Patent ID: 12215020

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Introduction

A typical MEMS device includes a substrate, typically made of crystalline silicon and a suspended element, which is often formed from a portion of the substrate using integrated circuit fabrication techniques. The suspended element is connected to a fixed part of the substrate by one or more flexures, which may also be formed from a portion of the substrate. The flexures are typically made long and thin to permit movement of the suspended element relative to a fixed part of the substrate. A drive beam connected to the suspended element provides a mechanical contact to an actuator, which may be a comb-drive actuator, a gap-closing actuator or other type of actuator. In some actuator designs, components of the actuator may also be fabricated from a portion of the substrate.

FIG.1is a simplified schematic top-view diagram of an example of a MEMS device100in which a microstructure which is suspended over an etched cavity and is able to mechanically actuate in rotation or in other degrees of freedom. Specifically, a suspended element101includes a large stiff portion that is tethered to anchors102and103via flexures104, e.g., in the form of torsion springs, which allow the suspended element101(also referred to as rotator) to rotate about an axis105, but mostly constrains the suspended element from lateral or vertical translation and rotation in all other directions. The diagram shows the top view of the structure which is typically defined by a photo-mask in a silicon etching process. Its thickness into the page of the figure is typically uniform, e.g. about 50 microns (μm), and is defined by the thickness of the silicon layer that is etched to form the rotator and, typically but not invariably, the anchors102,103and flexures104. In a typical embodiment the thickness of the rotator is the thickness of an SOI wafer device layer. The anchors102and103are attached to a fixed part110of substrate111, e.g., a SOI wafer handle layer. This fixed part110has a cavity106etched into it to allow the MEMS rotator structure its movement. The device shown inFIG.1may be used to provide some actuating force to another part of the MEMS device such as e.g. a MEMS mirror. A drive beam107, which is also suspended over the etched cavity106, conveys the rotation of the rotator shuttle101to a remotely attached structure (e.g. a MEMS mirror) which is not shown in the figure. The drive beam107may also be formed from the same substrate as the suspended element101. It should be noted that the rotation can be imparted to the suspended element by electrostatic, electromagnetic, thermal, or other means, the details of which are also not shown inFIG.1.

The MEMS rotator as schematically shown inFIG.1is typically used in standard air environment, at nearly 1 atm of pressure and room temperature. In special cases with hermetic packaging it may be surrounded with a different backfill gas such as N2, SF6, etc., or at different pressures. In all cases of nearly atmospheric pressure conditions the structure's mechanical damping is typically dominated by losses to the surrounding fluid (gas, air). These losses are relatively very low and therefore the structures typically have very high quality factors on the order of 100. Thus, they can be difficult to control when actuated as it is easy to initiate resonant/ringing response. Furthermore this results in high susceptibility to shock and vibrations which is not desired. Therefore, it is desired to lower the quality factor of the structure by increasing damping by mechanical means, possibly providing nearly critical underdamping or even exactly critical damping with Q=0.5.

To provide desired mechanical properties, a viscoelastic medium is sometimes incorporated into a gap between gap between sidewalls of both the fixed part of the substrate and either the suspended element or the drive beam or both the suspended element and drive beam. The viscoelastic medium provides a damping mechanism that controls mechanical properties such as resonance. The amount of damping is difficult to control.

The viscoelastic medium may be a silicone-based gel or oil or another type of viscoelastic gel or fluid. The choice of medium can be based on a number of important parameters of performance. Firstly the material should be possible to dispense in correct quantities to the appropriate locations on the device, therefore its viscosity cannot be too high. Secondly, the material should have a favorable viscoelastic damping “tangent delta” (tan δ) property. tan δ indicates how much energy is absorbed (changed into heat) by a material when it deforms. Namely, since the medium will exhibit both loss-less elastic properties and lossy viscous properties, it is important that there is a favorable high ratio of the lossy component of its viscoelasticity over the loss-less component. Namely it is preferred that the medium contributes to the overall damping coefficient of the MEMS device and that it has minor or no effect on the overall stiffness of the MEMS device.

