Micro-electro-mechanical variable capacitor for radio frequency applications with reduced influence of a surface roughness

A micro-electro-mechanical variable capacitor has a first and a second electrode, and a dielectric region arranged on the first electrode. An intermediate electrode is arranged on the dielectric region. The first electrode is fixed and anchored to a substrate, and the second electrode includes a membrane movable with respect to the first electrode according to an external actuation, in particular an electrostatic force due to an actuation voltage applied between an actuation electrode and the first electrode. The second electrode is suspended over the intermediate electrode in a first operating condition, and contacts the intermediate electrode in a second operating condition; in particular, in the second operating condition, a short-circuit is established between the second electrode and the intermediate electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a micro-electro-mechanical (MEMS) variable capacitor with reduced influence of a surface roughness, in particular for radio frequency (RF) applications, to which the following description will make explicit reference without this, however, implying any loss in generality.

2. Description of the Related Art

As is known, in the last few years MEMS devices have been developed for a wide variety of applications, in view of their size, cost and power consumption advantages. In particular, variable capacitors (the so called “varicaps”) manufactured using MEMS technology have been successfully introduced in RF applications (such as in wireless mobile communication), instead of conventional variable capacitors, such as junction diodes or MOS capacitors. For example, MEMS variable capacitors have been used in shunt antenna switches, tunable filters, and voltage-controlled oscillators.

FIG. 1illustrates a cross section of a variable capacitor1, of a MEMS type. In detail, the variable capacitor1comprises a fixed electrode2and a movable electrode3, of conductive material (e.g., aluminum, gold or nickel) or a combination of a dielectric material (e.g., oxide or nitride) and a conductive material, which constitute respectively the top and bottom plates of the variable capacitor having a capacitance C. The fixed electrode2is arranged on, and fixed to, a dielectric layer4(e.g., of silicon oxide) formed on a substrate5, for example of semiconductor material (silicon) or glass; a dielectric region6(e.g., of silicon oxide or nitride) locally coats the fixed electrode2. The movable electrode3is a membrane, which is suspended over the fixed electrode2, and is spaced apart from the dielectric region6by an interelectrode air gap7, having a thickness dg. The movable electrode3is electrically connected to actuation electrodes 8 which are arranged on the dielectric layer4laterally to the fixed electrode2; the actuation electrodes8mechanically anchor the movable electrode3to the substrate5. At least part of the membrane is perforated (in a not shown manner) in order to allow releasing of the membrane by etching of a sacrificial region, during a related manufacturing process.

During operation, a dc actuation voltage Vdcis applied across the plates of the variable capacitor1by means of the actuation electrodes8, resulting in an electrostatic force between the fixed electrode2(bottom plate) and the movable electrode3(top plate). This electrostatic force pulls the movable electrode3towards the fixed electrode2, determining a decrease of the thickness dgof the interelectrode air gap7and a corresponding increase of the capacitance value; in particular, the movable electrode3is pulled down to a position at which an equilibrium is reached between the electrostatic force due to the applied actuation voltage Vdcand an elastic force generated in the membrane. As shown inFIGS. 2a-2b,as long as the actuation voltage remains below a critical value, generally called the pull-in voltage (denoted with Vpi), the amount of displacement of the movable electrode3is a result of the equilibrium between the electrostatic force and the elastic force in the membrane. In this operating region (shown in the enlarged detail ofFIG. 2b), the variable capacitor1acts as a tuneable capacitor, and the capacitance C shows an increasing trend with the actuation voltage Vdc. When the actuation voltage Vdcexceeds the pull-in voltage Vpi, no equilibrium can be reached any more, and the movable electrode3collapses on the dielectric region6coating the fixed electrode2, as shown inFIG. 3.

This situation is unwanted for a tuneable capacitor, but it is the normal operation of a capacitive switch, which has two operating states: the on-state, for actuation voltages below the pull-in voltage Vpi, and the off-state, for actuation voltages above the pull-in voltage Vpi. In particular, a minimum capacitance value Cminis associated to the on-state, and a maximum capacitance value Cmaxis associated to the off-state; the ratio between the maximum capacitance value Cmaxand the minimum capacitance value Cminis generally called the switching ratio (denoted with SR) of the variable capacitor1, and is to be maximized for optimum operation of the capacitive switch (typically, the switching ratio is between 10 and 50).

