WO2005001863A1 - Dispositifs mems d'autoassemblage a actionnement thermique - Google Patents

Dispositifs mems d'autoassemblage a actionnement thermique Download PDF

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Publication number
WO2005001863A1
WO2005001863A1 PCT/US2004/017137 US2004017137W WO2005001863A1 WO 2005001863 A1 WO2005001863 A1 WO 2005001863A1 US 2004017137 W US2004017137 W US 2004017137W WO 2005001863 A1 WO2005001863 A1 WO 2005001863A1
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WIPO (PCT)
Prior art keywords
materials
leg
actuator
mems
beams
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PCT/US2004/017137
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English (en)
Inventor
Fedder Gary
Oz Altug
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Carnegie Mellon University
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Priority to US10/558,469 priority Critical patent/US7749792B2/en
Publication of WO2005001863A1 publication Critical patent/WO2005001863A1/fr

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B81—MICROSTRUCTURAL TECHNOLOGY
    • B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
    • B81B7/00—Microstructural systems; Auxiliary parts of microstructural devices or systems
    • B81B7/0003—MEMS mechanisms for assembling automatically hinged components, self-assembly devices
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G5/00—Capacitors in which the capacitance is varied by mechanical means, e.g. by turning a shaft; Processes of their manufacture
    • H01G5/16—Capacitors in which the capacitance is varied by mechanical means, e.g. by turning a shaft; Processes of their manufacture using variation of distance between electrodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G5/00—Capacitors in which the capacitance is varied by mechanical means, e.g. by turning a shaft; Processes of their manufacture
    • H01G5/40—Structural combinations of variable capacitors with other electric elements not covered by this subclass, the structure mainly consisting of a capacitor, e.g. RC combinations
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B81—MICROSTRUCTURAL TECHNOLOGY
    • B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
    • B81B2201/00—Specific applications of microelectromechanical systems
    • B81B2201/03—Microengines and actuators
    • B81B2201/038—Microengines and actuators not provided for in B81B2201/031 - B81B2201/037
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01H—ELECTRIC SWITCHES; RELAYS; SELECTORS; EMERGENCY PROTECTIVE DEVICES
    • H01H1/00—Contacts
    • H01H1/0094—Switches making use of nanoelectromechanical systems [NEMS]
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01H—ELECTRIC SWITCHES; RELAYS; SELECTORS; EMERGENCY PROTECTIVE DEVICES
    • H01H61/00—Electrothermal relays
    • H01H2061/006—Micromechanical thermal relay

Definitions

  • MEMS microeletromechanical systems
  • 00538309.DOC they require piezo-electric materials modified by high temperature steps.
  • devices based on electrothermal actuation can provide large forces, large displacements, and low area consumption. They can also operate in an integrated circuit voltage regime ( ⁇ 5N).
  • thermal actuators consume more power than electrostatic actuators.
  • thermal actuators are slower than the electrostatic actuators.
  • thermal time constants are longer than the electrical and mechanical time of constants. To alleviate this problem, the thermal mass of the actuators should be designed as small as possible.
  • Some of the early electrothermal actuator designs are based on the bimorph effect, which relies on the difference of thermal expansion coefficients between two adjacent layers on the device. By heating these layers, a bending moment is created. However such actuators produce deflection in the direction normal to the substrate.
  • One of the microactuator designs by Reithmuller and Benecke with 2.5 ⁇ m thick locally deposited Au layer achieved 90 ⁇ m displacement by using 200 mW power from 0.05mm 2 area.
  • An electrothermal design by Sun and Carr uses the out of plane actuators to produce in-plane deflections. By using both electrothermal and electrostatic actuation at the same time, this actuator design can produce 30 ⁇ m lateral deflection with 40mW power from 0.03 mm 2 area.
  • the displacement is increased from 3 ⁇ m to 33 ⁇ m, but the power is also increased from 40 mW to 375 W as multiple actuators are used.
  • Zero-standby power is achieved by the inchworm designs, which means that the power is only needed during the switching time, not for the on or off cases.
  • Sun, Farmer and Carr developed a similar zero-standby operation design by employing a mechanical latch structure.
  • a RF MEMS switch by Robert et al. is designed by a combination of thermal actuation and electrostatic latching. 400 mW of power is consumed for switching operation, but only 10 V is needed for the electrostatic latch mechanism with close to zero continuous power.
  • the switching time for the electrothermal actuation is 200 ⁇ s.
  • There are several examples of applications in MEMS utilizing electrothermal actuators including: RF MEMS tunable capacitors, RF MEMS switches, an optical fiber micro switch, rotary micro-engines, micro-tweezers, and positioners.
  • Fabrication steps of integrated MEMS, compatible with the present microactuators, may use CMOS post-processing techniques. Structures are made using the CMOS interconnect stack and released with a maskless CMOS micromachining process. The high-aspect-ratio CMOS micromachining technology begins with a conventional foundry CMOS process. After the foundry fabrication, three dry-etch steps, shown in Figure 1, are used to define and release the structure.
  • Figure 1(a) shows the cross section of the chip after regular CMOS fabrication.
  • dielectric layers are removed by an anisotropic CHF 3 /O 2 reactive ion etch (RIE) with the top metal layer acting as an etch
  • RIE reactive ion etch
  • Lakdawala et al made an infrared sensor from two in-plane bi-material beam elements.
  • a stress gradient is created by heating the beams with infrared radiation and is caused from the change in temperature coefficient of expansion of the materials within the beam.