Aspects of the present disclosure improve control of viscoelastic damping in MEMS devices by incorporating one or more fluid confinement structures on either the fixed part of the substrate and/or on the suspended element101(i.e. the fixed part, the suspended element or both) to confine a viscoelastic fluid in a limited part of a gap between one or more sidewalls of both the fixed part110of the substrate and either the suspended element101or the drive beam107or both the suspended element101and drive beam107. In this way a first portion of the gap is bridged by the fluid and second portion of the gap is not so bridged. The structures are configured to prevent flow of the fluid to other parts of the gap.

The fluid confinement structures may include one or more sharp edges on either the fixed part of the substrate and/or on the suspended element configured to restrict the flow of the fluid to prevent flow of the fluid to other parts of the gap.

The sharp edges may include corners formed by one or more of the sidewalls of the fixed part of the substrate and/or the suspended element and/or the drive beam. Such corners may include one or more 90 degree corners. In some implementations, the corners may include sidewall corners of the suspended element101or drive beam107that are opposite corresponding vertical sidewall corners of the fixed part110of the substrate. Other possible implementations include structures that protrude from or are recessed into the drive beam107, suspended element101or fixed part110of the substrate111or fingers that protrude into the gap from the drive beam107, suspended element101or fixed part110of the substrate111.

In some implementations, a region of the fixed part110of the substrate may be configured to act as a reservoir for the viscoelastic fluid so that the fluid can flow from the larger reservoir to the gap until the limited part of the gap is filled with the viscoelastic fluid.

In other implementations, a winglet structure may be connected to the drive beam107which in part is configured to be suspended above the fixed part110of the substrate111with a vertical gap between the winglet structure and the fixed part110of the substrate111such that the suspended element is movable vertically above the fixed part of the substrate without making contact with the fixed part of the substrate. A viscoelastic fluid may be placed in the vertical gap between the winglet structure and fixed part of the substrate.

According to aspects of the present disclosure certain mechanical structures (e.g., winglets), which can be designed as part of a MEMS device structure, can increase the structure's damping in the surrounding fluid. Moreover confinement structures can be designed to confine liquids or special viscoelastic fluids in such a way that the liquids remain in place after device manufacture and provide significantly higher damping to the structure above that provided by the surrounding gas itself.

In some implementations the suspended element101may be formed from the substrate111.

In some implementations the one or more flexures104may be formed from the substrate111.

In some implementations the drive beam107may be formed from substrate111.

In some implementations two or more of the suspended element101, the one or more flexures104, or the drive beam107may be formed from the substrate111.

In some implementations the suspended element101, the one or more flexures104, and the drive beam107may all be formed from the substrate111.

“Formed from the substrate” means that the respective element was originally part of the substrate and is formed using MEMS technique like etching.

In some implementations the limited part of the gap may correspond to no more than 20% of an outline of the suspended element, the flexures, and drive beam.

In some implementations, devices in accordance with aspects of the present disclosure may further include a structure configured to confine the viscoelastic fluid to the limited part of the gap. Such confinement structures may include, e.g., one or more winglets that protrude into the gap from the fixed part of the substrate, the suspended element, the flexures, or the drive beam.

There are a number of ways in which the viscoelastic fluid may contact the device. By way of example, and not by way of limitation, in some implementations, the viscoelastic fluid may contact the drive beam but not the suspended element. In some such implementations, but not all, the viscoelastic fluid may contact the drive beam close to an axis of rotation of the suspended element.

There are a number of different ways in which the viscoelastic fluid may be disposed in the limited part of the gap. By way of example and not by way of limitation, a device in accordance with aspects of the present disclosure may further include a fluid reservoir formed in the fixed part of the substrate in fluid communication with the limited part of the gap via channel also formed in the fixed part of the substrate. In some such implementations, but not all, the fluid reservoir may be sized to receive the viscoelastic fluid from an applicator device that is too large to apply the viscoelastic fluid directly to the limited part of the gap and is still small enough to confine the viscoelastic fluid by capillary forces. The channel may be configured to communicate fluid from the reservoir to the gap by capillary forces.