An important factor that influences the maximum capacitance value Cmax(and the switching ratio) is the roughness of the facing surfaces of both the movable electrode3and the dielectric region6. In particular, if surfaces were flat and without roughness, the maximum capacitance value Cmaxassociated to the off-state of the variable capacitor1would be:

Cmax=ɛ0·ɛr⁢Sd
wherein ∈0is the absolute dielectric constant (dielectric permittivity in vacuum), ∈ris the electric permittivity of the dielectric region6, S is the facing area of the electrodes, and d is the thickness of the dielectric region6. In particular, this maximum capacitance value Cmaxcorresponds to a capacitance Cdieldue to the dielectric region6.

However, if the above surfaces have a certain amount of roughness, as shown in the detail ofFIG. 4, they do not perfectly adhere to each other and gap regions10filled with air are formed between the movable electrode3and the dielectric region6, and an unwanted capacitance Cairdue to the air that fills the gap regions10, is generated in series to the capacitance Cdieldue to the dielectric region6. Since the electric permittivity of air is equal to1, the resulting value of the maximum capacitance C′maxis strongly decreased by the presence of air, according to the formula:

Accordingly, the switching ratio of the variable capacitor1is decreased with respect to a design value, and so are the electrical performances thereof, due to the presence of the surface roughness.

BRIEF SUMMARY OF THE INVENTION

One embodiment of the present invention provides a MEMS variable capacitor that will enable the aforementioned disadvantages and problems to be overcome, and in particular that will show a reduced dependence on the roughness of the surfaces of the electrode and dielectric region.

According to one embodiment of the present invention, a micro-electro-mechanical variable capacitor comprises a first and a second electrode, configured to be spaced apart at a distance varying according to an external actuation, and a dielectric region arranged on said first electrode; the variable capacitor further includes an intermediate electrode arranged on said dielectric region.

According to another embodiment of the present invention, a micro-electro-mechanical capacitive switch implemented on a RF transmission line, having a signal line and a ground line, comprises a dielectric region arranged on said signal line and a movable electrode electrically connected to said ground line, said movable electrode and said signal line configured to be spaced apart at a distance varying according to an external actuation; the capacitive switch further includes an intermediate electrode arranged on said dielectric region.

According to a further embodiment of the present invention, a process for manufacturing a micro-electro-mechanical variable capacitor comprises forming a first electrode on a supporting layer, forming a second electrode, suspended over said first electrode and free to move with respect to said first electrode, and forming a dielectric region on said first electrode; the process further includes forming an intermediate electrode on said dielectric region.

According to still another embodiment of the present invention, a process for manufacturing a micro-electro-mechanical variable capacitor comprises forming, on a supporting layer, a Metal Insulator Metal (MIM) capacitor having a top plate, and forming a movable electrode, suspended over said top plate and free to move with respect to said top plate.

According to another embodiment of the present invention, a process for manufacturing a micro-electro-mechanical capacitive switch on a RF transmission line, having a signal line and a ground line, comprises forming a dielectric region on said signal line, and forming a movable electrode electrically connected to said ground line and suspended over said signal line and free to move with respect to said signal line; the process further includes forming an intermediate electrode on said dielectric region.

According to a further embodiment of the present invention, a method for varying a capacitance value of a micro-electro-mechanical variable capacitor comprises varying said capacitance value between at least an on-state value and an off-state value, higher than said on-state value; wherein varying said capacitance value comprises assigning to said off-state value the capacitance value of a Metal Insulator Metal (MIM) capacitor.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 5, in which parts that are similar to those described previously are designated by the same reference numbers, shows a variable capacitor15according to an embodiment of the present invention. In particular, the variable capacitor15differs from the capacitor described previously for the presence of an intermediate electrode16of conductive material, which is arranged on the dielectric region6coating the fixed electrode2. The intermediate electrode16, for example made of aluminum, gold, or nickel (or any other suitable metallic material or combination of a dielectric and a metallic material), is formed on the dielectric region6so that it perfectly adheres to a top surface thereof, with no air gaps therebetween. In detail, the intermediate electrode16is formed via evaporation or sputtering (or other suitable deposition techniques), through a deposition mask which leaves uncovered only the top surface of the dielectric region6. A main dimension of the intermediate electrode16in the cross-section ofFIG. 4corresponds, for example, to a corresponding main dimension of the fixed electrode2.

