BACKGROUND OF THE DISCLOSURE

Field of the Disclosure

[0001] Embodiments of the present disclosure generally relate to a radio frequency (RF) digital variable capacitor (DVC) units for RF tuning and impedance matching.

Description of the Related Art

[0002] MEMS capacitors can show non linear behavior when operated as a capacitor. This is a problem for RF applications when signals transmitted at one frequency can leak into other frequency channels. One measure of this is the IP3 value or the value of input at which the third order nonlinearity times the input voltage or current is equal to the first order term times the input voltage or current.

[0003] With a MEMS capacitor, as the power increases on the RF line, an increasing voltage is dropped across the oxide between the RF line and the MEMS cantilever. Even though the MEMS device may be in mechanical contact with the oxide layer, any roughness or asperities at that interface can result in a small change in the gap (between the RF electrode and the MEMS device) as a function of applied power. This change in gap results in a change in the maximum capacitance as a function of power. Thus a modulation in power can then lead to a modulation in frequency and to more signals being found outside the desired frequency window.

[0004] Therefore, there is a need in the art for a MEMS DVC with a stable capacitance vs RF input power.

SUMMARY OF THE DISCLOSURE

[0005] The present disclosure generally relates to a MEMS DVC utilizing one or more MIM capacitors. The MIM capacitor may be integrated into the MEMS device itself and may be disposed on the anchor of the MEMS device. [0006] In one embodiment, a DVC comprises a substrate having at least one RF electrode and at least one anchor electrode disposed therein; an insulating layer disposed on the at least one anchor electrode; a conductive layer disposed on the insulating layer, wherein the at least one anchor electrode, insulating layer and conductive layer form a MIM capacitor; at least one MEMS bridge disposed over the substrate and coupled to the conductive layer, the at least one MEMS bridge movable from a position spaced a first distance from the RF electrode and a position spaced a second distance from the RF electrode that is less than the first distance.

[0007] In another embodiment, a method of making a DVC comprises forming a plurality of electrodes over a substrate, wherein at least one electrode is an anchor electrode and at least one electrode is an RF electrode; depositing an insulating layer over the plurality of electrodes; removing at least a portion of the insulating layer to expose at least a portion of the RF electrode; depositing a conductive layer over the insulating layer and exposed RF electrode; removing selected portions of the conductive layer; forming an ohmic contact over a portion of the conductive layer; forming a MEMS bridge in contact with the ohmic contact.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments.

[0009] Figure 1 is a schematic top view of a MEMS DVC according to one embodiment.

[0010] Figures 2A and 2B are schematic top and cross-sectional illustrations of the MEMS device of the MEMS DVC of Figure 1 . [0011] Figures 3A, 3B and 3C are schematic top and cross-sectional illustrations of an individual switching element in the MEMS device of the MEMS DVC of Figure 1 .

[0012] Figure 4 is a graph comparing the resistance for the MEMS DVC of Figure 1 and a fixed MIM-cap.

[0013] Figures 5A-5E are schematic illustrations of a MEMS DVC at various stages of fabrication according to one embodiment.

[0014] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.

DETAILED DESCRIPTION

[0015] The present disclosure generally relates to a MEMS DVC utilizing one or more MIM capacitors. The MI M capacitor may be disposed on the anchor of the MEMS device.

[0016] In this disclosure, the MEMS variable capacitor is converted into a resistive switch which then switches in a metal insulator metal (MI M) capacitor with conformal coatings of insulator and then metal over the first metal. Such a capacitor is more robust to high voltages and the high mechanical pressures induced by the voltage drop across the insulator, because the forces are evenly distributed over the interfaces. With the MEMS acting as a resistive switch, a low resistance is needed to get a good value for Q. To achieve a low resistance one normally requires a large MEMS device that can apply large forces to the contacts. The problem with a large MEMS device is that it switches slowly. Also each MEMS switch needs to be able to sustain the current through the MIM capacitor. With large values of the MIM capacitor, the current through the MEMS-switch can become very high which is a potential reliability issue. To avoid this, many small MEMS devices in parallel can be used. Each MEMS device connects to a small MIM capacitor and only needs to supply a limited current. Although each contact has a reasonably large resistance, the total combined value of all parallel devices is small.