  • the stress gradient produces a bending moment in each beam and causes the beams to move apart.
  • the air-gap capacitance between the beams changes from the beam motion and is detected with a capacitive sensing circuit.
  • MEMS technology has been widely used in applications such as optical communications, wireless systems, automotive sensors, aerospace systems, micro-robotics, chemical sensors, biotechnologies, and micro probes.
  • MEMS applications in the RF and microwave field have seen an enormous growth over the last decade stemming from the superior high frequency performance of RF MEMS switches.
  • other RF and microwave MEMS devices have been designed such as tunable capacitors, inductors, micro-machined transmission lines, micro-mechanical resonators and filters.
  • Q passive components such as inductors, tunable capacitors and switches in RF front-end circuits for low power and low noise receivers.
  • Q passive components such as inductors, tunable capacitors and switches in RF front-end circuits for low power and low noise receivers.
  • Q quality factor
  • VCOs voltage-controlled oscillators
  • RF filters frequency-controlled oscillators
  • MEMS tunable capacitors have advantages of lower loss, larger tuning range and more linear tuning characteristics.
  • MEMS-based RF tunable capacitors can also be classified according to their actuating mechanisms wliich are; electrostatic, electrothermal, and piezo-electric, discussed above.
  • Some of the early gap tuning designs have low tuning ranges, because of the parasitic capacitances coming from interconnects.
  • the parallel-plate capacitor designs with electrostatic actuators have a theoretical 50% tuning range limitation, because the electrodes snap after the gap between them becomes 2/3 of the initial gap.
  • the parallel-plate vertical gap device demonstrated by Young and Boser has a tuning range of 16% and quality factor of 60 at 1 GHz.
  • a NCO is implemented at 714MHz operating frequency with 14 MHz tuning range and a phase noise of -107 dBc/Hz at 100-kHz offset.
  • a modified parallel-plate RF tunable capacitor is designed to increase the tuning range larger than the 50% limit by using three parallel plates.
  • a tuning range of 87% with 4.4 N controlling voltage and Q of 15.4 at 1 GHz are achieved.
  • a NCO is also demonstrated with 24 MHz tuning at 1.336GHz operating frequency and phase noise of -98.5 dBc Hz at 100-kHz offset.
  • a parallel plate design by Zou et al. used a novel electrode design to achieve a tuning range larger than the 50% snap-in limit. For this design, larger gaps are used in the electrostatic actuation mechanism, compared to the gaps between the electrodes of the capacitor.
  • a tuning range of 69% is achieved by using 17 V driving voltages. Designs based on a cantilever beam also achieve tuning ranges larger than 50%.
  • the initial design by Hung and Senturia has a tuning range of 81.8% with 40V controlling voltage. Later designs have Q of 4 at 3 GHz and large tuning range of 354%. with 40 V controlling voltage.
  • Parallel-plate capacitor designs using electrothermal and piezo-electric actuation do not have the 50% tuning range limitation.
  • a parallel-plate capacitor by Feng et al. based on thermal actuation has lower driving voltages around 7 V, compared to the capacitor based on electrostatic actuation. It has also large tuning ranges of 270%. and high-Q factor of 300 at 10 GHz.
  • Yao et al. developed a capacitor based on piezo-electric actuation that has a Q factor of 210 at 1 GHz with a 6 V controlling voltage.
  • the area Inning RF MEMS capacitors are demonstrated to solve the snap- in tuning range limitation, when electrostatic actuation is used.
  • Early devices achieved a tuning range of 300% with 5 V controlling voltage.
  • Interdigitated finger structures with 30 ⁇ m thickness are used for capacitor electrodes and the electrostatic actuation mechanism.
  • Recent designs use thicker and longer finger blocks to increase the tuning range and the quality factor.
  • Tuning ratio of 8.4:1 with 8 V controlling voltage and Q factor of 35 at 2 GHz is demonstrated by using 40 ⁇ m thick finger electrodes. Having a 12 pF nominal capacitance value and Q-factor of 200 at 400 MHz enables UHF filter applications for these capacitor designs.
  • a UHF filter with tuning range of 225-400 MHz is developed with an insertion loss of 6.2 dB and Q factor of 100 for the operating frequency ranges.
  • the most recent capacitor design by Rockwell Science Center achieves a linear tuning characteristic by forming a completely electrically isolated capacitor. Two sets of electrostatic comb drive actuators in opposing directions are used to achieve a linear tuning characteristic.
  • Another area tuning design uses the idea that the dielectric between the electrodes is moved laterally to achieve high-Q factors. Using a dielectric that has large dielectric constant enables high-Q factors, because the same device would have bigger capacitance with the same resistance losses. Q factors of 291 at 1 GHz and a tuning range of 7% with 10 V controlling voltage is demonstrated.
  • the present disclosure is directed to a MEMS device comprising an in- plane, self-assembling beam.
  • Self-assembling means that the beam moves into a desired position upon being released from the substrate or upon the application of non-device specific heating, e.g. heating the entire die. The device may be locally heated as well.
  • Such a beam can be used to build larger devices, such as a MEMS actuator comprising a first leg having a fixed end and a movable end, the first leg having an in-plane, self-assembling construction, and a second leg having a first end
  • a MEMS actuator constructed using the disclosed beam is comprised of a first leg having a fixed portion and two movable ends, the first leg having an in-plane, self-assembling construction, and a second leg having a first end connected to one of the movable ends of the first leg and a second end connected to the other of the movable ends of the first leg, the second leg having an in-plane, self-assembling construction.