FIG.2is a schematic top-view diagram of a MEMS device200having a suspended element201connected to anchors202,203by torsion flexures204that allow the suspended element to rotate about an axis205and impart movement to a drive beam207. The aforementioned components may be configured as shown inFIG.1and described above, e.g., with the anchors attached to a fixed part210of a substrate and the suspended element201and drive beam207free to move in a cavity206formed in the fixed part of the substrate211. In this implementation, a viscoelastic fluid208is confined in the cavity206to a portion of a gap between the drive beam207and the fixed part of the substrate. Aspects of the present disclosure are not limited to this configuration; alternatively the fluid may be confined to part of a gap between the suspended element201and the fixed part210of the substrate211. In other configurations the fluid may be confined to part of one or more gaps between the drive beam207and the fixed part210of the substrate211and to one or more gaps between the suspended element201and the fixed part210of the substrate211. As noted above, rotation can be imparted to the suspended element201by any suitable form of actuator (not shown), e.g., electrostatic gap closing actuators, electrostatic comb-drive actuators, electromagnetic actuators, thermal actuators, and the like.

A number of variations on the device shown inFIG.2are possible. Specifically, one or more of the aforementioned MEMS device components may include confinement structures configured to confine the viscoelastic fluid to a limited part of the gap.FIG.3depicts one possible example, among others, of such a MEMS device300. In this example, a suspended element301is connected to anchors302,303by torsion flexures304that allow the suspended element to rotate about an axis305and impart movement to a drive beam307. The aforementioned components may be configured as shown inFIG.1andFIG.2and described above, e.g., with the anchors attached to a fixed part310of a substrate311and the suspended element301and drive beam307free to move in a cavity306formed in the fixed part310of the substrate311. In this implementation, a viscoelastic fluid308is disposed in a reservoir309that is connected to the cavity by a channel312. The fluid is confined in the cavity306to a portion of a gap between the drive beam307and the fixed part of the substrate. The relatively large size of the reservoir compared to the width of the gap facilitates application of the fluid to the device and also facilitates confinement of the fluid. The degree of confinement is largely a function of the viscosity and surface tension of the fluid and the size of the channel312and the gap between the drive beam and fixed part of the substrate. Aspects of the present disclosure are not limited to this configuration; alternatively the fluid may be confined to part of a gap between the suspended element301and the substrate311. In other configurations the fluid may be confined to part of one or more gaps between the drive beam307and the fixed part310of the substrate311and to one or more gaps between the suspended element301and the fixed part310of the substrate311.

Aspects of the present disclosure include other ways of confining viscoelastic fluid in gap. Some examples are shown inFIG.4A-4C. In these examples, viscoelastic fluid is confined to a portion of a gap between a moveable portion407of a MEMS device (e.g., a drive beam or suspended element or possibly both) and a fixed part410of a substrate. Confinement structures409,411,413are shown on both the moveable portion407and fixed part410; however, in some implementations it may be sufficient for the confinement structures to be on one or the other but not necessarily both.

By way of non-limiting example, in the implementation shown inFIG.4A, the confinement structures are in the form of structures409that protrude into a limited part of the gap in a direction perpendicular to an axis of the movable portion407. The protruding structures409provide corners C that confine the fluid through viscoelastic forces between the fluid and the moveable portion407and/or the fixed part410. By way of non-limiting example, the corners C may be right-angle corners. In the illustrate example, there are two protruding structure409on the moveable portion407and two corresponding protruding structures409on the fixed part410aligned with their counterparts on the moveable portion in a mirror-image fashion. Surface tension confines the fluid408to a limited part of the gap corresponding to the distance between inside corners C of the protruding structures409.

In the non-limiting example shown inFIG.4B, the confinement structures are in the form of recessed portions411of the moveable portion407and fixed part410. These structures provide corners C (e.g., right angle corners) that confine the fluid through viscoelastic forces between the fluid and the moveable portion407and/or the fixed part410. In the illustrate example, there are two recessed portions on the moveable portion407and two corresponding recessed portions411on the fixed part410aligned with their counterparts on the moveable portion in a mirror-image fashion. Surface tension confines the fluid408to a length of the gap corresponding to the distance between inside corners C of the recessed portions411.

The non-limiting example shown inFIG.4Cis a variation on the example ofFIG.4B. Here the confinement structures are in the form of a single protruding structure413on the moveable portion407and a corresponding mirror image protruding structure on the fixed part410. These structures413provide corners C (e.g., right angle corners) that confine the fluid through viscoelastic forces between the fluid and the moveable portion407and/or the fixed part410. Surface tension confines the fluid408to a length of the protruding structures413between outside corners C.

