WO2015035120A1

WO2015035120A1 - Internally generated dft stepped hysteresis sweep for electrostatic mems

Info

Publication number: WO2015035120A1
Application number: PCT/US2014/054213
Authority: WO
Inventors: James Douglas HUFFMAN; Cong Quoc Khieu; Robertus Petrus Van Kampen; Karl F. Smayling; Vikram Joshi
Original assignee: Cavendish Kinetics, Inc
Priority date: 2013-09-06
Filing date: 2014-09-05
Publication date: 2015-03-12

Abstract

The present invention 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 in a MEMS device to move. The power management circuit may utilize a charge pump, a comparator and a resistor ladder.

Description

INTERNALLY GENERATED DFT STEPPED HYSTERESIS SWEEP FOR

ELECTROSTATIC MEMS

BACKGROUND OF THE INVENTION

Field of the Invention

[0001] Embodiments of the present invention generally relate to a mechanism for testing a micro-electromechanical system (MEMS) hysteresis.

Description of the Related Art

[0002] 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.

[0003] 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.

[0004] Therefore, there is a need in the art for a method and device for effectively measuring the hysteresis curve for a MEMS DVC.

SUMMARY OF THE INVENTION

[0005] The present invention 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 to move between the electrodes in a MEMS device. The power management circuit may utilize a charge pump, a comparator and a resistor ladder. [0006] In one embodiment, a device comprises a first MEMS device having a first electrode, a second electrode, and a plate movable between a first position spaced a first distance from the first electrode and a second position spaced a second distance from the first electrode; a power source coupled to both the first electrode and the second electrode; an ammeter coupled to the first electrode; a voltmeter coupled to both the first electrode and the second electrode; a first switch coupled to the plate and to ground; and a second switch coupled to the plate and to a power management circuit.

[0007] In another embodiment, a MEMS DVC comprises at least one MEMS device, the MEMS device comprising a movable plate, an RF electrode, one or more pull-down electrodes and one or more pull-up electrodes; a first switch coupled to either the one or more pull-down electrodes or the one or more pull-up electrodes, wherein the first switch is additionally coupled to ground; a second switch coupled to either the one or more pull-down electrodes or the one or more pull-up electrodes; and a power management system coupled to the second switch, wherein the at least one MEMS device, the first switch, the second switch and the power management system are all disposed on a semiconductor chip.

[0008] In another embodiment, a method of testing a MEMS DVC, the MEMS

DVC including at least one MEMS device, the MEMS device comprising a movable plate, an RF electrode, one or more pull-down electrodes and one or more pull-up electrodes, is disclosed. The method comprises applying a first voltage to either the one or more pull-down electrodes or the one or more pull-up electrodes; measuring a capacitance of the MEMS device; applying a second voltage to either the one or more pull-down electrodes or the one or more pull-up electrodes; measuring the capacitance of the MEMS device; detecting the capacitance of the MEMS device equals a maximum capacitance of the MEMS device; removing the second voltage from either the one or more pull-down electrodes or the one or more pull-up electrodes; measuring the capacitance of the MEMS device; removing the first voltage from either the one or more pull-down electrodes or the one or more pull-up electrodes; measuring the capacitance of the MEMS device; and detecting the capacitance of the MEMS device is less than the maximum capacitance. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, 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 invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

[0010] Figure 1 is a schematic cross-sectional illustration of a MEMS DVC device in the free standing state.

[0011] Figure 2 is a schematic cross-sectional illustration of the MEMS DVC device in the C_max state.

[0012] Figure 3 is a schematic cross-sectional illustration of the MEMS DVC device in the C_min state.

[0013] Figure 4 is a schematic illustration of a waveform controller driving the MEMS DVC device.

[0014] Figure 5 is a graph showing the hysteresis curve for an electrostatically operated MEMS device.

[0015] Figure 6 is a schematic illustration of a two terminal MEMS device CV configuration.

[0016] Figure 7 is a schematic illustration of a three terminal MEMS device CV configuration.

[0017] Figure 8 is a schematic illustration of a DFT implementation to test a MEMS hysteresis according to one embodiment.

