Patent Description:
In operating a MEMS digital variable capacitor (DVC), a plate moves between a first position and a second position. The plate moves by applying a voltage to an actuation electrode. Once the electrode voltage reaches a certain voltage, oftentimes referred to as a snap-in voltage, the plate moves towards the electrode. The plate moves back to the original position once the voltage is lowered to a release voltage. The release voltage is typically lower than the snap-in voltage due to the higher electrostatic forces when the plate is close to the actuation electrode and due to stiction between the plate and the surface to which the plate is in contact once moved closer to the electrode.

Because the plate doesn't release at the same voltage as the snap-in voltage, the MEMS DVC has a hysteresis curve. The snap-in voltage and the release voltage, while different, should be known for the MEMS DVC to operate efficiently.

Therefore, there is a need in the art for a method and device for effectively measuring the hysteresis curve for a MEMS DVC. <CIT> relates to RF MEMS isolation, series and shunt DVC, and small MEMS. More specifically, <CIT> relates to an architecture for isolating an RF MEMS device from a substrate and driving circuit, series and shunt DVC die architectures, and smaller MEMS arrays for high frequency communications. The semiconductor device has one or more cells with a plurality of MEMS devices therein. The MEMS device operates by applying an electrical bias to either a pull-up electrode or a pull-down electrode to move a switching element of the MEMS device between a first position spaced a first distance from an RF electrode and a second position spaced a second distance different than the first distance from the RF electrode. The pull-up and/or pull-off electrode may be coupled to a resistor to isolate the MEMS device from the substrate. <CIT> relates to a MEMS stiction testing apparatus and method. More specifically, <CIT> relates to a MEMS stiction testing method which applies a first electrical signal to a MEMS device having two opposing surfaces to cause the two opposing surfaces to make physical contact. The two opposing surfaces produce a second electrical signal when in physical contact. The method then substantially mitigates the first electrical signal after detecting that the second electrical signal has reached a prescribed maximum value.

The present disclosure generally relates to a mechanism for testing a MEMS hysteresis. A power management circuit is coupled to the electrodes that cause the movable plate to move between the electrodes in a MEMS device. The power management circuit may utilize a charge pump, a comparator and a resistor ladder.

The present disclosure generally relates to a mechanism for testing a MEMS hysteresis. A power management circuit may be coupled to the electrodes that cause the movable plate that is disposed between the electrodes to move in a MEMS device. The power management circuit may utilize a charge pump, a comparator and a resistor ladder.

A MEMS DVC device may operate with electrostatic forces. As discussed herein, the mechanism operated by a force acting on the moveable MEMS element when a voltage V is applied between the movable MEMS element (e.g., movable plate) and a control electrode. This electrostatic force scales with (V/gap)<NUM>. The mechanical counter-balance force comes from a spring suspension system and typically scales linearly with the displacement. The result is that with an increasing voltage V the MEMS device moves a certain distance δ toward the control-electrode. This movement reduces the gap between the movable MEMS element (oftentimes referred to as a moveable plate) and the electrode, which in turn increases the electrostatic force further. For small voltages, an equilibrium position between the initial position and the electrode is found. However, when the voltage exceeds a certain threshold level (the pull-in voltage), the device displacement is such that the electrostatic force rises faster than the mechanical counterbalance force and the device rapidly snaps-in towards the control-electrode until it comes in contact.

The MEMS DVC device may have a control-electrode above (PU-electrode) and below (PD-electrode) the moveable MEMS element, as shown schematically in <FIG>. In addition an RF-electrode may be present below the moveable MEMS element. As shown in <FIG>, the PU-electrode, the PD-electrode and the RF electrode are all covered with dielectric material. During operation the MEMS element is either pulled-up or pulled-down in contact with the dielectric material to provide a stable minimum or maximum capacitance to the RF-electrode. In this way the capacitance from the moveable element to the RF-electrode (which resides below the moveable element) can be varied from a high capacitance Cmax when pulled to the bottom (See <FIG>) to a low capacitance Cmin (See <FIG>) when pulled to the top. The voltages applied to the PD-electrode (Vbottom) and to the top-electrode (Vtop) are typically controlled by a waveform controller (See <FIG>) to ensure a long-life stable performance of the DVC device. The moveable element is typically on DC-ground.

