Fig 19a] illustrates a first step in the process of producing an example of a component according to the invention;

[0075] [Fig 19b] illustrates a second step of the method of producing an example of a component according to the invention;

[0076] [Fig 19c] illustrates a third step of the method of producing an example of a component according to the invention;

[0077] [Fig 19d] illustrates a fourth step of the method of producing an example of a component according to the invention;

[0078] [Fig 19e] illustrates a fifth step of the method of producing an example of a component according to the invention;

[0079] [Fig 19f] illustrates a sixth step in the method of producing an example of a component according to the invention;

[0080] [Fig 19g] illustrates a seventh step in the method of producing an example of a component according to the invention.

[0081] [Fig 19h] illustrates an eighth step of the method for producing an example of a component according to the invention.

[0082] In general, the new capacitive MEMS-RF switch architecture according to the invention makes it possible to address the needs for high power and increased reliability. To this end, this new architecture offers the following characteristics, including the advantages described below in more detail: - a dissociation of the RF part and the DC part (for activation) to minimize stress on the RF line dielectric. The activation electrodes are remote to ensure their sizing, thus making it possible to target very high RF powers (» 30W). These activation electrodes are located at the ground planes, providing flexibility on their sizing to target high power levels while guaranteeing moderate actuation voltages; - a first dielectric material, with high dielectric strength and deposited on the activation electrodes, can advantageously, according to certain variants of the invention, be micro / nanostructured in order to improve the reliability of the component by limiting the injection of charges during switching thanks to a small surface area in contact with the membrane; - a second dielectric material deposited on the RF transmission line gives the component its RF properties (power handling and S parameters) when the membrane is in the low state. By the proposed architecture, the constraints on this material are minimized, making it possible to reduce its thickness and / or to use a material with a high dielectric constant whose properties (depending on the thickness) are not usually compatible with capacitive RF MEMS because these materials generally do not withstand the voltages applied for actuation; - a membrane composed of a flexible part to allow its movement and a rigid part sufficiently thick to maintain structural rigidity makes it possible to limit its deformation (keep the membrane straight) under different stresses (RF power and actuation) and can withstand high RF powers.

[0083] In general, according to the present invention it is proposed to move the activation electrodes to the level of the ground planes. The latter are suspended, locally, above the activation electrodes, these two elements being separated by an air gap (e g). Figure 5a illustrates a 3D view and Figure 5b illustrates a top view of an example of a component according to the present invention. Activation electrodes 200a and 200b are thus produced on a substrate 100, extended by connection pads 600a and 600b, covered with elements 300a and 300b made of first dielectric material D1 (elements 300a / D1 and 300b / D1 shown in Figure 5b). Ground planes 400a-i, 400bi, 400a2 and 400b2 are distributed on either side of the membrane having a lower element 501 which rests on said ground planes, this lower element 501 being locally covered by an upper element 502. The RF line 110 is locally covered by an element 111 made of second dielectric material D2. According to a variant illustrated in figure 5b, the element 111 can itself be covered with a metallization 120 in metal M6 (this constituting a variant of the component illustrated in figure 5a).The RF line is also thickened at the periphery by 112i and 1122- metallizations.

[0084] The ground planes 400a-i, 400bi, 400a2 and 400b2 and the metallizations 112i and 1122 can advantageously be made of the same metal referenced M3. The lower element 501 of the membrane is made of a metal referenced M4, the upper element 502 of the membrane being made of a metal referenced M5.

[0085] The movable metal membrane is suspended and separated by an air gap (e g) of the two metal activation electrodes M2 and the thin metal RF line 100 made of metal M1 (not shown because it is covered by the element 111 in dielectric D2). The first dielectric material referenced D1 (located above the electrodes) is chosen for its good voltage resistance (high dielectric strength and thickness). For example, first dielectric materials such as silicon nitride (SiN), silicon oxide (SiO2), aluminum oxide (AI2O3), polymers, etc., deposited by different techniques (LPCVD, PECVD, PVD, CVD, ALD, etc.), can have dielectric strengths greater than 6 MV / cm, as illustrated in Figure 6 which shows the evolution of the strengths as a function of the dielectric constant according to the reference: Pushkar Jain et al., “Embedded Thin Film Capacitors - Theoretical Limits”, IEEE Transactions on Advanced Packaging, 2002.