FIG.5illustrates a MEMS device500incorporating another variation on the concepts illustrated inFIGS.4A-4C. In this implementation, a suspended element501is connected to anchors502,503on a fixed part510of a substrate by torsion flexures504that allow the suspended element to rotate about an axis505and impart movement to a drive beam507. The aforementioned components may be configured as shown inFIG.1,FIG.2andFIG.3and as described above, e.g., with a drive beam507free to move in a cavity506formed in a fixed part510of a substrate. Here one or more fluid confinement structures include recessed portions511of the sidewall of the fixed part510of the substrate. The recessed structures511form an extension of a gap between the drive beam507and the fixed part510. One or more structures513protrude from the drive beam into a limited part of a gap defined by the recessed structures511. Viscoelastic fluid508is disposed in the limited part of the gap and is confined between ends of the protruding structures513and nearby sidewalls of the fixed part510corresponding to the recessed structures.

InFIG.5, the device500is depicted with potential locations for inclusion of winglet structures513. The drive beam507, which actuates another part of a device (not pictured), has two winglets,513attached to it. By way of example, and not by way of limitation, the winglets may be manufactured in the same steps as the rest of the structure and may be monolithically integral part of it, their thickness (into the page of the figure) would also be the same as the rest of the rotator and would be defined by the silicon layer thickness. The moving portion of a winglet may be fully suspended over the portion of the etched cavity506corresponding to the recessed structures511to allow out-of-plane movement.

FIG.6illustrates a MEMS device600incorporating another variation on the concepts illustrated inFIGS.4A-4CandFIG.5. In this implementation, a suspended element601is connected to anchors602,603on a fixed part610of a substrate by torsion flexures604that allow the suspended element to rotate about an axis605and impart movement to a drive beam607. The aforementioned components may be configured as shown inFIG.1,FIG.2andFIG.3and as described above, e.g., with a drive beam607free to move in a cavity606formed in a fixed part610of a substrate. Here, the one or more fluid confinement structures include one or more beams613that protrude into the limited part of the gap611from the suspended element in a direction perpendicular to the axis605on an opposite side of the suspended element from the drive beam607. Viscoelastic fluid608is confined between an end614of the beam613and a nearby vertical sidewall of the fixed part610in this example.

The design illustrated inFIG.6may be practical to add such structures for applications where it is desirable not to interfere with the drive beam507and its attachment to an actuated device (e.g. a micromirror). The portion of the etched cavity606, corresponding to the gap611may extend underneath the beam613to allow for out-of-plane movement.

FIG.7illustrates a MEMS device700incorporating yet another variation on the concepts illustrated inFIGS.4A-4C,FIG.5, andFIG.6. In this implementation, a suspended element701is connected to anchors702,703on a fixed part710of a substrate by torsion flexures704that allow the suspended element to rotate about an axis605and impart movement to a drive beam707. The aforementioned components may be configured as shown inFIG.1,FIG.2andFIG.3and as described above, e.g., with a drive beam707free to move in a cavity706formed in a fixed part710of a substrate. Here the one or more fluid confinement structures include a plurality of fingers714that protrude from a beam713that extends into a limited portion of the gap711from the suspended element701in a direction perpendicular to the axis705on a side of the suspended element opposite the drive beam707. The fingers714are configured to entrain a portion of viscoelastic fluid708next to a vertical sidewall of the fixed part710and between adjacent fingers of the plurality of fingers714. In alternative implementations the fingers714may extend from the drive beam707, or the fixed part710or some combination of two or more of the suspended element, the drive beam and the fixed part.

In the device700, in the example depicted inFIG.7the beam713is not used as the drive beam and the comb fingers714can be on its sides as depicted or at its end. The fingers714act as damping structures and a portion of the etched cavity706corresponding to the gap711may extend underneath the suspended element710, beam713and fingers714to allow for out-of-plane movement.

There are a number of variations on the design of the fluid confinement structures that may be used in the implementation shown inFIG.7and other implementations.FIGS.8A-8Fillustrate a number of possible variations, among others of structure for fluid confinement and mechanical damping. Such damping structure designs aim to have a relatively large area and narrow gap as shown. InFIGS.8A-8F, an etched cavity formed in a fixed part810of a substrate includes a small gap portion811that surrounds an end of a beam813configured similarly to the beam713inFIG.7. The beam may be connected to a suspended element that is in turn anchored to the fixed part810, which may be part of a silicon layer.