As shown in <FIG>, the MEMS DVC device may comprise a movable plate disposed in a cavity. The movable plate is coupled to ground and moves between a free standing state shown in <FIG> to a Cmax state shown in <FIG> and a Cmin state shown in <FIG>. A voltage may be applied to one or more pull-in or pull-down electrodes to pull the plate into close proximity of the RF electrode. The electrodes are covered by a dielectric material. A pull up or pull off electrode may be disposed opposite the pull-in electrodes.

<FIG> shows a typical response of the MEMS DVC device to an applied control voltage to the PD-electrode. Initially, the device is in the free-standing state as in <FIG> and has a capacitance Cfree. As the voltage on the bottom control electrode is ramped up, the capacitance slowly increases as the movable plate slowly moves closer to the RF electrode until the snap-in point p1 is reached at a voltage Vpi (pull-in voltage). At this point the device (i.e., movable plate) quickly snaps in and the capacitance goes to its maximum value Cmax. Because the gap between the MEMS element and the PD-electrode is now much smaller, the electrostatic force has increased and the voltage has to be reduced down to Vrl (release voltage) in order for the MEMS device to release from the bottom at point p2. The capacitance of the MEMS device is at the maximum value when the MEMS element is in contact with the dielectric material that is disposed on the RF electrode.

<FIG> shows a typical response of the MEMS DVC device to an applied control voltage to the PU-electrode. Initially, the device is in the free-standing state as in <FIG> and has a capacitance Cfree. As the voltage on the top control electrode is ramped up, the capacitance slowly decreases as the movable plate slowly moves away from the RF electrode until the snap-in point p3 is reached at a voltage Vpu (pull-up voltage). At this point the device (i.e., movable plate) quickly snaps in and the capacitance goes to its minimum value Cmin. Because the gap between the MEMS element and the PU-electrode is now much smaller, the electrostatic force has increased and the voltage has to be reduced down to Vrlu (release voltage) in order for the MEMS device to release from the top at point p4. The capacitance of the MEMS device is at the minimum value when the MEMS element is in contact with the dielectric material that is disposed on the PU electrode.

The Vpi, Vpu, Vrl and Vrlu are important parameters for the MEMS DVC device. If the pull-in voltage Vpi or Vpu is too high then the waveform controller may not be able to pull the MEMS devices into contact intimately, which can impact the obtainable Cmin (upward actuation) or Cmax (downward actuation). If the release voltage Vrl or Vrlu is too low this could indicate stiction which impedes proper device operation. Also, if the release voltage Vrl from the bottom is too low then this will impede the device to be released from the RF-electrode in the presence of an RF signal.

Both Vpi, Vpu, Vrl and Vrlu depend on material parameters (Young's Modulus) as well as geometrical parameters, such as layer thicknesses and CD-control of various layers. Therefore, in production, the MEMS devices will exhibit a certain distribution in these voltages. In order to screen functional devices that meet all required product specs, it is key to test the Vpi, Vpu, Vrl and Vrlu on every device. As discussed herein, a built-in test methodology can be used facilitate the test.

The built-in test methodology is termed as "hysteresis testing. " Hysteresis, because the pull-in and release voltages are separated or the pull-in and the release curves do not overlap as shown in <FIG>. For a reliable part (MEMS DVC) Vpi, Vpu, Vr/ and Vrlu are designed to be in a certain range. Otherwise they can result in non performance as explained in above paragraphs. Unlike, Cmax and Cmin, Vpi, Vpu, Vrl and Vrlu are not product specs, i.e. they are not listed on a product sheet but they are the best gauges for estimating the reliability or robustness of the part. Due to process variations, certain parts on a wafer or across the lot may fall outside the range and, if escape screening, can lead to failures in the field. So hysteresis testing allows for screening the bad parts from good.

A typical method for performing a hysteresis test on an electrostatic MEMS device is to perform a CV (capacitance - voltage) sweep. A typical CV sweep can be performed using a CV meter, which uses a combination of a DC source and an AC source to provide the DC bias and the AC signal. The measurements are performed by a combination of an AC voltmeter and an AC ammeter. The basic test configuration to perform a test on a two terminal electrostatic MEMS device is shown in <FIG>.