[0086] It should be noted that for the choice of this first dielectric material D1, the value of the dielectric constant is a parameter that is not taken into consideration for the RF performance of the component. In order to withstand high voltages, the thickness of this material is calculated according to its dielectric strength. For example, AI2O3 deposited by plasma-assisted ALD can have a dielectric strength of 10 MV / cm. To withstand a voltage of 200V, the necessary thickness of AI2O3 is of the order of 200nm. Generally speaking, the minimum thickness of this dielectric must be sufficient to withstand the voltage imposed to switch the membrane to the low state. By applying a sufficient voltage between the activation electrodes and the ground plane / membrane, the membrane will switch from a high state to a low state.

[0087] Failures due to the charging of the dielectric during the different activations will affect the first dielectric material D1. Depending on the material used and the actuation mode (applied voltage and actuation frequency), the first dielectric material D1 charges more or less, which can cause a failure of the component either by shifting the actuation voltage or by sticking of the membrane to the dielectric, as described in the following references: David Mardivirin et al., “Evidence of Successive Fowler-Nordheim and Frenkel-Poole Conductions in Si3N4 Based RF-MEMS Capacitive Switches”, Proceedings of the 40th European Microwave Conference, 2010, Negar Tavassolian et al., “Dielectric Charging in Capacitive RF MEMS Switches: The Effect of Extended Durations of Electric Stress”, IEEE Microwave and Wireless Components Letters, 2011, S. Mellé et al., “Modeling of the dielectric charging kinetic for capacitive RF- MEMS”, IEEE MTT-S International Microwave Symposium Digest, 2005, Anne- Charlotte Amiaud et al., “Modeling tool for capacitive RF MEMS operating bias voltage optimization”, IEEE Transactions on Devices and Materials Reliability, 2019, W. A. de Groot et al., “Review of device and reliability physics of dielectrics in electrostatically driven MEMS devices,” IEEE Transactions on Devices and Materials. Reliability, 2009, W. M. van Spengen, “Capacitive RF MEMS switch dielectric. charging and reliability: A critical review with recommendations,” Journal of Micromechanics and Microengineering, 2012.

[0088] The first dielectric material D1, at the activation electrodes, acts as an insulator when the membrane is in the low state. In the context of the present invention, the dielectric constant for this first dielectric material D1 is only a parameter to be considered for the RF performance of the device. The material D1 is essential to support the actuation voltages necessary to switch the device (depending on the defined power handling), it is therefore subject to the same failure rules as those cited previously. These failures are all the more pronounced as the material D1 is stressed (applied voltage, time in the low state, actuation frequency, etc.), as described in: RW Herfst et al., “Time and voltage dependence of dielectric charging in RF MEMS capacitive switches”, IEEE International Reliability Physics Symposium Proceedings, 2007.

[0089] When a potential difference is applied between the membrane (0 V) and the activation electrodes (+V), an electrostatic force is generated which attracts the (mobile) membrane to the material D1. During this contact, there is a transfer of charges (electrons e) from the membrane into the material D1. These charges then accumulate at the defects of the material D1 to be trapped there, as the switching occurs. This injection of charges into the dielectric material D1 depends on several factors such as the applied electrostatic field, the contact surface, the temperature, etc. This accumulation of charges in the dielectric material D1 generates a "parasitic" electric field opposite to the electric field resulting from the actuation and inducing a drift in the actuation voltage (RW Herfst et al., “Characterization of dielectric charging in RF MEMS capacitive switches”, IEEE International Conference on Microelectronic Test Structures, 2006), see a permanent bonding of the membrane on the D1 dielectric (https: / / menlomicro.com and https: / / www.analog.com).

[0090] Depending on the dielectric material D1 used (T. Lisec et al., “Dielectric Material Impact on Capacitive RF MEMS Reliability”, 34th European Microwave Conference, 2004), and the electrical stress applied to it, the accumulation of charges is localized in the dielectric material over a certain depth (A. Ziaei et al., “Fast high power capacitive RF-MEMS switch for X-Band applications”, European Solid State Device Research Conference, 2015).