  • An actuator is carried by the second leg.
  • the legs of the actuator may each be either a single serially connected pair of beams or a plurality of serially connected pairs of beams in parallel with one another.
  • the present disclosure is also directed to a MEMS device comprised of at least one beam having at least two materials arranged so as to provide an in-plane stress gradient of sufficient magnitude to enable the beam to move in-plane upon the beam's release.
  • the two materials may include, for example, a metal and a dielectric arranged about a dividing line such that there is more metal than dielectric on one side of the line and more dielectric than metal on the other side of the line. With such a construction, lateral stress can be predetermined as a function of beam length.
  • the two materials maybe selected so that one of the two materials has a greater compressive force, also known as compressive residual stress, than the other of the two materials, one of the two materials has a greater tensile force, also known as tensile residual stress, than the other of the two materials, or one of the two materials has a compressive force and the other of the two materials has a tensile force.
  • a thermal heater is provided to thermally induce a stress gradient in a direction opposite to the in-plane residual stress gradient to cause movement in the opposite direction.
  • one of the two materials has a coefficient of thermal expansion greater than a coefficient of thermal expansion of the other of the two materials. It is also possible to choose the two materials such that the in-plane thermal stress gradient is in the same direction as the in-plane residual stress gradient.
  • two such devices may be constructed, with the second device positioned with respect to the first device such that the actuator of the second device may inhibit movement of the first device.
  • the second device may be positioned orthoganally with respect to the first device.
  • the actuator of the second device may carry a surface for engaging the first device.
  • the actuators disclosed herein may be used to build devices such as capacitors and devices having nano-scale gaps.
  • a MEMS variable capacitor has a plurality of capacitive surfaces connected to an actuator of a MEMS device constructed according to any of the embodiments disclosed herein.
  • a MEMS device defining a nano-meter sized gap may be comprised of a stop surface having a recess formed therein so as to define one side of the gap.
  • An actuator or piston of a MEMS actuator constructed according to any of the embodiments disclosed herein is designed to engage the stop surface and to provide the other side of the gap.
  • the present disclosure is also directed to a method of building an in-plane, self-assembling MEMS beam.
  • the method is comprised of: selecting two materials; forming the two materials into a beam such that the beam has an in-plane stress gradient of sufficient magnitude to enable the beam to move in-plane upon the beam's release.
  • the two materials may include a metal and a dielectric arranged about a dividing line such that there is more metal than dielectric on one side of the line and more dielectric than metal on the other side of the line.
  • the lateral stress gradient may be formed so as to be a function of beam length.
  • the two materials may be selected such that one of the two materials has a greater compressive force than the other of the two materials, one of the two materials has a greater tensile force than the other of the two materials, or such that one of the two materials has a compressive force and the other of the two materials has a tensile force.
  • the two materials may additionally be selected such that upon heating, a thermally induced stress gradient is generated that is in a direction opposite to the in-plane stress gradient. In such an embodiment, one of the two materials has a coefficient of thermal expansion greater than a coefficient of thermal expansion of the other of the two materials.
  • the two materials may be selected, in an alternative embodiment, such that the in-plane
  • DOC thermal stress gradient is in the same direction as the in-plane residual stress gradient. Beams with more than two materials may be fabricated with these same general self- assembly properties.
  • the present disclosure is broadly directed to a method for designing new MEMS micro-movers, particularly suited for, but not limited to, CMOS fabrication techniques, that are capable of large lateral displacement for tuning capacitors, fabricating capacitors, self-assembly of small gaps in CMOS processes, fabricating latching structures and other applications where lateral micro-positioning on the order of up to 10 ⁇ m is desired. Lateral micro-positioning larger than 10 ⁇ m is possible by making larger micro-movers, because the displacement scales with device area. Principles of self-assembly and electro-thermal actuation are used for designing micro-movers. In self-assembly, motion is induced in specific beams by designing a lateral effective residual stress gradient within beams.
  • the lateral residual stress gradient arises from purposefully offsetting certain layers of one material versus another material.
  • lower metal layers may be side by side with dielectric layers, both of which are positioned beneath a top metal layer of a CMOS-MEMS beam.
  • TCE temperature coefficient of expansion
  • the lateral TCE gradient is achieved in the same manner as with self-assembly, by purposefully offsetting the lower metal layers with layers of dielectric with respect to the top metal layer of a CMOS-MEMS beam.
  • a heater resistor usually made from a CMOS polysilicon layer, is embedded into the beam or into an adjacent assembly to heat the flexure. When heated, the TCE gradient will cause a stress gradient in the beam, resulting in the electro-thermal actuation.
  • An addition to the beam design is to include vias, available in the CMOS or BiCMOS process, inside the beam between the offset metal layers.
  • the presence of the vias offset in the same manner as the lower metal layers has been shown to create greater stress gradients and bending than designs without the vias.
  • the present disclosure represents an advancement over prior methods of desigmng electro-thermal actuators and micro-movers that enables high displacement magnitude with small geometry. The new ability is to design from layout, and
  • This design is capable of up to 25 ⁇ m lateral displacement with a length of 220 ⁇ m and a width of 22 ⁇ m.
  • the 25 ⁇ m lateral deflection for such dimensions is more area efficient than previous designs.
  • one recent prior actuator design produced 2.7 ⁇ m deflection with 800 ⁇ m diameter circular area.