The MEMS device capacitance is given by the equation, where f is the frequency of the AC voltage source: <MAT>.

A three or more terminal electrostatic MEMS device does not allow for the same straightforward CV test as shown in <FIG>. The bias electrodes on the MEMS device provide the actuation bias and are separate from the capacitor electrodes. A CV sweep is performed on this configuration by using the same configuration as shown in <FIG>, but also including a DC source, Vbias, as shown in <FIG>. The DC source is used in the same manner as the DC source in <FIG>.

A semiconductor chip can be composed of one or more MEMS transducers and monolithically integrated CMOS control and power management circuitry. This allows for the Vbias power supply in <FIG> to be generated internal to the semiconductor chip and not in an external power supply. This is shown in <FIG> and <FIG> as a plurality of MEMS transducers each with separate switches to the power management. In <FIG>, the power management and the MEMS elements are all disposed on a common semiconductor chip. In <FIG>, the MEMS elements are disposed on semiconductor chip <NUM> and the power management is disposed on semiconductor chip <NUM>. The Vbias voltage is generated in the integrated power management circuit and passes to the MEMS transducers through switch S2. The actuation voltage for the MEMS transducers is controlled through the power management, where the level of the output voltage is controlled by the digital control bits C<<NUM>:n>, as shown in <FIG> and <FIG>. The hysteresis sweep can be performed by changing the digital control bits in the power management circuit to the desired actuation voltage. A primary difference between the external power supply version in <FIG> and the internal DFT mode in <FIG> and <FIG> is that the digital control bits hold the value at a discrete number of fixed levels (n) instead of a continuous sweep that can be performed by the external version.

One representation of a power management circuit that can allow for discrete levels of output voltage is a charge pump with a regulator. In the simple case in <FIG>, the charge pump clock is gated by the output of the comparator. The output voltage of the charge pump is divided by the resistor ladder and compared to the bandgap voltage reference. If the voltage reference at the resistor ladder is lower than the value of the bandgap voltage reference voltage, the charge pump clock is on. This condition will allow the charge pump clock to be toggling and the charge pump voltage will be increasing if the charge generation is greater than the output load current. If the voltage reference at the resistor ladder is higher than the value of the bandgap voltage reference, the charge pump clock is off. As shown in <FIG>, the programming of the voltage level is produced by switching in discrete resistors in the resistor ladder, effectively changing the voltage on the compare node to produce a higher or lower output voltage set point. In this manner, the value of the output voltage is programmed by the address bits C<<NUM>:n>.

As shown in <FIG>, the value of Vactuation will be programmed to be a resistor ratio as compared to the Vbandgap voltage as shown by the following equation for a programmation of c<<NUM>>: <MAT>.

The value for Vactuation with a c<<NUM>> programmation is: <MAT>.

The value for Vactuation with a c<n> programmation is: <MAT>.

For a discrete hysteresis curve using this DFT method, the test method consists of a voltage programmation using the C address bits, a wait time for settling, and a measurement strobe of the capacitance.

The test is implemented in the hardware configuration shown in <FIG>. The device under test, or DUT, is preset using the address bits to the regulator to output a voltage level to the MEMS that is lower than the Vpi. After the DUT is powered up, a wait time, or Tw, for voltage and MEMS settling is implemented in the test sequence before the capacitance level is measured by the CV meter at time Ts. After the capacitance is measured, the address bits are incremented to the next voltage level and the measurement is performed using the same timing. Once the Vpi is detected, the address bits are decremented and the measurements taken until the capacitance meter detects Vrl. By utilizing this test sequence, along with the internal DFT, a continuous hysteresis curve can be represented by discretizing the voltage levels as shown in <FIG>.

<FIG> and <FIG> are schematic illustrations of a DFT implementation to test a MEMS hysteresis according to additional embodiments. <FIG> shows an embodiment where the test is performed for voltage applied to the pull-down electrode <NUM> while <FIG> shows an embodiment where the test is performed for voltage applied to the pull-up electrode <NUM>. It is contemplated that the test may be performed on both the pull-down electrode <NUM> and pull-up electrode <NUM>.