[0091] In order to limit the injection of charges into the dielectric material D1, it may be particularly interesting to drastically reduce the surface area that will be in contact with the membrane (in the low state) by structuring the material D1. This involves generating structures such as pillars, holes, lattices, etc., from a few micrometers to a few hundred nanometers in the dielectric material D1. For the same electrical stress, the quantity of charge Q' accumulated at the level of the micro / nano structures (by electron transfer e from the membrane) is lower than the quantity of charge Q for a flat surface (as shown in Figures 7a and 7b relating respectively to configurations with charge accumulation in a flat dielectric material and in a structured dielectric material, when the membrane is in the low state).

[0092] By structuring the dielectric material D1 in this way, the contact surface is limited when the membrane is in the low state and therefore the reliability of the component linked to charge injection is improved. This reliability will be all the higher as the density of the structures is low.

[0093] Current technologies allow micro / nanostructuring of insulating materials such as SiN, SiO2, AI2O3 ... over several hundred nanometers in depth and for resolutions lower than 100 nm. It should be noted that it is possible to carry out a localized deposition of an insulating material to generate micro / nano structures with the same dimensional specifications on the activation electrodes which can be, beforehand, already passivated with another insulator.

[0094] The choice of the second dielectric material D2 defines the RF performance (S parameters and power handling) of the component when the membrane of the MEMS-RF switch is in the low state. For this, its dielectric constant is taken into consideration as well as the thickness necessary to withstand the voltage corresponding to the power injected when the membrane is in the low state according to the formula U = (RP) where U is the voltage in volts, R the characteristic impedance of the component (50 ohms for an RF circuit and for the calculation in this table) and P the power in Watts injected into the component. Table 2 below shows, for different power levels, the equivalent voltage likely to be applied to the second dielectric material D2 when the membrane is in the low state. Table 2

[0095] The dielectric material D2 is only subjected to the stress related to the admitted RF power and, possibly, to a voltage of a few volts (applied between the RF line and the ground plane / membrane) to optimize the plating of the membrane on the dielectric material D2. Equation 1 of the electrostatic force, below,

[0096] Equation 1:

[0097] with "S" : the surface facing the membrane and the RF line below, E0: the permittivity of the vacuum, V: the voltage applied to the terminals of the electrodes and z: the position of the membrane along the z axis (between 0 and e g );

[0098] The formula shows that the force F applied to the membrane is proportional to 1 / z 2, where z represents the distance of the membrane from the RF line. This implies that this electrostatic force becomes very significant when the membrane approaches the RF line covered with the dielectric material D2, even for a low applied voltage. According to a variant of the invention, a metal can be deposited on the dielectric material D2 so that a simple contact of the membrane on this metal makes it possible to obtain the desired capacitance, in the low state, and without the membrane needing to be correctly plated (the application of a secondary voltage on the RF line is no longer required). Figures 8a and 8b represent schematic views of the actuation mode of the capacitive MEMS-RF switch without the presence of an additional metal on the dielectric material D2, respectively in the “high state” and in the “low state” and with the application of electrostatic fields E C h on the activation electrodes and E' Ch on the RF line. Figures 8c and 8d show schematic views of the actuation mode of the capacitive MEMS-RF switch with the presence of an additional metal on the dielectric material D2, respectively at 'High state' and at 'Low state' and this with application of an electrostatic field E ch on the activation electrodes. In this case, the application of an electric field on the RF line is no longer required.