Accordingly, a MIM (Metal-Insulator-Metal) capacitor18is formed on the dielectric layer4, having the intermediate electrode16as top plate, the fixed electrode2as bottom plate, and the dielectric region6as interelectrode dielectric; the capacitance of the MIM capacitor18is determined by the thickness d of the dielectric region6, and can be accurately set in a design stage.

During operation, when an actuation voltage Vdc, having a value higher than a pull-in voltage Vpi, is applied to actuation electrodes8, the movable electrode3collapses, for the reasons previously described, on the intermediate electrode16(as shown inFIG. 5). Therefore, a short-circuit is established between the movable electrode3and the intermediate electrode16(the top plate of the variable capacitor15being formed by the assembly of the moveable electrode3and the intermediate electrode16), and hence the maximum capacitance value Cmaxassociated to the off-state of the variable capacitor15is equal to the capacitance of the MIM capacitor18. In other words, the series capacitance due to the presence of the air-filled gap regions due to the surface roughness, is no more involved in the determination of the off-state capacitance of the variable capacitor15.

Since the maximum capacitance value, and accordingly the switching ratio of the variable capacitor15, can be set in the design stage and it is not reduced by the above discussed roughness issues, the variable capacitor according to the present invention can be advantageously used as a capacitive switch for RF applications, for example as a shunt switch in a mobile phone30, seeFIG. 6, to allow/block transmission of a received RF signal. The variable capacitor15can be integrated in a chip32with other suitable electronics, and the chip32can be coupled to a receiving (RX) circuit34of the mobile phone30(in particular, both the chip32and the receiving circuit34being connected to a printed circuit board35arranged inside a housing of the mobile phone30).

As shown inFIG. 7, in this application, the variable capacitor15is implemented on a co-planar waveguide (CPW) transmission line36, which is provided for the transmission of the received RF signal; the CPW transmission line36is arranged on the dielectric layer4and includes a signal line (including the fixed electrode2), and ground lines (including the actuation electrodes8) arranged laterally with respect to the signal line (both the signal and ground lines being formed at a same metal level). The movable electrode3is electrically connected, and mechanically anchored, to the ground lines, and it is arranged as a bridge crossing over the signal line. When the bridge is up (i.e., the actuation voltage Vdchas a value which is lower than the pull-in voltage Vpi), the capacitance of the signal line to ground is low (e.g., on the order of 10-100 fF), the capacitive switch is in the on-state, and hardly affects the impedance of the line, so allowing passage of the RF signal. Upon activation (actuation voltage Vdchigher than the pull-in voltage Vpi), the bridge is pulled down onto the intermediate electrode16, the capacitance becomes high and the capacitive switch turns to the off-state, shunting the RF signal to ground. In particular, during operation, the RF signal and the actuation voltage Vdcare superimposed and applied to the signal line; in any case, the RF signal does not influence the displacement of the movable electrode3, since it is filtered by the large mechanical time constant of the capacitor structure.

The advantages of the described micro-electro-mechanical variable capacitor are clear from the foregoing description.

In particular, the variable capacitor15is no more roughness dependent, and the maximum capacitance value Cmaxassociated to the off-state (and so the switching ratio) can be accurately set in a design stage, by properly sizing the MIM capacitor18.

The manufacturing of the variable capacitor requires a simple additional manufacturing step, and in particular only one more deposition mask is required.

Moreover, the intermediate electrode16offers a mechanical protection for the underlying dielectric region6.

Finally, it is clear that modifications and variations may be made to what is described and illustrated herein, without thereby departing from the scope of the present invention, as defined in the annexed claims.

For example, in a per se known manner, actuation of the movable electrode3could be provided by electro-thermal actuators, and/or the movable electrode could be anchored to the substrate via additional elastic elements.

The dielectric layer could not be envisaged, in case substrates made of glass or other insulating material were used.

Furthermore, in the CPW implementation, the variable capacitor15could provide a capacitive switch arranged in series to the signal line; in this case, the condition of maximum capacitance would be associated to the on-state of the switch, allowing passage of the RF signal.

The capacitive switch could also be implemented on a micro-strip transmission line, or any other kind of transmission line.

Furthermore, the variable capacitor can be used both as a tuneable capacitor and as a switched capacitor. In particular, in the described mobile phone application, the variable capacitor could also be used in a tuning stage of the receiving circuit34.