WO2011131249A1

WO2011131249A1 - Mems device having a membrane and method of manufacturing

Info

Publication number: WO2011131249A1
Application number: PCT/EP2010/055474
Authority: WO
Inventors: Dennis Mortensen; Morten Ginnerup
Original assignee: Epcos Ag
Priority date: 2010-04-23
Filing date: 2010-04-23
Publication date: 2011-10-27
Also published as: CN102834347A; CN102834347B; KR20130092995A; KR101703379B1

Abstract

A MEMS deviceis disclosed, comprising a micro-machined bulk body (BB) and a membrane (MM) fixed thereto. The membrane is cut preferably by laser from a flat sheet of metal and bonded to the bulk body.

Description

MEMS device having a membrane and method of manufacturing MEMS devices (MEMS = micro electro mechanical system) incorporating membranes are used in a variety of different applications. Examples are microphones, sensors, capacitors, switches and other types of devices employing a flexible membrane in a passive or active way.

Usually, a MEMS device is fabricated on silicon or ceramic substrate by use of planar processing and bulk micro- machining techniques. Such techniques comprises steps like physical and chemical etching, depositing of thin film layers, forming conductor lines, forming through-conducts and also mechanical methods like polishing and grinding. Further, micro-structuring can comprise a bonding step where two wafers are bonded together by a respective method. A MEMS device having a membrane needs a structuring method where free spaces are formed on both sides of the membrane. The free space on the substrate-side can e.g. be formed by etching a sacrificial layer deposited below the membrane. The above technique give a rise to problems when the aspect ratios between horizontal and vertical dimensions become too large as is the case for most membrane structures. Then, the removal of sacrificial layers becomes increasingly difficult. Furthermore, high selectivity between sacrificial and permanent layers can be difficult to achieve, potentially imposing additional constraints on the design. Finally, it can be difficult to produce a planar region on which to apply a membrane when preceding layers have topography consisting e.g. of protrusions and depressions. It is an object of the invention to provide a MEMS device having a membrane that bypasses these mentioned problems.

This problem is solved by a MEMS device according to claim 1. Further embodiments of the invention as well as a method for manufacturing a micro-mechanical device are given in further claims .

A MEMS device is provided comprising a micro-machined bulk body and a membrane fixed thereto. The membrane is not deposited or sputtered but is cut from a plane metal foil. A metal foil usable as a membrane may be structured by cutting elsewhere than directly on the surface of the MEMS device in question. This means that the process of structuring the membrane is fully independent from the process of structuring the bulk body. Hence, problems arising while processing the micro-structured bulk body do not have any impact on the process of forming the membrane. Further, the membrane may be formed in one step into an arbitrary two-dimensional shape without any problem and without having to take care of the topography of the surface of the micro-machined bulk body on which the membrane is mounted in a later step.

The material of the membrane can be chosen from all

appropriate metal foils. Hence, the selection of the metal is not restricted by the demand of forming an etch selectivity like it is in known methods. There is no restriction on the thickness of the metal foil and hence the thickness of the membrane. Without any problem, a metal foil can be used having a thickness between 1 and 50 μη covering the demands of most of the devices using such membrane. In some cases, metal foils having a smaller thickness or a thickness extending the given range may be used, too. In one embodiment, the membrane comprises a plane member. In most applications of the MEMS device, the plane member serves as plane electrode of the device. Hence the area of the plane member is important for defining the properties and

electrical parameters of the MEMS device.

The second electrode of the MEMS device is usually a fixed electrode and may be integrated into or fixed to the bulk body.

In a further embodiment, suspension arms laterally extending from the plane member are foreseen. At their outermost ends, the suspension arms are widened to suspension arm pads that are used to fix the suspension arms to respective contact pads on the bulk body.

The suspension arms can be present in an arbitrary number. A number of three suspension arms is sufficient to fix the membrane at a stable position relative and parallel to the surface of the bulk body. Depending on the operation mode of the micro-mechanical device, the number of suspension arms may exceed three and may be four or a greater number.

The suspension arms usually have a width small enough to provide sufficient flexibility. The width is selected to provide a cross-sectional area of the suspension arms that is sufficient to provide enough mechanical strength for fixing the membrane in a start position. In a further embodiment, the suspension arms are bent to extend over/below the plane of the plane member. Further, the suspension arms may be bent or angled in the plane of the plane member, thereby providing additional length that can be used to compensate for a translational movement of the membrane within the plane or to compensate strains working in lateral direction. The suspension arms can comprise straight sections as well as curved sections.