[0099] The RF performance of the device, in the low state, is therefore linked to the capacitance generated at the level of the dielectric material D2 and whose value depends on the thickness of the dielectric material D2, its dielectric constant (at the frequency considered), the contact surface B*A and the quality of the contact of the membrane on the dielectric material D2 (which can be ideal by adding a metal M6 on the dielectric material D2). Figure 9 illustrates an example of an electrolytic gold growth (equivalent to the metal M5) of a few micrometers, in lattice, to reinforce the rigidity of the membrane, this electrolytic deposition having been carried out on a metallization (type M4). [000100] In summary, the change of state of the membrane (from a high state to a low state) is ensured by the remote electrodes and it is the dielectric material D1 which supports the voltage necessary to switch the membrane. The dielectric material D2 ensures the performance of the component in the low state and the power handling. The DC part (to actuate the component) is thus separated from the RF part. [000101] For topographical reasons, the thickness of the dielectric material D1 can advantageously be chosen as a function of the height of the dielectric material D2. The thickness of the activation electrode with the dielectric material D1 can be equal to the thickness of the RF line under the membrane with the dielectric material D2. [000102] Concerning the mechanical (mobile) part of the capacitive MEMS-RF switch, the self-supporting membrane is divided into two zones, supported by several anchoring points (at least some of which are located at the ground planes for the case of parallel MEMS-RF). The so-called "flexible" zones, to allow the membrane to switch from the high state to the low state (figures 8a and 8b), are located at the anchoring points and consist of the M4 metallization. The so-called "rigid" zone consists of the M4 metallization on which a thick M5 metallization (greater than 2 micrometers) is grown by electrodeposition. [000103] This M5 metallization can be structured (e.g., in a lattice) in order to keep the structure robust and light. The image of an example of electrolytic gold growth (equivalent to the M5 metal) of a few micrometers, in a lattice, to reinforce the rigidity of the membrane (electrolytic deposition carried out on an M4 metallization) is illustrated in figure 9. [000104] The aim is for this area of ​​the membrane to remain flat / rigid, as illustrated in Figure 10b showing the so-called “rigid” area ZR, Figure 10a highlighting the so-called “flexible” areas ZF. The rigid area of ​​the membrane does not deform under the effect of the incident power. Deformation of the membrane is thus only permitted at the anchoring points (flexible areas). [000105] The mechanical properties of the capacitive MEMS-RF switch are therefore provided by the flexible zones of the membrane which are found at the ground planes. In this configuration, we have all the flexibility necessary to vary the dimensions of the flexible zones in order to adjust the power handling of the component. For example, by reducing the length (F) of the flexible zones (illustrated in Figure 11), we make them more rigid to bending and it will therefore be necessary to have a higher voltage (DC and RF) to switch the membrane to the low state. While making the self-supporting structure more rigid to bending at the anchor points, we can reduce the DC voltage necessary to actuate the component by increasing the dimensions (C and E) of the activation electrodes and the dimensions (C' and E') of the parts of the rigid membrane facing each other. By this methodology, several advantages are obtained: - there are no space constraints for defining the activation electrodes because they are offset from the central transmission line; - high power levels can therefore be addressed. The electrostatic force, for a given voltage, is proportional to the surface area of ​​the electrodes (Equation 1). The power handling of the component is defined by the level of flexibility of the flexible zones and the height of the air gap (e g ) ; - the control lines can then be outside the device (contrary to the work of Hao Wei: Hao Wei et al, “High on / off capacitance ratio RF MEMS capacitive switches”, Journal of Micromechanics and Microengineering, 2017), thus limiting their impact on the RF performance of the device when the membrane is in the high state; - the RF architecture is not impacted and the RF performance is the same as for a more conventional architecture. Indeed, the modifications generated are located at the level of the ground planes and will therefore not affect the transmission of the RF signal. [000106] Figures 12a and 12b show an example of the architecture of the present invention produced on the basis of the technological steps of microelectronics. Figure 12a illustrates an optical microscope image of a component according to the invention and Figure 12b illustrates a 3D view with an optical profilometer of a component according to the invention. This component