  • prior work bent-beam electro-thermal actuator designs 800 ⁇ m long and 13.9 ⁇ m wide demonstrated deflection of 5 ⁇ m.
  • CMOS interconnect stack using a maskless CMOS micromachining process.
  • 1st generation actuators were fabricated using Austria Microsystems (AMS) 0.6 ⁇ m CMOS process and Agilent 0.5 ⁇ m CMOS process.
  • 2 nd generation electro-thermal actuator designs in Taiwan Semiconductor Manufacturing Company (TSMC) 0.35 ⁇ m CMOS process and the jazz Semiconductor 0.35 ⁇ m BiCMOS/SiGe process have more area efficiency than the 1st generation designs.
  • TSMC Taiwan Semiconductor Manufacturing Company
  • BiCMOS/SiGe BiCMOS/SiGe process
  • This application further discloses new CMOS-compatible tunable capacitors using the disclosed electro-thermal actuators.
  • Previous designs use separately fabricated CMOS electronics for potential applications like VCOs or tunable filters, as the capacitors have not been CMOS compatible.
  • An advantage of this approach is that CMOS electronics for VCOs and other possible applications can be integrated on the same chip, thereby eliminating losses coming from interconnects
  • Figure 1 illustrates a prior art CMOS-compatible process used to create MEMS devices.
  • Figure 2(a) is a top view looking down on one embodiment of an actuator, including an electro-thermal heater, according to the present disclosure, before the release step;
  • Figure 2(b) illustrates the same actuator after release, demonstrating the self assembly aspect of the present disclosure;
  • Figure 2(c) illustrates the same actuator after electrothermal heating, demonstrating actuation;
  • Figure 2(d) is a cross-section view of two of the beams of the actuator.
  • Figure 2(e) is a cross section view of two of the beams of the actuator using vias offset within the beam.
  • Figure 3 illustrates the results of a Finite Element Analysis simulation used to calculate the lateral displacement of an actuator designed in accordance with the present disclosure.
  • Figures 4(a) through 4 (d) illustrate the operation of a simple latching scheme that can use actuators of the designs disclosed herein to move between and latch in either of two pre-determined positions.
  • Figures 5(a) and 5(b) illustrate the use of actuators of the designs disclosed herein to create nanometer-scale gaps.
  • Figure 6 illustrates how angled surfaces may be used to form a self- aligning latch for use in creating a nanometer-scale gap.
  • Figure 6 (a) illustrates an actuated device as initially released.
  • Figure 6 (b) illustrates the same device moved into position by a process such as electro-thermal actuation, and latched into the desired configuration by the use of an angled sidewall and an angled peg.
  • Figure 7 illustrates how self-assembly mechanisms may be used to hold a micro-mirror or other mechanism actuated out of plane.
  • Figure 7 (a) is a top layout
  • Figure 7 (b) is a side view of the mirror illustrating the placement of the actuator piston clamping onto the mirror.
  • Figure 8 is a scanning electron micrograph (SEM) of a tunable capacitor made in accordance with the 1 st generation design disclosed herein and which uses heaters for electro-thermal actuation.
  • Figure 19 shows SEMs of a tunable capacitor made in accordance with the
  • Figure 9 (a) presents an overview of the tunable capacitor.
  • Figure 9 (b) shows more details of the small finger beams in a disengaged configuration, while Figure 9 (c) shows the small finger beams in an engaged configuration.
  • Figure 10 is an SEM of an actuator made in accordance with the 1 st generation design disclosed herein.
  • Figure 11 is an SEM of an actuator which is anchored from both sides.
  • Figure 12 is a graph showing both the predicted and the measured lateral displacement of an actuator made using the 1 st generation design.
  • Figure 13 is an SCM of an early actuator made in accordance with the 2 nd generation design disclosed herein. Because compressive and tensile residual stresses were different from what was anticipated, the actuator displaced in the direction opposite that which was anticipated.
  • Figure 14 shows the measured lateral deflection of actuators made in accordance with the 2 nd generation actuators. Displacements as a function of input power supplied to both "half size actuator” and “full size actuator” designs are shown.
  • Figure 15 is an SEM of a capacitor made in accordance with the 2 nd generation design disclosed herein. This capacitor uses the "full size" actuators.
  • Figure 16 is an SEM of another capacitor made in accordance with the 2 generation design disclosed herein. This capacitor uses the "half size” actuators.
  • Figure 17 illustrates the measured tuning characteristics of the 2 nd generation design using full size actuators.
  • Figure 18 shows an SEM of another tunable capacitor made in accordance with the 2 nd generation micromover design disclosed herein.
  • Electro-thermal CMOS-MEMS designs are capable of large lateral displacement for tuning capacitors, self-assembly of small gaps, switches and other applications where micro-positioning on the order of 1 to 20 ⁇ m is desired.
  • Motion is induced in specific beams by designing a lateral stress gradient within beams.
  • the lateral stress gradient arises from purposefully offsetting the lower metal layers with respect to the top metal layer of a CMOS-MEMS beam.
  • Of particular benefit is the ability to tailor the lateral stress gradient, and therefore lateral moment, as a function along the beam length. This ability to set an internal moment along the beam arises from different offset and width of the embedded layers.
  • the microstructures can be made from the CMOS interconnect stack using a maskless CMOS micromachining process, however these beams could be made in alternate custom processes, and it is not intended that this disclosure be limited to CMOS processes.
  • the microstructures can also be made in hybrid CMOS/SiGe bipolar processes.