The cutting process is used to structure and separate a single membrane from a large area metal foil. The complete structure of the membrane that may include the plane member, the suspension arms and the suspension arm pads are cut in one piece from the metal foil.

In an embodiment, the membrane is suspended between anchor means formed on top of the bulk body. The plane member itself or alternatively the suspension arms may be fixed to the anchor means on the bulk body. In this embodiment, the membrane can be flat having no extensions above the plane of the plane member.

The suspension of the membrane at the anchor means comprises mechanical fixation and an electrical contact as well. The anchor means can be formed from structured backend layers and are in that case part of the bulk body.

In all embodiments, if not regarding the membrane, the MEMS device may be formed like a known MEMS or CMOS device. The bulk body may comprise a semiconductor with or without an integrated circuit integrated within the semiconductor body. In an embodiment, the bulk body may be a ceramic body. Back-end layers are deposited on top of the semiconductor or ceramic body, one of which being a conductive layer arranged opposite to the membrane and functioning as a back electrode that works together with a first electrode formed by the membrane .

The integrated circuit is in electrical contact to the back electrode and to the membrane by respective through-contacts through the back-end layers. Electrical connections lines other than through-contacts may also be part of the

respective electrical connection. The bulk body comprises the semiconductor or ceramic body and the back-end layers deposited thereon. The back-end layers can comprise a stack of layers of the same or different materials. These materials may be selected from mechanically stable layers, electrically conducting layers and

electrically isolating layers. Preferred layers for the back- end layers are those known from CMOS devices, e.g. silicon oxide or silicon nitride for electrically isolating and for forming a given three-dimensional shape, and metal layers comprising aluminium and tungsten as the most preferred metals for micro-structuring processes. Other insulators, poly-Silicon, and metals may be used as well.

Through-contacts through back-end layers may comprise other materials as well that are used for filling the holes produced for the through-contacts in an isolating back-end layer. Tungsten is the preferred metal for forming through- contacts as it can be deposited at sidewalls and at the bottom of holes having large aspect ratios. In one specific embodiment the MEMS device is designed and used as a microphone. Hence the membrane is fixed to the bulk body and may vibrate or get deflected by acoustic sound. For best operation, a back volume is provided by the device. The back plate is perforated with back plate holes, in order to reduce the damping between membrane and back plate.

Directly below the back plate a back volume or cavity can be formed in the bulk body. The back plate holes and cavity are structured by means of micro-machining. The back volume can be formed by a through going hole or recess that is closed at its bottom by a lid member. In the bulk body, at any location but elsewhere than directly beneath the back plate, an integrated circuit (IC) could be integrated if a semiconduc- tor bulk body is used. The membrane and back plate could be electrical connected to the ICs, thereby creating a

monolithic microphone design.

The air-gap distance between membrane and back plate could be defined by stand-off structures on-top the back-end layers. The Stand-off structures could consist of a circular closed or partially closed rim forming a stand-off rim at the edge of the back plate. If one or more openings are realized in the stand-off rim, they can be used as ventilation holes equalizing static pressure differences. Ventilation holes could also be structured in the membrane.

A method for manufacturing the MEMS device described above may comprise the following steps:

- providing a metal foil

providing a laser

cutting the foil with the laser to receive a membrane providing a micro-machined bulk body for the device, transferring the foil with the designed membrane to the bulk body and fixing it thereto.

Alternatively roll-to-roll techniques can be used to handle the foil during laser cut-out and membrane to device bonding. The laser is guided along the perimeter of the membrane to be cut out from the metal foil. Controlling means are to control the scan of the laser over the surface of the foil. The scanning speed is selected in dependence of the power amount per area (W/cm²) necessary to make a cut through the foil.

The spot diameter with which the laser is focused onto the metal foil is selected in dependence of a desired smoothness of the cutting edges at the later membrane. A spot diameter of 1-30 m is appropriate. Preferred diameter are between 5 and 20 μιη The metal foil may be selected from a suitable metal

providing desired properties like mechanical strength, elasticity and electrical conductivity. For most

applications, aluminium is a preferred material for the metal foil. The metal foil has a thickness preferably between 1 and 20 μιη, but even thicker sheets could be used as well.