was produced on a high resistivity silicon substrate with 2 pm of thermal silicon oxide. For this production, the dielectric material D1 is silicon nitride (500 nm) deposited by PECVD and the dielectric D2 is metal oxide deposited by evaporation. The flexible part of the membrane is made of a multilayer based on refractory material (by cathode sputtering) and Au (by evaporation). The rigid part of this membrane is made of the same multilayer with an electrolytic Au overthickness of a few micrometers. [000107] Figures 13a, 13b and 13c show optical profilometer acquisitions of 3 capacitive MEMS-RF switches based on the principle of the present invention. The three structures shown are from the same fabrication on a high resistivity silicon substrate and with 2 pm of thermal oxide. A variation on the length of the flexible zones was made in order to modify the voltage necessary for actuation from one component to another. [000108] Figures 14a and 14b show optical profilometer acquisitions of the switching of an architecture with the shortest flexible zones, Figure 14a relates to a state in which the membrane is in the high state and Figure 14b relates to a state in which the membrane is in the low state. [000109] In order to validate this concept, these new architectures were characterized on an RF bench and their S parameters were compared to those of a standard architecture from the same manufacturer. Figure 15 presents these results and shows equivalent S parameters for the two types of architectures. Insertion losses are of the order of 0.1 dB at 10 GHz for an isolation of the order of 15 dB for the same frequency. [000110] Figure 16 illustrates a top view of an example of architecture according to the invention showing 2 cutting axes A and B. [000111] Figures 17a and 17b relate to section A and relate to the high state and the low state respectively. [000112] Figures 18a and 18b relate to section B and relate to the high state and the low state respectively. [000113] Generally speaking, the offsetting of the activation electrodes to the ground planes makes it possible to manage high power levels while maintaining moderate actuation voltages and keeping the component compact. The dimensions of the activation electrodes, as well as the dimensions of flexible parts, can be adjusted according to the need in terms of power level and constraint on the actuation voltage. The micro / nanostructuring of the dielectric material on the activation electrodes makes it possible to limit the injection of charges into this dielectric and therefore to improve the reliability of the component for RF power / actuation voltage compared to an unstructured dielectric.The DC stress is applied on the dielectric material D1 and the dielectric material D2 is only subjected to RF stress and therefore the dielectric material D2 can be chosen (thickness and dielectric properties) according to the expected specifications (power handling and S parameters). [000114] This new architecture is composed of activation electrodes about a hundred nanometers thick made of metallization carried out by a thin layer deposition (cathode sputtering, evaporation, etc.). These activation electrodes are located at the level of the component's ground planes. [000115] The dielectric material D1 above the activation electrodes is chosen for its good voltage resistance, either with a suitable thickness and / or high dielectric strength. This material can be, for example, SiO2, SiN, AI2O3, etc., deposited by PECVD, by ALD, by cathode sputtering, etc., for a thickness of around a hundred nanometers, capable of withstanding voltages of several hundred volts. This material can be micro / nanostructured either by etching or by making a localized deposit of another dielectric (possibly the dielectric D2) on top. This structuring thus makes it possible to limit the contact surface when the membrane is in the low state. [000116] The mobile membrane is composed of a flexible part to allow its movement. This flexible part can be based on a metallic multilayer deposited by cathodic sputtering and / or evaporation. The membrane is also composed of a rigid part which can be produced using a thick electrolytic deposit (of around ten micrometers) which can be gold, copper, etc., for example. This growth is carried out on the multilayer used for the flexible part in the areas which are to be kept flat under the effect of the injected power. The deformation of the membrane is therefore only permitted at the flexible areas defined by the multilayer. This electrolytic growth can be structured in the form of a lattice in order to minimize the weight of the membrane while ensuring mechanical rigidity. [000117] The dielectric material D2 is located on the RF line and below the membrane. It can be deposited by different techniques (evaporation, CVD, ALD, etc.) for thicknesses ranging from a few tens of nanometers to a few hundred nanometers. The choice of the material and its thickness will be made according to its properties and the targeted performances. A thin metallic layer of a few tens of nanometers and floating