The MEMS device <NUM> includes the pull-down electrodes <NUM>, the pull-up electrode <NUM>, an RF electrode <NUM> and ground electrodes <NUM>. The ground electrodes <NUM> are connected to ground and to the movable plate <NUM>. A dielectric layer <NUM> is disposed over the pull-down electrodes <NUM> and the RF electrode <NUM>. Another dielectric layer <NUM> is disposed between the pull-up electrode <NUM> and the cavity <NUM> within which the movable plate <NUM> is disposed.

As shown in <FIG>, the pull-down electrodes <NUM> are coupled to multiple switches <NUM>, <NUM>. The first switch <NUM>, when engaged, connects the pull-down electrodes <NUM> to ground. In <FIG>, when the first switch <NUM> is engaged, the pull-up electrode <NUM> is connected to ground. The second switch <NUM>, in <FIG>, is connected to a power management device <NUM>. Similarly, in <FIG>, the second switch <NUM> is connected to the power management device <NUM>. Thus, when the second switch <NUM> is engaged, the pull-down electrode <NUM> (<FIG>) or the pull-up electrode <NUM> (<FIG>) is connected to the power management device <NUM>. The power management device <NUM> and the MEMS device <NUM> are both disposed in a single package represented by box <NUM>. It is to be understood that the power management device <NUM> and the MEMS device <NUM> are disposed in a single package. In one embodiment, the single package may comprise a single chip having both the power management device <NUM> and MEMS device <NUM> disposed thereon. In another embodiment, the single package may comprise separate chips that operate collectively as a single entity wherein the MEMS device <NUM> is on a first chip and the power management device <NUM> is disposed on a second chip.

The power management device <NUM> includes a charge pump <NUM> that is coupled to a gate <NUM>. The gate <NUM> is coupled to both the Vclock node and the output from a comparator <NUM>. The comparator has inputs from the Vbandgap node and the resistive ladder. The resistive ladder is what divides the output of the charge pump <NUM>. The resistive ladder includes a plurality of resistors R0. Rn which are coupled together in series. The address bits c<<NUM>>, c<<NUM>>, c<n> are programmed to incrementally "actuate" or operate such that the next voltage level is achieved and the capacitance of the MEMS device <NUM> is measured. Hence, an incremental voltage is applied by operating the address bits c<<NUM>>, c<<NUM>>, c<n>. Based upon the incremental voltage increase, the actuation voltage Vpi to the pull-in electrode (for <FIG>) or the actuation voltage to the pull-up electrode (for <FIG>) is determined. Similarly, by decrementally decreasing the voltage (i.e., operating the address bits c<<NUM>>, c<<NUM>>, c<n>), the capacitance is again measured and the release voltage Vrl from the bottom electrode (for <FIG>) or the release voltage from the top electrode (for <FIG>) is detected. As such, the hysteresis curve for the particular MEMS device <NUM> is determined. It is to be understood that multiple MEMS devices may be coupled to the power management device <NUM>. The multiple MEMS devices may collectively operate as a DVC.

<FIG> shows an embodiment with four switches 1118A, 1118B, 1120A, 1120B whereby both the pull up-electrode <NUM> and the pull-down electrode <NUM> are coupled to the power management device <NUM> and to ground. The pull-up electrode <NUM> is coupled to ground through switch 1118A and to the power management device <NUM> through switch 1120A. The pull-down electrodes <NUM> are coupled to ground through switch 1118B and to the power management device <NUM> through switch 1120B. During operation, when the movable plate <NUM> is pulled down by the pull-down electrodes <NUM>, switch 1120B is connected to the power management device <NUM> and switch 1118B is disengaged from ground. Simultaneously, the pull-up electrode <NUM> is grounded whereby switch 1118A is connected to ground and switch 1120A is disengaged from power management device <NUM>. When the movable plate <NUM> is pulled up, the pull-up electrode <NUM> is coupled to the power management device <NUM> by switch 1120A which is engaged with the power management device <NUM> while switch 1118A is decoupled from ground. Simultaneously, pull-down electrodes <NUM> are coupled to ground through switch 1118B while switch 1120B is disengaged from power management device <NUM>.