  • the CMOS-MEMS beams are made from metal layers embedded within a dielectric (silicon oxide and silicon nitride). The offset layers do not have to be metal, and do not have to be embedded in dielectric. However, this particular design technique is particularly advantageous in CMOS-MEMS, because no special processing must be done to implement the designs.
  • the actuator 10 is comprised of a first leg 12 having a first movable end 14 and a second movable end 16.
  • the first leg is anchored in the middle to an anchor 18.
  • the actuator 10 is further comprised of a second leg 20 having a first movable end 22 connected to the first movable end 14 of the first leg 12 and a second movable end 24 connected to the second movable end 16 of the first leg 12.
  • the second leg carries a movable piston 26.
  • the first leg 12 and second leg 20 may each be comprised of a pair of single beams 28, 28 and 28', 28', respectively, or a plurality of pairs of parallel beams may be provided as shown to provide increased stiffness.
  • the metal layers inside the flexure beams 28, 28 'of each leg are offset to one side of the beam, e.g. arranged around an imaginary dividing line 30 as seen in
  • CMOS processes generally have lower residual stress in both the aluminum and silicon oxide layers. Some CMOS processes provide aluminum layers with compressive residual stress. In all cases, efficient CMOS-MEMS electrothermal actuators can be designed.
  • the stiffness of the flexure in Figure 2 can be modified independent of the deflection.
  • the particular design in Figure 2 employs 12 beams. Additional beams, ideally in a symmetric arrangement, may be added to increase the stiffness. This is a great advantage over electrostatic microactuation schemes.
  • a simplified version of the actuator 10 may be constructed by reducing leg 12 to one beam 28 extending from anchor 18 to movable end 14 and reducing leg 20 to one beam 28', connected at one end to end 14 and carrying the actuator 26 at substantially the other end of beam 28'.
  • Such a design is sometimes referred to herein as a "half size actuator".
  • additional beams may be added in parallel with beams 28, 28' to provide increased stiffness.
  • the particular form of the actuator, half size vs full size, single vs multiple beams in parallel, etc. are matters of design choice dictated by such factors as layout space available, geometry of the available space, required force, etc. The present disclosure is not to be limited by such matters of
  • Electrothermal actuation can be applied to any designed actuator.
  • the heating is implemented in CMOS-MEMS by embedding a polysilicon resistor 32 inside the beam as shown in Figure 2(c). Current flowing through the resistor generates the heating power. Motion is induced from the different Temperature Coefficient of Expansion (TCE) of the metal offset layers and the rest of the beam material (silicon oxide).
  • TCE Temperature Coefficient of Expansion
  • the offset aluminum layers have a much larger TCE than the surrounding silicon oxide.
  • Figure 2(e) illustrates a modified beam construction in which a plurality of vias are added, as closely packed as desired, to increase the amount of metal. Other methods for increasing the amount of metal, e.g. filling a trench, maybe employed depending upon the fabrication process being employed.
  • Displacement magnitudes of the lateral actuation can be verified quantitatively by finite element analysis (FEA). For FEA, a simulation temperature, T Set , is calculated to model the lateral actuation magnitude upon release of the actuator.
  • FEA finite element analysis
  • T S j m is the simulator initial temperature, usually 273 K
  • Td is the ambient temperature.
  • T 0 denotes the characteristic temperature at which the beam has zero deflection.
  • Figure 3 shows a simulation result for lateral displacement of the 1 st generation actuator design in AMS 0.6 ⁇ m CMOS process with a length of 220 ⁇ m and a width of 22 ⁇ m at 113 °C. As it can be seen from Figure 3, the lateral displacement magnitude at the tip of the actuator is 3.78 ⁇ m. Lateral displacements
  • Electro-thermal actuators consume more power in continuous operation compared to electrostatic actuators.
  • the continuous power required to sustain displacement can be reduced to mW levels, however this is still too large for some applications.
  • Many variations of latch mechanisms can be designed.
  • the latch principle shown in Figure 4 is not meant to be comprehensive, but illustrates the general concept.
  • the simplest latch uses two actuators 10. One actuator actuates a device, for example, a tunable capacitor plate (not shown) while the second actuator latches the latch 40.
  • the latch 40 shown in Figure 4 is a peg/slot configuration, although any type of latch may be implemented, including a clutch-like latch in which one surface, which may be corrugated or smooth, on the movable piston 26 of one actuator engages the other actuator and prevents further movement.
  • the peg can be any shape that mates together with the corresponding slot shape.
  • a simple rectangular shape is shown in Figure 4.
  • the peg may be located on the "latch” actuator with the slot located on the "device” actuator, as shown in Figure 4, or their respective locations may be swapped.
  • the two actuators may be of the same design, or may be of a different design from one another, and may take any of the previously discussed designs, i.e. they need not be limited to the full size design shown in the figure.
  • the process starts at zero power with the peg and one of the slots in an engaged position.
  • This engaged position may be formed through self- assembly of the actuators, or may be formed as drawn in the layout if there is little or no motion from self-assembly.
  • the device is then set to a new position by a set of sequential steps.
  • the latch actuator is electro- thermally actuated, pulling the peg away from the first slot.
  • the device actuator is electro-thermally actuated to a new position corresponding to a different second slot.
  • heating power to the latch actuator is turned off, and the peg becomes engaged with the second slot.
  • the fourth step which results in a configuration identical to that shown in Figure 4(d)
  • heating power to the device actuator is turned off, and the slot and peg remain in contact with each other keeping the device in its new position.