can be deposited on the dielectric material in order to optimize the capacitance when the membrane is in the low state. A simple contact of the membrane on this metallization makes it possible to obtain the maximum capacitance while freeing itself from the contact quality of the membrane on the RF line (which can be induced by the roughness or the plating rate). [000118] The RF line, passing under the membrane, is a metallization a few hundred nanometers thick deposited by a thin layer technique (evaporation, cathodic sputtering, etc.) and which can be gold. Outside the area under the membrane, this RF line can be thickened with an electrolytic deposit which also serves to define the ground planes as well as the anchors of the flexible parts of the membrane. [000119] The air gap between the activation electrodes and the membrane and between the RF line and the membrane can be from a few hundred nanometers to a few micrometers in height depending on the desired power handling and the desired switching time. The thickness of the activation electrodes with its dielectric can be adjusted according to the thickness of the RF line with its dielectric so that the air gap is similar between the two contact areas. [000120] Example of a method for producing an architecture according to the invention: [0001211 Step 1: A metallization (metal M2) is deposited to define the activation electrodes 200a and 200b extended by connection pads 600a and 600b, on a substrate 100, as illustrated in FIG. 19a. [0001221 Step 2: A dielectric material (D1) is deposited on the activation electrodes for the DC performance of the component as illustrated in Figure 19b which shows the dielectric elements 300a and 300b above the electrodes 200a and 200b. [0001231 Step 3: A micro / nanostructuring operation of the dielectric is carried out above the activation electrodes so as to define the micro / nanostructured elements 300a' and 300b' above the elements 200a and 200b, as illustrated in figure 19c. [000124] Step 4: A metallization (metal M1) is deposited in order to define the RF 110 line intended to pass under the membrane as illustrated in figure 19d. [000125] Step 5: Dielectric D2 is deposited on the RF line for the RF performance of the component, defining a dielectric element 111 above the line 110, as illustrated in Figure 19e. [0001261 ​​Step 6: A first electrolytic growth operation (metal M3) is carried out to thicken the RF line with elements 112-i and 1122 outside the area where the membrane must lower and to also define ground planes 400a-i, 400bi, 400a2 and 400b2, as illustrated in figure 19f. [000127] Step 7: The lower element 501 of the membrane is produced. To do this, the deposition and structuring operation of the metallization (metal M4) are carried out, defining the geometry of the membrane and the flexible parts of the membrane 501 as illustrated in figure 19g. [000128] Step 8: [000129] The upper element 502 of the membrane is produced. To do this, a localized electrolytic deposition operation (metal M5) is finally carried out over the metallization M4 in order to define the rigid zones of the membrane as illustrated in figure 19h. [000130] The capacitive shunt MEMS-RF switch architecture described above can be adapted to an ohmic contact, a metallic contact between the membrane and the RF line, without changing anything in the mechanical nature of the membrane or in the way of activating it. The architecture differs in that there will be no dielectric above the RF line as it passes under the membrane and that the contact will be made directly between the RF line and the membrane when the latter is in the low state. [000131] Similarly, the capacitive shunt MEMS-RF switch architecture described above can be adapted to a micro-strip line instead of coplanar. The architecture differs in that the ground plane is not located on either side of the central signal line but on the rear face of the manufacturing substrate, and that metallized vias, holes in the substrate connecting the front face to the rear face and lined / filled with a conductive material thus making it possible to electrically connect the two faces, electrically connect the membrane to the ground plane on the rear face. [000132] Similarly, the capacitive shunt MEMS-RF switch architecture described above can be adapted to a series architecture of the MEMS-RF switch. The architecture differs in that the RF line is discontinuous and the membrane is not connected to the ground planes, it is suspended above the first part of the RF line and is connected to the second part of the RF line. When the membrane is in the low state, the two pieces of the RF line are connected, which allows the passage of the RF signal. [000133] The combination of these characteristics allows the realization of the following eight types of structures: MEMS-RF capacitive coplanar shunt, MEMS-RF capacitive coplanar series, MEMS-RF ohmic coplanar shunt, MEMS-RF ohmic coplanar series, MEMS-RF capacitive micro-strip shunt, MEMS-RF capacitive microstrip series, MEMS-RF ohmic micro-strip shunt, MEMS-RF ohmic micro-strip series.