<FIG> is a flow chart <NUM> illustrating a method of testing a MEMS DVC according to one embodiment. Initially, a low voltage is applied to the pull-down electrode in step <NUM>. The voltage applied to the pull-down electrode is to pull the movable plate closer to the RF electrode and will result in an increase of the MEMS RF-capacitance. After the voltage has been applied, the capacitance of the MEMS device is measured in step <NUM>. If the capacitance is equal to the maximum capacitance of the MEMS device, then the pull-in voltage Vpi has been determined in step <NUM>. If the measured capacitance is not equal to the maximum capacitance, then the voltage is incrementally increased in step <NUM>, with the capacitance measured with each incremental voltage increase in step <NUM>, until the maximum capacitance is reached and the pull-in voltage has been determined in step <NUM>.

Once the pull-in voltage Vpi has been determined, the release voltage is determined. The release voltage is determined by reducing the voltage applied to the pull-down electrode in step <NUM>. The capacitance is then measured in step <NUM>. If the capacitance is equal to the free-standing capacitance, then the release voltage has been determined. If, however, the measured capacitance is still larger, then the voltage is decrementally reduced in step <NUM>. The capacitance is measured for each decremental voltage reduction. If the measured capacitance is equal to the free-standing capacitance in step <NUM>, then the release voltage has been determined in step <NUM>.

<FIG> is a similar flow chart <NUM> illustrating a method of testing a MEMS DVC according to one embodiment. Initially, a low voltage is applied to the pull-up electrode in step <NUM>. The voltage applied to the pull-up electrode is to pull the movable plate away from the RF electrode and will result in a decrease of the MEMS RF-capacitance. After the voltage has been applied, the capacitance of the MEMS device is measured in step <NUM>. If the capacitance is equal to the minimum capacitance of the MEMS device, then the pull-up voltage Vpu has been determined in step <NUM>. If the measured capacitance is not equal to the minimum capacitance, then the voltage is incrementally increased in step <NUM>, with the capacitance measured with each incremental voltage increase in step <NUM>, until the minimum capacitance is reached and the pull-up voltage has been determined in step <NUM>.

Once the pull-up voltage Vpu has been determined, the release voltage is determined. The release voltage is determined by reducing the voltage applied to the pull-up electrode in step <NUM>. The capacitance is then measured in step <NUM>. If the capacitance is equal to the free-standing capacitance, then the release voltage has been determined. If, however, the measured capacitance is still larger, then the voltage is decrementally reduced in step <NUM>. The capacitance is measured for each decremental voltage reduction. If the measured capacitance is equal to the free-standing capacitance in step <NUM>, then the release voltage has been determined in step <NUM>.

It is to be understood that the embodiments disclosed herein are not limited to the MEMS DVC using the MEMS such as shown in <FIG>. The embodiments disclosed herein are applicable to MEMS DVC using MIM capacitors in the MEMS device. <FIG> show a MEMS device using a MIM capacitor that is applicable to the embodiments discussed herein.

<FIG> is a schematic cross-sectional illustration of a MEMS DVC device <NUM> having a MIM capacitor in the free standing state. <FIG> is a schematic cross-sectional illustration of the MEMS DVC device <NUM> of <FIG> in the Cmax state. <FIG> is a schematic cross-sectional illustration of the MEMS DVC device <NUM> of <FIG> in the Cmin state. The MEMS DVC device <NUM> includes pull-in electrodes <NUM>, <NUM> and an RF line <NUM>. The RF line <NUM> extends throughout the cavity of the MEMS DVC and is common to one or more MEMS devices within the cavity. The MEMS bridge includes a layer <NUM> that lands on bumps <NUM> that overlie the pull-in electrodes <NUM>, <NUM>. The top layer <NUM> of the MEMS bridge is connected to the bottom layer <NUM> by one or more posts <NUM>. The layers <NUM>, <NUM> and posts <NUM> comprise a conductive material. The top layer <NUM> may not extend all the way to the ends of the structure, making layer <NUM> shorter in length than layer <NUM>. The grounded MEMS bridge is connected to the underlying metallization though via <NUM>. An insulating layer <NUM> is capped with metal electrode <NUM> which is used to pull the MEMS bridge up to the roof for the Cmin state. This helps reduce the capacitance of the switch in the Cmin state. A top insulating layer <NUM> which fills the etch holes used to remove the sacrificial layers. The top insulating layer <NUM> 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.