  • the latch actuator must be designed with adequate mechanical stiffness to hold the peg in place.
  • the stiffness of the latch mechanism in the direction of the device actuator's displacement should be at least 10 times greater than the stiffness of the device actuator.
  • a bistable latch mechanism with two slots is shown in Fig. 4, however any number of slots can be designed as long as the actuator stroke can accommodate the slots. Any number of latch mechanisms can be used with a given device, as long as there is layout area to fit the required actuators.
  • An application of the actuators and latch mechanisms is in assembling lateral nanometer-scale sidewall gaps for large capacitance and large electrostatic force per area. Desirable sizes range from about 50 nm or less to 500 nm. These nanometer-scale gaps are particularly useful for improving the performance of high- frequency nanoresonator devices.
  • Conventional optical lithography limits gap width in the CMOS microstructures to around 0.5 ⁇ m. Smaller gaps have been made in some other micromachining processes, for example by forming a thin sacrificial oxide layer between silicon or polysilicon electrodes. In our assembly approach, the gap as drawn in layout is much larger than the final nanometer-scale dimension, as shown in Figure 5.
  • One electrode of the gap may be a nanoresonator, exemplified by the fixed- fixed beam 50 in Figure 5(a).
  • the beam 50 has a recess 52 formed thereinbetween limit stops 54, the recess 52 defining one wall or electrode of the gap.
  • DOC wall/electrode of the gap is defined by the piston 26 of the actuator 10.
  • the actuator 10 shifts the piston/electrode 26 upward in the figures to narrow the gap.
  • the rigid limit stops 54 set the desired final gap value or dimension as shown in Figure 5(b). Because the limit stop edge and the beam edge face the same direction, any overetch or underetch in the processing will not affect the gap dimension. Theoretically, the gap can be set to an arbitrarily small value, however the practical minimum gap is determined by the surface roughness of the sidewalls, estimated to be less than 50 nm. A self-assembly actuator of the present disclosure is ideal for this application, requiring zero power.
  • CMOS processes have such low residual stress gradients that making self-assembly actuators is impractical.
  • an electro-thermal actuator shifts the piston/electrode 26 and closes the gap.
  • a self- aligning latch mechanism can be included.
  • FIG. 6 One embodiment of a self-aligning latch is shown in Figure 6.
  • -An angled sidewall surface is located on the side of the piston/electrode 26.
  • An angled peg is designed to mate with the sidewall surface.
  • the peg is actuated with a second thermal actuator (not shown).
  • a second thermal actuator (not shown).
  • Both actuators are actuated as shown in Figure 6(a).
  • the peg moves away from the device and allows the first actuator to narrow the gap.
  • the latch actuator is then turned off, moving the peg into place and mating with the angled sidewall surface as shown in Figure 6(b).
  • the device displacement is designed to be slightly smaller than the distance between the mating surfaces as drawn in layout.
  • the two angled surfaces self-align and keep the electrode pushed into its final position creating the nanometer-scale gap.
  • the latch actuator must be designed with adequate mechanical stiffness to hold the peg in place.
  • the combination of moving electrode with limit stop and latch can be used for any other application requiring a static deflection on the order of up to 10 ⁇ m with zero power.
  • Another application of the disclosed self-assembly mechanisms and actuators is to hold at a fixed position mechanisms that are actuated out-of-plane.
  • One example is in holding micro-mirror mechanisms 60 at a specific angle as shown
  • the micro-mirror is positioned at an angle out of-plane using electro-thermal or electrostatic actuators 66.
  • One or more self-assembly actuators 10 are actuated to release the micro-mirror 60 while it is being actuated out- of-plane. Once the micro-mirror 60 is positioned at the desired angle, actuation of the self-assembly actuators 10 is turned off.
  • the pistons 62, 64 of the self-assembly actuators 10 are designed to clamp onto the micro-mirror 60 in this off state. This use of self-assembly actuators 10 to hold mechanisms in place extends to micro- mechanisms other than micro-mirrors 60.
  • Interdigitated beams are used to create sidewall capacitors in our designs, as our structures are currently constrained to a single mechanical layer.
  • Some previous tunable capacitor designs use parallel plates with gaps out of the plane of the substrate.
  • Our tunable capacitor designs can be classified into two categories based on their tuning schemes. Both comb “gap tuning” and “area tuning” are used for 1 st generation capacitors, and only gap tuning is used for the 2 nd generation capacitors.
  • Tunable capacitors were fabricated using the AMS 0.6 ⁇ m and Agilent 0.5 ⁇ m CMOS processes. Elecfro-thermal actuators are used for our MEMS tunable capacitor design (See Figure 8). In contrast, some previous works used electrostatic actuators.
  • Polysilicon resistors act as heaters inside the inner frame. Upon heating the structure, the interdigitated beams curl down vertically (out of the substrate surface plane) and also curl laterally (within the substrate surface plane). The vertical curling changes the area between interdigitated beams for tuning, and is called "area tuning.” As mentioned previously, the reason of this curling behavior is that metal and oxide layers inside the beams have different TCE values. The lateral bending changes the gap between interdigitated beams for tuning, called “gap tuning.” Gap tuning is achieved by exploiting the new elecfro-thermal actuator designs. [0062] Other capacitors fabricated using TSMC 0.35 ⁇ m CMOS process are shown in Figure 9. Gap tuning is again achieved by use of electro-thermal actuators.