[0018] Figure 9 is a schematic illustration of a power management implementation for MEMS hysteresis testing according to one embodiment. [0019] Figure 10 is a schematic illustration of test methodology for a discrete capacitance hysteresis test using an internal DFT according to one embodiment.

[0020] Figures 1 1A and 1 1 B are schematic illustrations of a DFT implementation to test a MEMS hysteresis according to additional embodiments.

[0021] Figure 12 is a flow chart illustrating a method of testing a MEMS DVC according to one embodiment.

[0022] 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

[0023] The present invention 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.

[0024] 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)². 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.

[0025] The MEMS DVC device may have a control-electrode above (PU- electrode) and below (PD-electrode) the moveable MEMS element, as shown schematically in Figure 1 . In addition an RF-electrode may be present below the moveable MEMS element. As shown in Figures 1 -3, 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 C_max when pulled to the bottom (See Figure 2) to a low capacitance C_min (See Figure 3) 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 Figure 4) to ensure a long-life stable performance of the DVC device. The moveable element is typically on DC-ground.

[0026] As shown in Figure 1 -3, 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 Figure 1 to a C_max state shown in Figure 2 and a C_min state shown in Figure 3. 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.

[0027] Figure 5 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 Figure 1 and has a capacitance Cf_ree- 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 C_max- 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 between the MEMS element is in contact with the dielectric material that is disposed on the RF electrode.

[0028] The Vpi and Vrl are important parameters for the MEMS DVC device; both for upward and downward actuation. If the pull-in voltage Vpi is too high then the waveform controller may not be able to pull the MEMS devices into contact intimately, which can impact the obtainable C_min (upward actuation) or C_max (downward actuation). If the release voltage Vrl 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.

[0029] Both Vpi and Vrl 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 both Vpi and Vrl. In order to screen functional devices that meet all required product specs, it is key to test the Vpi and Vrl on every device. As discussed herein, a built-in test methodology can be used facilitate the test.

[0030] The built-in test methodology is termed as "hysteresis testing." Hysteresis, because the Vpi and Vrl are separated or the pull-in and the release curves do not overlap as shown in Figure 5. For a reliable part (MEMS DVC) Vpi and Vrl are designed to be in a certain range. Otherwise they can result in non performance as explained in above paragraphs. Unlike, C_max and C_min, Vpi and Vrl 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, a measure of Vpi and Vrl, allows for screening the bad parts from good. [0031] 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 Figure 6.

[0032] The MEMS device capacitance is given by the equation, where f is the frequency of the AC voltage source:

CMEMS = iac/2nf^*v_ac

[0033] A three or more terminal electrostatic MEMS device does not allow for the same straightforward CV test as shown in Figure 1 . 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 Figure 1 , but also including a DC source, Vbias, as shown in Figure 7. The DC source is used in the same manner as the DC source in Figure 6.

[0034] 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 Figure 7 to be generated internal to the semiconductor chip and not in an external power supply. This is shown in Figure 8 as a plurality of MEMS transducers each with separate switches to the power management. 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<0:n>, as shown in Figure 8. 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 Figure 7 and the internal DFT mode in Figure 8 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.

[0035] 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 Figure 9, 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 Figure 9, 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<0:n>.

[0036] As shown in Figure 9, 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<0>:

Vactuation = ((Rs+R0)/R0)^*Vbandgap

[0037] The value for Vactuation with a c<1 > programmation is:

Vactuation = ((Rs+R0+R1 )/(R0+R1 ))^*Vbandgap

[0038] The value for Vactuation with a c<n> programmation is:

Vactuation = ((Rs+R0+R1 +R2...+Rn)/(R0+R1 +R2...+Rn))^*Vbandgap [0039] 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.

[0040] The test is implemented in the hardware configuration shown in Figure 10. 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 Figure 10.

[0041] Figures 1 1A and 1 1 B are schematic illustrations of a DFT implementation to test a MEMS hysteresis according to additional embodiments. Figure 1 1A shows an embodiment where the test is performed for voltage applied to the pull-down electrode 1 102 while Figure 1 B shows an embodiment where the test is performed for voltage applied to the pull-up electrode 1 104. It is contemplated that the test may be performed on both the pull-down electrode 1 102 and pull-up electrode 1 104.