To form the MIM, landing posts <NUM> are present that are conductive and make contact with the conducting underside of the MEMS bridge. A surface material, such as a metal feature <NUM> is disposed on the conducting post <NUM> that provides good conductivity, low reactivity to the ambient materials and high melting temperature and hardness for long lifetime. The underside of the MEMS bridge may be coated with an insulator but a window is opened on the underside of the MEMS bridge to provide a conducting region <NUM> for the conducting post <NUM> to make electrical contact with when the MEMS bridge is pulled down. A dielectric layer <NUM> is formed over the pull-in electrodes <NUM>, <NUM>, but not the RF line <NUM>.

<FIG> shows the MEMS bridge pulled in with voltages applied to pull-in electrodes <NUM>, <NUM> so that the layer <NUM> lands on the insulated bumps <NUM>. The conducting region <NUM> of the MEMS bridge lands on the two conducting post <NUM> (only one shown as the other is behind it), which gives the low resistance state. <FIG> shows the MEMS bridge after it has been pulled to the roof using electrode <NUM>. The MEMS bridge makes contact with the insulating layer <NUM>. This prevents any electrical contact between the pull up electrode <NUM> and the MEMS bridge. The region in the dotted rectangles is shown in <FIG>.

Although not shown in these figures, there may be an insulating layer over the top and most of the underside of the MEMS bridge. A hole is made in the insulator on the underside of the cantilever to allow it to make contact with the conducting post <NUM>. In this state the resistance of the MEMS bridge to the RF line is very large and the capacitance coupling to that line is small.

The embodiments discussed herein are also applicable to hybrid ohmic-MIM devices. <FIG> is a schematic cross-sectional illustration of a MEMS DVC device <NUM> according to another embodiment. In the embodiment shown in <FIG>, a surface material, such as a metal feature <NUM> is disposed on the conducting post <NUM> that provides good conductivity, low reactivity to the ambient materials and high melting temperature and hardness for long lifetime. The dielectric layer <NUM> that covered only the pull-in electrodes <NUM>, <NUM> is replaced with a dielectric layer <NUM> that is deposited on top of pull in electrodes <NUM>, <NUM> and on top of RF line <NUM>. The metal feature <NUM>, the dielectric layer <NUM> and the RF line <NUM> implement a MIM capacitor. The top electrode of this MIM is either electrically floating, when the MEMS bridge is in UP position, or grounded via the ohmic contact between surface material <NUM> and conducting region <NUM>, when the MEMS bridge is in DOWN position.

Claim 1:
A device for testing MEMS hysteresis, comprising:
at least one MEMS digital variable capacitor, DVC, device (<NUM>), the MEMS device comprising a movable plate (<NUM>), an RF electrode (<NUM>), one or more pull-down electrodes (<NUM>) and one or more pull-up electrodes (<NUM>);
a first switch (<NUM>) coupled to either the one or more pull-down electrodes (<NUM>) or the one or more pull-up electrodes (<NUM>), wherein the first switch (<NUM>) is additionally coupled to ground;
a second switch (<NUM>) coupled to either the one or more pull-down electrodes (<NUM>) or the one or more pull-up electrodes (<NUM>); and
a power management system (<NUM>) coupled to the second switch (<NUM>),
wherein the at least one MEMS device (<NUM>), the first switch (<NUM>), the second switch (<NUM>) and the power management system (<NUM>) are all disposed in a single package (<NUM>), and the first switch (<NUM>) and the second switch (<NUM>) are both coupled to the one or more pull-down electrodes (<NUM>), or the first switch (<NUM>) and the second switch (<NUM>) are both coupled to the one or more one or more pull-up electrodes (<NUM>) ;
wherein the power management system causes the movable plate (<NUM>) to move between the one or more pull-down electrodes (<NUM>) and the one or more pull-up electrodes (<NUM>) for testing the hysteresis behaviour of the MEMS digital variable capacitor, DVC device (<NUM>).