  • Figure 9(a) shows a 2 nd generation released tunable MEMS capacitor with half-sized electrothermal actuators. This "half-actuator" design uses only one half of the folded-flexure
  • Fig. 9(b) and 9(c) show a closer view of the finger parts of the tunable capacitors.
  • the disengaged mechanism is shown in Fig. 9(b) after release without elecfro-thermal actuation.
  • Fig. 9(c) shows the engaged mechanism with actuation of 12N control voltage.
  • lateral elecfro-thermal actuators are used for implementation of lateral latch structures.
  • the latch structures in the first TSMC 0.35 ⁇ m chip did not work, as the intended self-actuation displaced in the opposite direction from the other CMOS processes used in our past experiments.
  • Functional latches in future designs will require no power to operate statically at a given capacitance value. Previous designs consume continuous power for tuning.
  • the lateral displacements are measured with an optical profilometer and an MIT microvision system.
  • the actuators in AMS 0.6 ⁇ m CMOS displaced 6.8 ⁇ m, upon release.
  • Figure 10 shows the scanning electron micrograph (SEM) of the released actuator. When this actuator is heated, the structure refracts back toward its layout shape. For a controlled actuation of 4.0 ⁇ m, the structure must be heated to 150°C.
  • Figure 11 shows a different elecfro-thermal actuator design, which is anchored from both sides.
  • this design Upon release, this design with a length of 100 ⁇ m and a width of 11 ⁇ m displaced 1.2 ⁇ m, which is much less compared to the folded-flexure design.
  • the extensional stress in the fixed-fixed design limits the effectiveness of the actuation.
  • Figure 12 shows the simulated and measured lateral displacements for the actuator at different temperature values. There is a match between experiment and simulation data of within 10%. Table 2, shows measured lateral displacement for different size of actuators in the AMS 0.6 ⁇ m process.
  • the actuator designs in the TSMC 0.35 ⁇ m process were tested by applying DC voltage to the polysilicon heaters, as shown in Fig. 14. There are two types of actuators designated as half size and full size. The full size actuator is shown in Figure 15, and the half size actuator is shown in Figure 16. Half-size actuators have advantages compared to full-size actuators of more area efficiency and less power consumption. For a half-size actuator with a length of 124 ⁇ m and a width of 22 ⁇ m, 5.5 ⁇ m actuation is measured with 8.4mW of heating power in air. [0067] Second-generation elecfro-thermal actuator designs were fabricated in TSMC 0.35 ⁇ m CMOS.
  • Figure 15 and Figure 16 show SEMs of the two different actuator designs as implemented for RF MEMS tunable capacitors.
  • full- size actuators include embedded polysilicon heaters.
  • the half-size actuators are used. The desired tuning operation was successfully tested with polysilicon heaters.
  • Half-size actuators have advantages compared to full-size actuators of two times more area efficiency and almost half of the power consumption. The disadvantage of the half-size actuators is less stiffiiess. Additional beams, ideally in a symmetric arrangement, may be added into half-size actuators to
  • Sn parameters of the 1 st and 2 nd generation tunable capacitors are measured using an Agilent E8364A network analyzer from 45 MHz to 3 GHz.
  • the 1 st generation designs have low tuning ratios compared to the 2 nd generation designs.
  • the reason for this low tuning range in AMS 0.6 ⁇ m CMOS process is the excessive lateral beam curling, which causes the fingers to snap together.
  • the reason for the low tuning range in the Agilent 0.5 ⁇ m CMOS process is the behavior of the elecfro-thermal actuators in that process; they displaced the opposite way of the intended direction, upon release, as noted above.
  • Fig. 17 shows the measured tuning characteristics of 2 nd generation design in TSMC process with full-size actuators.
  • the tuning looks linear, but the problem is the high tuning range is coming from the ratio of the capacitances at 12 N and 6 N control voltages. So with zero control voltage, the capacitance value is 95 fF, then it goes to 42 fF at 6 N control voltage, and achieves the highest value, 148 fF, at 12 N.
  • the increase in capacitance for voltage above 6 N is due to the parallel-plate gap closing between adjacent banks of combs.
  • This particular actuator design did not self- assemble in the intended fully engaged position due to the low residual stress. However, the elecfro-thermal actuation worked successfully as intended.
  • the implementation in high-frequency electronics processes is particularly attractive for making tuning passives for RF applications.
  • a difference between the current work on actuators and prior similar work is the ability to design from layout, and thereby tailor, the lateral sfress gradients and gradients of temperature coefficient of expansion into the actuation beams. This ability enables high displacement magnitude with small geometry and the generation of lateral movement without rotation.
  • the large actuation stroke in CMOS processes is particularly attractive for future tunable RF capacitors with large C on :C 0ff ratio, and is an area of our active research.
  • the impact of successfully creating such tunable on-chip passive components is in enabling the design of system-on-chip receivers.
  • One of the biggest difficulties in designing tunable passives on chip is obtaining adequate area efficiency.
  • Figure 18 illustrates an SEM of another tunable capacitor made in accordance with the 2 nd generation micromover design disclosed herein using the Jazz Semiconductor 0.35 ⁇ m BiCMOS/SiGe process.