The micro-machined bulk body is preferably a semiconductor or a ceramic body that could comprise an integrated circuit like a CMOS device. The integrated circuit can provide electrical functions necessary for the operation of the MEMS device. These functions can comprise detecting and measuring an electrical parameter like a voltage, a capacity, a

resistance, a current or respective other electrical

parameters. Further, the integrated circuit may provide voltages to be applied to the membrane and the back plate.

In another application, the integrated circuit is able to provide a frequency to be applied to the electrodes of the device, the frequencies being selected from RF frequencies or from acoustic frequencies. A logic circuit can be integrated to rule or govern the operational modes of the micro- mechanical device. An amplifier may be included to amplify a measured parameter as mentioned above. Any other computatio^¬ nal operation necessary for the operation of the MEMS device or its periphery can implemented within the IC inside the bulk body. In a further embodiment, the step of fixing the membrane to the bulk body may be done by ultrasonic bonding. By this step, the membrane is brought into electrical contact with contact pads arranged on the top surface of the bulk body. The bonding locations on the membrane can be on the plane member or on the suspension arms extending from the plane member, preferably at suspension arm pads at the outermost ends of the suspension arms.

In an embodiment, the method of manufacturing a MEMS device comprises the following steps

the foil is fixed and restrained to a handling frame before cutting

the cutting is done while the foil is fixed to the handling frame

- the membrane is transferred to the bulk body with the aid of the handling frame

the membrane is fixed to the bulk body

the handling frame is released from the membrane and the membrane is thereby cut and separated from any remaining foil not being part of the membrane.

In this embodiment, the metal foil is fixed and restrained to a handling frame before cutting. The fixation is preferably done in a way that allows an easy release of the membrane in a later step. The frame is used to handle a single membrane as well as an array of membranes structured in one step from the metal foil.

The cutting is done while the foil is fixed to the handling frame. In this step, membranes are cut sequentially and the membranes are at least partially cut from the remaining foil. It is preferred that a remaining holding structure fixes the cut membrane to the remaining foil and hence to the handling frame. This allows an easy transfer of the membrane to the bulk body with the aid of the handling frame. The membrane is fixed to the bulk body while still being connected to the handling frame.

In a last step, the membranes are released from the handling frame. Connecting structures that may still be present at this stage are cut or broken and thus, the membrane is separated from any remaining foil not being part of the membrane.

The handling frame comprises a plate of a hard material like glass or metal. The plate has preferably an array of

openings, each providing an area to accommodate just one membrane. The openings are cut-outs or may be of a finite depth .

According to an embodiment, the micro-mechanical device is used and designed as a miniature microphone with an

integrated IC. In this application, the membrane is excited to acoustical vibrations by incoming sound. The vibrations are detected by measuring the electrical parameter changed by the vibration of the membrane. This parameter may be a voltage or a capacitance between the two electrodes, the first electrode being the membrane and the second electrode integrated in the back plate. In another embodiment, the micro-mechanical device is designed and used as a tunable capacitor. In this

application, the membrane is constructed to function as a movable electrode that can be moved in a direction vertical to the surface of the plane member towards and away from the second electrode integrated into the back plate. The movement of the membrane can be initiated by electrostatic forces that are due to a voltage applied to the two electrodes.

The tunable capacitor can be tuned to have at least two different capacitances, one of which being a basic

capacitance where no forces are impacting on the membrane. A second capacitance may be a state where the membrane has moved and is now at a position that is closest to the counter electrode integrated in the back plate. It is possible as well that the tunable capacitor may be tuned continuously so that any position between the two extreme positions can be used to provide a desired capacitance of the tunable

capacitor. One or more sets of stand-off structures could be formed in the bulk body between the membrane and back plate. By increasing the voltage between membrane and back plate the membrane can be collapsed onto the different sets of

standoffs, thereby obtaining several discrete capacitances.

In a further embodiment, the device is designed and used as a switch. Here, too, a movement of the membrane can be used. At the end of the movement, a contact between the membrane at a contact point on the bulk body is made. In all applications, it is preferable that the second electrode integrated in the back plate is electrically isolated against the membrane by at least one isolating layer on the second electrode.

In the following, the invention is explained in more detail by the description of embodiments and the accompanying figures. The figures are only for illustration purposes and hence are drawn schematically. Some dimensions can be distorted. Hence, it is impossible to take any dimension or a ratio of dimensions directly from the figures.