FIG.8Adepicts a T-shaped damping structure814A attached to the beam813within the gap portion811of the cavity. The beam813with the T-shaped structure may be directly connected to a suspended element, e.g., as shown inFIG.7and the cavity and gap portion may be configured so that the suspended element, beam, and damping structure can have out-of-plane movement. Viscoelastic fluid808may be confined to the gap portion811of the etched cavity. By providing for moving and non-moving surfaces and a narrow gap between them, a significantly higher fluid drag can be obtained to increase damping.

The device of claim1, wherein the one or more fluid confinement structures include one or more Y-shaped structures formed on the drive beam or the suspended element.

FIG.8Bdepicts an example of a Y-shaped damping structure814B within a modified gap portion811of the etched cavity. The gap portion811has been etched so that there is a uniform narrow gap between the Y-shaped damping structure814B and the surrounding fixed part810along the length of the damping structure.

FIG.8Cschematically illustrates an example that uses a rectangular damping structure814C with an array of small holes etched through it. As a result of the holes, there is an increased surface area of the winglet that is exposed to the surrounding fluid and therefore higher fluid drag. The gap portion has been etched so that there is a uniform narrow gap between the outer edges of the rectangular structure814C and nearby fixed parts810of the substrate.

InFIG.8Dis a T-shaped damping structure814D has teeth extensions815within a modified gap portion811of the etched cavity. The teeth extensions815significantly increase the surface area of the damping structure814D that is exposed to the surrounding fluid808, resulting in higher fluid drag.

As shown inFIG.8Edamping structure814E similar to the one shown inFIG.8Dmay further include teeth816extending from the fixed part that interdigitate with the teeth815on the damping structure814E. This addition effectively narrows the gap between the damping structure814E and the fixed part810, resulting in stronger surface forces and higher fluid drag.

FIG.8Fis a schematic top-view diagram showing a proposed rectangular damping structure814F within a modified gap portion811of the etched cavity. The rectangular winglet utilizes a smaller surface area without sacrificing the narrow gap and therefore high fluid drag, and may be versatile for devices with smaller structures.

There are a number of ways in which the suspended element may be actuated in MEMS devices in accordance with aspects of the present disclosure. By way of example, and not by way of limitation,FIG.9is a photomicrograph of an example embodiment in which a combdrive actuator and a T-shaped damping structure914are attached to a suspended element901by a beam913. The damping structure moves in a portion911of a cavity etched in a fixed part910of a substrate. The combdrive includes comb fingers915extending from the suspended element that interdigitate with comb fingers916that extend from nearby portions of a fixed part910.

FIGS.10A-10Cdepict some possible variations on the design shown inFIG.8A. Specifically,FIG.10Ais a schematic top-view diagram of a portion of a MEMS device in which a T-shaped damping structure1014similar to that shown inFIG.8Ais attached to a beam1013. In this example, fluid1008is laterally confined within the indicated area by careful design of the damping structure, using strong surface forces between the fluid and the silicon sidewalls of a gap portion1011of a cavity formed within a fixed part1010of a substrate. Namely the strong forces aim to keep the fluid confined within the narrow uniform gap section and do not easily spill beyond the edges where the gap is changed. In the depth direction, the fluid1008is also confined by the thickness of the structure itself (silicon device layer) since the top and bottom of the structure have sudden gap increases with sharp angles.

FIG.10Bis a schematic top-view diagram of a portion of MEMS device similar to that shown inFIG.10Ain which the damping structure1014is fully immersed in viscoelastic fluid1008, and both the damping structure1014and fluid1008are suspended within the gap portion1011of the etched cavity. As in the example shown inFIG.10A, the fluid1008is laterally confined within the indicated area by careful design of the damping structure1014and the portion1011of the cavity, and is vertically confined by the thickness of the structure. Strong surface forces at the narrow uniform gap sections between the winglet-shaped part of damping structure1014and surrounding fixed part1010near the intersection of the beam1013and damping structure1014keep the fluid1008laterally confined. The head of the damping structure inFIG.10Bis modified with smooth and round edges to ensure it is laterally fully immersed in the fluid1008.