[0017] Figure 1 shows a possible implementation of the resistively switched digital variable capacitor shown from the top. It contains an array of small hybrid Ohmic-MIM cells 3. The RF connections 1 and 4 to each cell are on opposite ends. Each cell contains an array of (5 to 40) small Ohmic-MIM switches 5 working in parallel. All switches 5 in a single cell 3 are actuated at the same time and provide a minimum capacitance when turned off or a maximum capacitance when turned on. Multiple cells can be grouped to result in a binary control-scheme so that the total capacitance between the RF connections 1 and 4 can be tuned with a digital control of 1 to 8 bits resolution.

[0018] Figure 2A shows the top view of the Ohmic-MIM cell marked as 3 in Figure 1 . The cell contains an array of Ohmic-MIM switches 5. Underneath the switches there is an RF electrode 1 1 and pull-in electrodes 12 and 13 to actuate the switches to the down-position (switch closed).

[0019] Figure 2B shows the side view with pull up electrode 15 to actuate the switches to the up-position (switch open), cavity 16 and underlying substrate layer 17. The substrate can contain multiple metal levels for interconnect and also CMOS active circuitry to operate the device.

[0020] Figure 3A shows a top view of one of the switches in the array marked as 5 in Figures 1 and 2A. The pull-in electrodes are marked with 12 and 13 and the RF-electrode is marked with 1 1 .

[0021] Figure 3B shows a cross-section view of the switch. The switch element contains a stiff bridge consisting of conductive layers 20, 22 which are joined together using an array of vias 21 . Layer 20 may not extend all the way to the end of the structure, making layer 20 shorter in length than layer 22. The MEMS bridge is suspended by legs 14 formed in the lower layer 22 of the MEMS bridge and anchored with via 23 onto conductor 8. This allows for a stiff plate-section and compliant legs to provide a high contact-force while keeping the operating voltage to acceptable levels. Conductors 8 and 10 together with dielectric layer 9 form a Metal-lnsulator-Metal (MIM) capacitor. The MEMS bridge is connected to the top- metal 8 of the MIM-capacitor through the anchor vias 23. This scheme allows to set the maximum on-capacitance of the switch by sizing the MIM capacitor in the anchor accordingly. The off-capacitance of the switch is dominated by the dimensions of the small RF electrode 1 1 and is to a large degree independent of the maximum capacitance set by the MIM.

[0022] Landing post 16 is conductive and makes contact with the conducting underside of the cantilever. 16B is a surface material on the conducting post that provides good conductivity, low reactivity to the ambient materials and high melting temperature and hardness for long lifetime. Although not shown in these figures, there may be an insulating layer over the top and underside of the conductive layers 20, 22. A hole can be made in the insulator on the underside of layer 22 in the landing post area to expose a conducting region 16C for the conducting post to make electrical contact with when the MEMS is pulled down. Typical materials used for the contacting layers 16, 16B, 16C include Ti, TiN, TiAI, TiAIN, AIN, Al, W, Pt, Ir, Rh, Ru, Ru0 ₂ , ITO and Mo and combinations thereof. In the actuated down state layer 22 of the MEMS bridge may land on multiple bumps 15A, 15B, 15C and 15D, which are provided to avoid landing the MEMS bridge on the dielectric layer 9 above the pull-in electrodes 12, 13 which can lead to reliability issues. These bumps are generated at the same time as the top-plate 8 of the MIM capacitor and landing post 16B, 16C.

[0023] Above the MEMS bridge there is a dielectric layer 19 which is capped with metal 18 which is used to pull the MEMS up to the roof for the off state. Dielectric layer 19 avoids a short-circuit between the MEMS bridge and the pull-up electrode in the actuated-up state and limits the electric fields for high reliability. Moving the device to the top helps reduce the capacitance of the switch in the off state. The cavity is sealed with dielectric layer 17 which fills the etch holes used to remove the sacrificial layers. It enters these holes and helps support the ends of the cantilevers, while also sealing the cavity so that there is a low pressure environment in the cavities.

[0024] Figure 3C shows an alternative embodiment of the switch, where the MEMS bridge consists of two layers 20, 22 which are joined together with an intermediate dielectric layer 24 and some vias 21 to electrically connect layer 20 and 22. Suitable materials for the dielectric layer 24 include silicon based materials including silicon-oxide, silicon-dioxide, silicon-nitride and silicon-oxynitride. The legs 14 are again defined in the lower layer 22 of the MEMS bridge. This allows for a stiff plate-section and compliant legs to provide a high contact-force while keeping the operating voltage to acceptable levels.