Claims

CLAIMS 1. Capacitive radiofrequency electromechanical microsystem comprising a metal membrane suspended above an RF transmission line (110) arranged along a first axis X on the surface of a substrate (100), activation electrodes and ground planes on which said membrane rests, characterized in that: - said metal membrane comprises: o a lower element (501) having a first part located partially above said electrodes (200a, 200b) covered by elements (300a, 300b) comprising at least a first dielectric material (D1) and partially above said RF line covered by an element (111) made of a second dielectric material (D2), said lower element having second parts located partially in contact with said ground planes; o an upper element (502) located above said first part of the lower element; - the activation electrodes being arranged on either side of the RF transmission line; - said ground planes being arranged on either side of the RF line and along axes parallel to said first axis X; - said membrane comprising flexible zones (ZF) constituted by a part of the second parts of the lower element and a rigid zone (ZR) constituted by the stacking of the first part of the lower element and the upper element.

2. Capacitive radiofrequency electromechanical microsystem according to claim 1, characterized in that said elements comprising at least a first dielectric material (D1) are structured and have structuring patterns, with dimensions between 50 nanometers and 5 micrometers.

3. Capacitive radiofrequency electromechanical microsystem according to one of claims 1 or 2, characterized in that the first dielectric material (D1) has a rigidity greater than or equal to 5MV / cm, and is for example made of SiC>2 or SiN or AI2O3.

4. Capacitive radiofrequency electromechanical microsystem according to one of claims 1 to 3, characterized in that the element made of second dielectric material (D2) is covered with an upper metallic element (120).

5. Capacitive radiofrequency electromechanical microsystem according to one of claims 1 to 4, characterized in that the total thickness of an activation electrode and of an element made of first dielectric material is equal to the total thickness of the thickness of the part of said RF line under the membrane and of an element made of second dielectric material.

6. Capacitive radiofrequency electromechanical microsystem according to one of claims 1 to 5, characterized in that the thickness of the lower element of the membrane is between 500 nanometers and 2 micrometers.

7. Capacitive radiofrequency electromechanical microsystem according to one of claims 1 to 6, characterized in that the thickness of the upper element of said membrane is greater than 2 micrometers.

8. Capacitive radiofrequency electromechanical microsystem according to one of claims 1 to 7, characterized in that the upper element of said membrane is structured, said structure being for example in lattice form and having patterns of dimensions greater than one micrometer.

9. Capacitive radiofrequency electromechanical microsystem according to one of claims 1 to 8, characterized in that the second dielectric material has a dielectric constant EQ greater than that of silicon oxide, said second dielectric material being for example HfO2 or ZrO2 or Y2O3 or AI2O3 or SiN.

10. Capacitive radiofrequency electromechanical microsystem according to one of claims 1 to 9, characterized in that the lower element of the membrane is made of multilayers based on refractory material(s) for example W, Mo, Ta, TaN, TiN, TiW and conductive material(s) for example Au, Al, Cu, the upper element of the membrane being made of conductive material for example Au or Cu.

11. Method for manufacturing a capacitive radiofrequency electromechanical microsystem according to one of claims 1 to 10, comprising: - a step of producing activation electrodes on the surface of a substrate; - a step of producing elements in at least one first dielectric material (D1) on the surface of a portion of said activation electrodes; - a step of producing the RF line located between said activation electrodes; - a step of producing an element in second dielectric material (D2) above a part of the RF line; - a step of producing said ground planes on either side of said RF line; - a step of producing the lower element of said membrane; - a step of producing the upper element of said membrane.

12. Method for manufacturing a capacitive radiofrequency electromechanical microsystem according to the preceding claim 11, in which the step of producing the activation electrodes on the surface of a substrate is carried out by cathode sputtering or by evaporation.

13. Method for manufacturing a capacitive radiofrequency Electromechanical Microsystem according to one of claims 11 or 12, in which the lower element of the membrane is produced by multilayer depositions comprising at least one metallic layer of Au or Al or Cu and at least one other metallic layer, for example of refractory material, carried out by cathodic sputtering and / or evaporation.

14. Method of manufacturing a capacitive radiofrequency electromechanical microsystem according to one of claims 11 to 13, in which the upper element of said membrane is produced by electrolytic deposition.

15. Method for manufacturing a capacitive radiofrequency Electromechanical Microsystem) according to one of claims 11 to 14, in which the element made of second dielectric material on the surface of said RF line is produced by physical vapor deposition (PVD) or by chemical vapor deposition (CVD) or by deposition of atomic thin layers (ALD) with thicknesses for example between 20 nanometers and 300 nanometers.

16. Method for manufacturing a radiofrequency electromechanical microsystem according to one of claims 11 to 15, comprising carrying out a step partial thickening of said RF line in regions located on the periphery of said membrane.

17. Method for manufacturing a radiofrequency electromechanical microsystem according to claim 16, in which the step of partially thickening said RF line and the step of producing said ground planes are carried out during the same metal deposition step.

18. Method for manufacturing a radiofrequency electromechanical microsystem according to claim 17, in which the production of the ground planes or the production of the ground planes and the step of partial thickening of said RF line are carried out by electrolytic growth.

19. Method for manufacturing a radiofrequency electromechanical microsystem according to one of claims 11 to 18, in which the structuring patterns of said elements made of at least a first dielectric material (D1) and present above said electrodes are produced by etching a first dielectric material (D1) or by localized deposition of a dielectric material on a first dielectric material (D1).

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