FIG.10Cillustrates an example similar to that shown inFIG.10Bbut in which the fixed part1010includes a reservoir1009in fluid communication with the gap portion of the cavity via a fluid channel. The size and location of the reservoir and the dimensions of the channel may be chosen to facilitate dispensing of fluid and communication of the fluid from the reservoir to the gap portion by capillary forces.

Aspects of the present disclosure include implementations in which the damping structure is configured to enhance vertical damping forces. By way of example, and not by way of limitation,FIGS.11A-11Cdepict an implementation of a portion of a MEMS device in which a suspended element1101is connected to anchors1102,1103on a fixed part1110of a substrate by torsion flexures1104that allow the suspended element to rotate about an axis1105and impart movement to a drive beam1107. The aforementioned components may be configured as shown inFIGS.1-3andFIG.5and as described above, e.g., with a drive beam1107free to move in a cavity1106formed in a fixed part1110of a substrate. Here one or more fluid confinement structures include recessed portions1111of the sidewall of the fixed part1110of the substrate that communicate with the cavity1106. The recessed structures1111form an extension of a gap between the drive beam1107and the fixed part1110. One or more winglet structures1113protrude from the drive beam into a limited part of recessed structures1111. Viscoelastic fluid1108is disposed in a gap portion defined by the recessed structures1111and is confined between ends of the winglet structures1113and nearby vertical and horizontal sidewalls of the fixed part1110corresponding to the gap portion. As may be seen in the cross-sectional view inFIG.11Band the three-dimensional view inFIG.11C, the fluid1108is disposed at least partly underneath the winglet structures1113, which encounter vertical damping forces when undergoing vertical motion (i.e., motion into the plane ofFIG.11A.

As noted above, viscoelastic fluid may be dispensed into a gap portion or nearby reservoir.FIG.12Ais a schematic diagram illustrating a dispensing technique used to deliver droplets of viscoelastic fluid1201to an entire wafer1202. Initially stored in a reservoir1203, the fluid is deposited via contact-less droplet ejection method through a dispenser head1204onto the wafer. The wafer is mounted on a stage1205that allows for its biaxial lateral movement. Manual movement of the stage is possible; however, for a more precise droplet ejection, an automated machine can be programmed to dispense exact pico-liter droplets of viscoelastic fluid1201on particular locations on the wafer1202.

FIG.12Bis a schematic of the dispensing technique used to deliver viscoelastic fluid801to specific portions of a die1206. Similarly toFIG.12A, the fluid is initially stored in a reservoir1203and deposited via contact-less droplet ejection method through a dispenser head1204. The die1206is mounted on a stage that allows for biaxial lateral movement to correctly position the dispensing device, and the stage can be manually repositioned or programmed for biaxial lateral movement to deposit droplets in the correct location.

Fluid dispensing systems like, e.g., the jetlab 4® printing system from MicroFab Technologies, Inc. of Plano Texas or the he BioJet Ultra™ Piezo Dispenser MD-K-140 from Biodot Inc of Irvine, California are widely available commercially, and have been used for applications such as serial dilution of picoliter drops of fluid. Presently these products can dispense fluids of lower viscosity not exceeding 20 centipoise (cP). Viscoelastic fluids exceeding these parameters may still be dispensed via these methods after their preparation by preheating or premixing with solvents to lower their viscosity as they are ejected from the dispensing device.

While the above is a complete description of the preferred embodiments of the present invention, it is possible to use various alternatives, modifications, and equivalents. Therefore, the scope of the present invention should be determined not with reference to the above description but should, instead, be determined with reference to the appended claims, along with their full scope of equivalents. Any feature, whether preferred or not, may be combined with any other feature, whether preferred or not. In the claims that follow, the indefinite article “A” or “An” refers to a quantity of one or more of the item following the article, except where expressly stated otherwise. The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase “means for”. Any element in a claim that does not explicitly state “means for” performing a specified function, is not to be interpreted as a “means” or “step” clause. Although certain process steps may appear in a certain order in the claims, the steps are not required to be carried out in any particular order unless a particular order is otherwise specified by the claim language.