[0025] There are several advantages in implementing the full device with a large number of small ohmic-MIM switches. In general, breaking up the device in a large number of branches each one made of an ohmic switch with a very small MIM capacitor in series relaxes the requirements for the ohmic resistance value of each switch in order to achieve an overall small equivalent series resistance (ESR) and high device Q factor. Also only a small RF current will flow through each switch for a given rms voltage, because the small MIM capacitors limit the current through each switch, which minimizes reliability issues. Additionally, positioning the small MIM-capacitors in the anchor of the ohmic switch allows to size the maximum capacitance of the switch in the closed-state to a large degree independent of the minimum-capacitance in the open-state.

[0026] Figure 4 is a plot generated by a simulated analysis comparing an device implemented by just MIM capacitors (therefore of fixed capacitance value) with a device which introduces ohmic switches in series to all MIM capacitors in order to obtain a programmable C value; the device ESR of the MIM capacitors is 0.3ohm; adding the ohmic switches increases the ESR, but in order to have an ESR penalty of less than 0.1 ohm it is sufficient to have each ohmic switch resistance to be below 60 ohms; this is taking advantage of the parallelization in the architecture made of a large number of very small ohmic-MIM switches. MEMS DVC Fabrication

[0027] Figures 5A-5D are schematic illustrations of a MEMS DVC 1 100 at various stages of fabrication according to one embodiment. As shown in Figure 5A, the substrate 1 102 has a plurality of electrodes 1 104A-1 104E formed therein. Electrodes 1 104A, 1 104E will form the bottom metal of the MIM capacitor, electrodes 1 104B, 1 104D will form the pull-in electrodes and electrode 1 104C will form the RF electrode.

[0028] It is to be understood that the substrate 1 102 may comprise a single layer substrate or a multi layer substrate such as a CMOS substrate having one or more layers of interconnects. Additionally, suitable material that may be used for the electrodes 1 104A-1 104E include titanium-nitride, aluminum, tungsten, copper, titanium, and combinations thereof including multi-layer stacks of different material.

[0029] As shown in Figure 5B, an electrically insulating layer 1 106 is then deposited over the electrodes 1 104A-1 104E. Suitable materials for the electrically insulating layer 1 106 include silicon based materials including silicon-oxide, silicon- dioxide, silicon-nitride and silicon-oxynitride. As shown in Figure 5B, the electrically insulating layer 1 106 is removed over the RF electrode 1 104C to expose the underlying electrode 1 104C.

[0030] Electrically conductive material 1 108 may then be deposited over the electrically insulating layer 1 106 as shown in Figure 5C. The electrically conductive material 1 108 provides the direct electrical connection to the RF electrode 1 104C. Additionally, the electrically conductive material 1 108 provides the upper "metal" in the MIM capacitors located above electrodes 1 104A, 1 104E. Suitable materials that may be used for the electrically conductive material 1 108 include titanium, titanium nitride, tungsten, aluminum, combinations thereof and multilayer stacks that include different material layers. On top of conductive material 1 108 a thin layer of conductive contact material 1 1 10 is deposited which will provide the Ohmic contact to the MEMS bridge in the landed-down state. Suitable materials that may be used for the electrically conductive contact material 1 1 10 include W, Pt, Ir, Rh, Ru, Ru0 ₂ , ITO and Mo.

[0031] During patterning of the electrically conductive materials 1 108, 1 1 10 also multiple electrically isolated bumps 1 1 12 can be formed over the electrically insulating layer 1 106. These provide additional mechanical support to the plate in the actuated-down state. Once the electrically conductive materials 1 108, 1 1 10 have been patterned, the remainder of the processing may occur to form the MEMS DVC 1 100 shown in Figure 5D or the alternative embodiment in Figure 5E which uses a dielectric stiffener 1 125 between the 2 plates of the MEMS bridge.

[0032] As noted above, the switching element 1 1 14 may have insulating material coating the bottom surface thereof and thus, an area 1 124 of exposed conductive material may be present that will land on the surface material 1 1 10. An additional electrically insulating layer 1 1 18 may be formed over the pull-off (i.e., pull-up) electrode 1 120, and a sealing layer 1 122 may seal the entire MEMS device such that the switching element 1 1 14 is disposed within a cavity. During fabrication, sacrificial material is used to define the boundary of the cavity.

[0033] While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.