FIG. 1 shows a cross-sectional view of a membrane mounted on top of a bulk body,

FIG. 2 shows a membrane from a top view, shows a cross-section through the device membrane , shows a handling frame, shows a schematic cross-section through another device with a membrane, shows a membrane with suspension arms having a different shape in a top view.

FIG. 1 shows a schematic cross-section through a MEMS device comprising a bulk body BB and a membrane MM mounted on top thereof. In the figure, the bulk body BB has a plane surface but this is not necessary for a device according to the invention. The membrane MM is a metal foil that has been laser cut to the desired shape necessary for the given application. The membrane comprises a plane member PM mainly being a large area member and arranged in parallel to the surface of the bulk body BB .

In a peripheral region of the membrane MM that can be the suspension arm pads or the plane member itself the membrane is fixed to the bulk body at membrane connection points MCP. In FIG. 1, the membrane connection point is on top of an anchor means AN protruding from the surface of the bulk body BB . It may also be at a same height level as the surface of the bulk body BB. FIG. 2 shows a schematic top view onto a membrane showing some more detail of the membrane structure. The plane member PM of the membrane MM is here depicted as a circular area but is not restricted to this shape. Connected to the perimeter of the plane PM, there are provided suspension arms SA. These can be formed as strips that may be bended in and/or within the plane of the plane member PM. In FIG. 2, the suspension arms are formed by three linear sections, enclosing an angle between each two sections such that the character "z" is formed by the three sections. The suspension arms may also comprises rounded sections without corners.

The suspension arms SA are to provide a flexible connection between the plane member PM and the suspension arm pad SP used to fix the suspension arm to the bulk body BB. The suspension arm SA may also function as a spring providing a flexible connection of the plane member. The spring may force and hold the plane member PM in a base position relative to the surface of the bulk body.

The suspension arms SA can be made as short as possible but long enough to provide a flexible but sufficient fixation of the plane member PM. The suspension arm pad SP at the end of every suspension arm can be a widened section of the arm having a width to provide enough area allowing a secure bonding process of the suspension arm pad SP to the surface of the bulk body BB, respectively, to a respective membrane connection point MCP.

FIG. 3 shows a micro-mechanical device embodied as a

microphone. In this embodiment, the bulk body comprises a semiconductor body SB on top of which a stack of back-end layers BL are deposited to form a rigid structure. The back- end layers BL may be layers that are necessarily present on top of a CMOS device integrated in the semiconductor body SB. Alternatively, the backend layers BL are provided only for forming a proper mechanically stiff structure that may comprise electrically conductive lines and areas.

Within the semiconductor body SB, an integrated circuit IC is formed. The back-end layers BL mainly comprise layers of insulating materials like oxides or nitrides or any other insulating inorganic material and metals that can be

deposited and structured with known methods. At least one through-contact TC is formed through the back-end layers providing an electrical conductive connection from a terminal of the IC to a contact pad on a surface on top of the back- end layers BL . Other structures may be conductive or

insulating may be arranged and patterned in the back-end layers BL . The whole bulk body BB comprising back-end layers and the semiconductor or ceramic body SB is structured to form a depression in the semiconductor body SB. The bottom of the depression is formed by semiconductor or ceramic material or a lid member LM closing or covering the depression from the opposite side of the bulk body like depicted in the figure.

The back plate section BP is formed by a number of back-end layers BL . This number may be smaller than the number of back-end layers in sections besides the back plate section to result in a smaller thickness of the back plate section. The depth of the first depression in the top surface of the back- end layers can comply with the number of layers omitted in the back plate section.

In the back plate sections BP, back plate holes VH are arranged that connect the second depression in the

semiconductor body with the free space above the back plate section. In the back plate section, at least one electrically conductive layer CL is arranged, preferably near the top surface of the back plates but isolated against the surface of the back plate section by an insulating layer. The conductive layer CL forms an electrode of the microphone and is preferably connected to the integrated circuit IC in the semiconductor body SB by respective electric connections like conductor lines and through-contacts TC.

The back volume BV is formed between backend layer BL, back plate BP, depression in the semiconductor body SB and may be closed at its bottom by a lid member LM.