[0042] The MEMS device 1 100 includes the pull-down electrodes 1 102, the pull- up electrode 1 104, an RF electrode 1 106 and ground electrodes 1 108. The ground electrodes 1 108 are connected to ground and to the movable plate 1 1 10. A dielectric layer 1 1 12 is disposed over the pull-down electrodes 1 102 and the RF electrode 1 106. Another dielectric layer 1 1 14 is disposed between the pull-up electrode 1 104 and the cavity 1 1 16 within which the movable plate 1 1 10 is disposed.

[0043] As shown in Figure 1 1 A, the pull-down electrodes 1 102 are coupled to multiple switches 1 1 18, 1 120. The first switch 1 1 18, when engaged, connects the pull-down electrodes 1 102 to ground. In Figure 1 1 B, when the first switch 1 1 18 is engaged, the pull-up electrode 1 104 is connected to ground. The second switch 1 120, in Figure 1 1 A, is connected to a power management device 1 122. Similarly, in Figure 1 1 B, the second switch 1 120 is connected to the power management device 1 122. Thus, when the second switch 1 120 is engaged, the pull-down electrode 1 102 (Figure 1 1 A) or the pull-up electrode 1 104 (Figure 1 1 B) is connected to the power management device 1 122. The power management device 1 122 and the MEMS device 1 100 are both disposed on the semiconductor chip represented by box 1 124.

[0044] The power management device 1 122 includes a charge pump 1 128 that is coupled to a gate 1 130. The gate 1 130 is coupled to both the Vclock node and the output from a comparator 1 132. The comparator has inputs from the Vbandgap node and the resistive ladder. The resistive ladder is what divides the output of the charge pump 1 128. The resistive ladder includes a plurality of resistors R0... Rn which are coupled together in series. The address bits c<0>, c<1 >, c<n> are programmed to incrementally "actuate" or operate such that the next voltage level is achieved and the capacitance of the MEMS device 1 100 is measured. Hence, an incremental voltage is applied by operating the address bits c<0>, c<1 >, c<n>. Based upon the incremental voltage increase, the Vpi (for Figure 1 1 A) or Vrl (for Figure 1 1 B) is determined. Similarly, by decrementally decreasing the voltage (i.e., operating the address bits c<0>, c<1 >, c<n>), the capacitance is again measured the Vrl (for Figure 1 1 A) or Vpi (for Figure 1 1 B) is detected. As such, the hysteresis curve for the particular MEMS device 1 100 is determined. It is to be understood that multiple MEMS devices may be coupled to the power management device 1 122. The multiple MEMS devices may collectively operate as a DVC.

[0045] Figure 12 is a flow chart 1200 illustrating a method of testing a MEMS

DVC according to one embodiment. Initially, a low voltage is applied to either the pull-down electrode or the pull-up electrode in step 1202. The voltage applied to the pull-down electrode is to pull the movable plate closer to the RF electrode. Voltage applied to the pull-up electrode, on the other hand, is to push the movable plate closer to the RF electrode. After the voltage has been applied, the capacitance of the MEMS device is measured in step 1204. If the capacitance is equal to the maximum capacitance for the MEMS device, then the pull-in (or push-in for voltage applied to the pull-up electrode) has been determined in step 1206. If the measured capacitance is not equal to the maximum capacitance, then the voltage is incrementally increased in step 1208, with the capacitance measured with each incremental voltage increase in step 1204, until the maximum capacitance is reached and the pull-in voltage has been determined in step 1210.

[0046] Once the pull-in voltage (or push-in voltage) has been determined, the release voltage is determined. The release voltage is determined by reducing the voltage applied to either the pull-down electrode or the pull-up electrode in step 1212. The capacitance is then measured in step 1214. If the capacitance is less than the maximum capacitance, then the release voltage has been determined. If, however, the measured capacitance is equal to the maximum capacitance, then the voltage is decrementally reduced in step 1218. The capacitance is measured for each decremental voltage reduction. If the measured capacitance is less than the maximum capacitance in step 1216, then the release voltage has been determined in step 1220.