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Abstract

L'invention concerne de manière générale un procédé de conception de nouveaux éléments micromoteurs MEMS, particulièrement destinés, entre autres, à des techniques de fabrication CMOS, qui peuvent opérer un déplacement latéral important pour des condensateurs d'accord, des condensateurs de fabrication, l'autoassemblage de petits espacements dans des procédés CMOS, des structures de blocage et d'autres applications pour lesquelles un micropositionnement latéral de l'ordre de 10 µm ou plus est désiré. Les principes de l'autoassemblage et de l'actionnement électrothermiques sont utilisés pour la conception d'éléments micromoteurs. Pour l'autoassemblage, le mouvement est induit dans des montants spécifiques par conception d'un gradient latéral de contrainte résiduelle efficace à l'intérieur des montants. Ce gradient latéral de contrainte résiduelle est produit par le décalage intentionnel de certaines couches d'un matériau par rapport à un autre. Par exemple, des couches métalliques inférieures peuvent être disposées côte à côte avec des couches diélectriques, lesdites couches métalliques et diélectriques étant positionnées sous une couche métallique supérieure d'un montant CMOS-MEMS. Pour l'actionnement électrothermique, le mouvement est induit dans des montants spécifiques par conception d'un gradient latéral de coefficient de température de dilatation (TCE) à l'intérieur des montants. Le gradient latéral de TCE est obtenu de la même manière que pour l'autoassemblage, par décalage intentionnel des couches métalliques inférieures et de couches diélectriques par rapport à la couche métallique supérieure d'un montant CMOS-MEMS. Une résistance chauffante, généralement constituée d'une couche polysilicium CMOS, est intégrée dans le montant ou dans un ensemble adjacent afin que le montant soit chauffé. Lorsque ce dernier est chauffé, le gradient TCE provoque un gradient de contrainte dans le montant, ce qui entraîne l'actionnement électrothermique.
PCT/US2004/017137 2003-06-02 2004-06-02 Dispositifs mems d'autoassemblage a actionnement thermique WO2005001863A1 (fr)

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2884960A1 (fr) * 2005-04-25 2006-10-27 Commissariat Energie Atomique Micro-condensateur electromecanique a capacite variable et procede de fabrication d'un tel micro-condensateur
CN103346699A (zh) * 2013-06-17 2013-10-09 上海大学 基于电热驱动的v+u型复合梁微致动器及其制备方法
US10332687B2 (en) 2017-10-23 2019-06-25 Blackberry Limited Tunable coplanar capacitor with vertical tuning and lateral RF path and methods for manufacturing thereof
US10497774B2 (en) 2017-10-23 2019-12-03 Blackberry Limited Small-gap coplanar tunable capacitors and methods for manufacturing thereof
DE102020214445B3 (de) 2020-11-17 2022-05-05 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung eingetragener Verein Mems-aktor und verfahren zum steuern einesmems-aktors

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US6114794A (en) * 1996-12-16 2000-09-05 Cronos Integrated Microsystems, Inc. Thermal arched beam microelectromechanical valve
WO2000067268A1 (fr) * 1999-05-03 2000-11-09 Cronos Integrated Microsystems, Inc. Structures et reseaux d'actionneurs microelectromecaniques echelonnables a capacite de deplacement en plusieurs dimensions
EP1089109A2 (fr) * 1999-09-27 2001-04-04 JDS Uniphase Inc. atténuateur optique variable MEMS

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US6114794A (en) * 1996-12-16 2000-09-05 Cronos Integrated Microsystems, Inc. Thermal arched beam microelectromechanical valve
WO2000067268A1 (fr) * 1999-05-03 2000-11-09 Cronos Integrated Microsystems, Inc. Structures et reseaux d'actionneurs microelectromecaniques echelonnables a capacite de deplacement en plusieurs dimensions
EP1089109A2 (fr) * 1999-09-27 2001-04-04 JDS Uniphase Inc. atténuateur optique variable MEMS

Cited By (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2884960A1 (fr) * 2005-04-25 2006-10-27 Commissariat Energie Atomique Micro-condensateur electromecanique a capacite variable et procede de fabrication d'un tel micro-condensateur
EP1717830A1 (fr) * 2005-04-25 2006-11-02 Commissariat A L'Energie Atomique Micro-condensateur électromécanique à capacité variable et procédé de fabrication d'un tel micro-condensateur
US7558046B2 (en) 2005-04-25 2009-07-07 Commissariat A L'energie Atomique Variable-capacitance electromechanical micro-capacitor and method for producing such a micro-capacitor
CN103346699A (zh) * 2013-06-17 2013-10-09 上海大学 基于电热驱动的v+u型复合梁微致动器及其制备方法
CN103346699B (zh) * 2013-06-17 2016-12-07 上海大学 基于电热驱动的v+u型复合梁微致动器及其制备方法
US10332687B2 (en) 2017-10-23 2019-06-25 Blackberry Limited Tunable coplanar capacitor with vertical tuning and lateral RF path and methods for manufacturing thereof
US10497774B2 (en) 2017-10-23 2019-12-03 Blackberry Limited Small-gap coplanar tunable capacitors and methods for manufacturing thereof
US10770540B2 (en) 2017-10-23 2020-09-08 Nxp Usa, Inc. Small-gap coplanar tunable capacitors and methods for manufacturing thereof
US10825612B2 (en) 2017-10-23 2020-11-03 Nxp Usa, Inc. Tunable coplanar capacitor with vertical tuning and lateral RF path and methods for manufacturing thereof
DE102020214445B3 (de) 2020-11-17 2022-05-05 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung eingetragener Verein Mems-aktor und verfahren zum steuern einesmems-aktors
WO2022106492A1 (fr) 2020-11-17 2022-05-27 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Actionneur de mems et procédé de commande d'actionneur de mems

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