[0047] In one particular embodiment of determining the pull-in and release voltages, a first voltage is applied to either the one or more pull-down electrodes or the one or more pull-up electrodes. The capacitance of the MEMS device is measured. A second voltage is then applied to either the one or more pull-down electrodes or the one or more pull-up electrodes. The capacitance is again measured. The capacitance of the MEMS device is detected to equal the maximum capacitance for the MEMS device. The second voltage is then removed from either the one or more pull-down electrodes or the one or more pull-up electrodes. The capacitance of the MEMS device is then measured. If the measured capacitance equals the maximum capacitance, then the first voltage from either the one or more pull-down electrodes or the one or more pull-up electrodes is removed. The capacitance is again measured and if the measured capacitance is less than the maximum capacitance for the MEMS device, the release voltage has been determined. [0048] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

Claims:

1 . A MEMS DVC, comprising:

at least one MEMS device, the MEMS device comprising a movable plate, an RF electrode, one or more pull-down electrodes and one or more pull-up electrodes; a first switch coupled to either the one or more pull-down electrodes or the one or more pull-up electrodes, wherein the first switch is additionally coupled to ground;

a second switch coupled to either the one or more pull-down electrodes or the one or more pull-up electrodes; and

a power management system coupled to the second switch, wherein the at least one MEMS device, the first switch, the second switch and the power management system are all disposed on a semiconductor chip.

2. The MEMS DVC of claim 1 , wherein the power management system comprises:

a charge pump; and

a resistive ladder coupled between the charge pump and the second switch.

3. The MEMS DVC of claim 2, wherein the power management system further comprises a comparator coupled between the charge pump and the resistive ladder.

4. The MEMS DVC of claim 3, wherein the power management system further comprises a gate coupled between the comparator and the charge pump.

5. The MEMS DVC of claim 4, wherein the comparator is coupled to a bandgap voltage node on the semiconductor chip.

6. The MEMS DVC of claim 5, wherein the gate is coupled to a clock voltage node on the semiconductor chip.

7. The MEMS DVC of claim 6, wherein the resistive ladder comprises a plurality of resistors.

8. The MEMS DVC of claim 7, further comprising a plurality of address bits coupled to the resistive ladder.

9. The MEMS DVC of claims 8, wherein the second switch is coupled to the one or more pull-down electrodes.

10. The MEMS DVC of claim 8, wherein the second switch is coupled to the one or more pull-up electrodes.

1 1 . The MEMS DVC of claim 1 , wherein the second switch is coupled to the one or more pull-down electrodes.

12. The MEMS DVC of claim 1 , wherein the second switch is coupled to the one or more pull-up electrodes.

13. The MEMS DVC of claim 1 , wherein the power management system includes a resistive ladder having a plurality of resistors.

14. The MEMS DVC of claim 13, wherein the resistive ladder is coupled to one or more address bits.

15. The MEMS DVC of claim 14, wherein the plurality of resistors are coupled together in series.

16. A method of testing a MEMS DVC, the MEMS DVC including at least one MEMS device, the MEMS device comprising a movable plate, an RF electrode, one or more pull-down electrodes and one or more pull-up electrodes, the method comprising: applying a first voltage to either the one or more pull-down electrodes or the one or more pull-up electrodes;

measuring a capacitance of the MEMS device;

applying a second voltage to either the one or more pull-down electrodes or the one or more pull-up electrodes;

measuring the capacitance of the MEMS device;

detecting the capacitance of the MEMS device equals a maximum capacitance of the MEMS device;

removing the second voltage from either the one or more pull-down electrodes or the one or more pull-up electrodes;

measuring the capacitance of the MEMS device;

removing the first voltage from either the one or more pull-down electrodes or the one or more pull-up electrodes;

measuring the capacitance of the MEMS device; and

detecting the capacitance of the MEMS device is less than the maximum capacitance.

17. The method of claim 16, wherein the first voltage and the second voltage are applied incrementally.

18. The method of claim 17, wherein the second voltage and the first voltage are removed decrementally.

19. The method of claim 16, wherein the first voltage and the second voltage are applied to the one or more pull-down electrodes.

20. The method of claim 16, wherein the first voltage and the second voltage are applied to the one or more pull-up electrodes.