The cut membrane MM is fixed to the top of the back-end layers BL such that the plane member PM of the membrane MM is arranged above but distant to the back plate section. The membrane is fixed by bonding the suspension arm pads SP to respective metallic pads on the surface of the back-end layers. A membrane MM working in a microphone like depicted preferably comprises an ventilation hole for equalizing static pressure differences between back volume BV and the atmosphere above the plane member PM. But this opening may be omitted such that venting is performed in the area where the membrane is supported by the stand-off structures.

FIG. 4 shows a top view onto a handling frame HF used to hold the metal foil during cutting and, after cutting, to transfer the cut membranes to a wafer where a plurality of MEMS devices are preformed. The handling frame HF comprises a plurality of openings OP, each having a diameter that is slightly bigger than the diameter of the membrane. The diameter of the openings may be from 1 to 3 mm.

The metal foil MF can be releasable fixed to the handling frame HF at fixation points FP by bonding, for example, with the aid of an adhesive. The laser cutting of the metal foil may be done while the metal foil is fixed to the handling frame, for example, with the aid of an adhesive. Membrane-to- bulk body alignment can be eased by the alignment protrusions FP, if matching holes or recesses are structured in the bulk body BB.

FIG. 5 shows a schematic cross-section through a micro- mechanical device designed to be used as a switch. Here again, the MEMS device comprises a bulk body BB and a membrane, the plane member PM of which is depicted in the figure. The plane member PM is fixed to anchor means AN arranged on the surface of the bulk body BB by spring elements SE that may be formed by suspension arms (see e.g. FIG. 2) . In the bulk body BB, one or more of the conducting layers are used to form a back plate BP electrically

connected to a terminal T2 arranged on a bottom or a top surface of the back plate BP. The plane member PM is

connected to another terminal Tl that is arranged at any place of the device, preferably on top of the anchor means AN or on top of the bulk body BB . Preferably, both of the conductive layer CL and the plane member PM are electrically connected to respective contact areas on the same surface of the bulk body BB by respective contacting lines and through- contacts, respectively. Plane member PM and conductive layer CL form two electrodes to which a voltage may be applied. A movement of the flexibly fixed plane member PM towards the surface of the bulk body and thus towards the conductive layer CL can be induced by electrostatic forces. When the plane member PM is moved towards the bulk body BB, respective contact points CP1 on the plane member and CP2 on the surface of the bulk body come into contact, thus closing the switch by providing an electric connection between the plane member PM and another contact point CP2 on the surface of the bulk body BB and a third terminal T3 connected to the contact point CP2.

In another embodiment not shown in FIG. 5, the contact point CP1 on the bottom surface of the plane member PM may be electrically isolated to the plane member but electrically connected to (not shown) a respective contact pad on the surface of the bulk body or the anchor means AN.

FIG. 6 shows a further embodiment of a membrane MM having suspension arms SA comprising straight and curved sections. The number of suspension arms is not restricted to three and may be higher or lower too.

A MEMS device according to the invention is not restricted to any of the embodiments depicted in the figures or described in connection therewith. An inventive device may deviate in real structure or real dimension from the described

embodiments. With the exception of the cut membrane, the micro-mechanical device may be an arbitrary one as known from the art meaning that the structure, besides the membrane, may be the same as in known devices. The invention consists of combining micro-mechanical structures with a cut membrane to result in the advantages described above.

List of terms and reference symbols

AM anchor means

BB bulk body

BL backend layers

BP back plate

BV back volume

CL conductive layer

CP contact point

FP Alignment protrusions

HF handling frame

IC integrated circuit

MCP membrane connection point

MM membrane

OP opening

PM plane member,

SA suspension arms

SB semiconductor body

SP suspension arm pad

T1-T3 terminals

TC through contacts

TL terminal line

VH back plate holes

Claims

1. MEMS device, comprising a micro-machined bulk body (BB) and a membrane (MM) fixed thereto, characterized in that the membrane is laser cut from a flat sheet of metal and bonded to the bulk body.

2. Device according Claim 1,

wherein the membrane (MM) comprises a plane member (PM) and suspension arms (SA) laterally extending from the plane member, the suspension arms ending in suspension arm pads (SP) bonded to the bulk body (BB) .

3. Device according to one of the preceding claims, comprising

a semiconductor or ceramic body (SB) with or without an integrated circuit integrated (IC) therein, a stack of backend layers (BL) deposited on top of the semiconductor body, the bulk body being formed by the backend layers and the semiconductor body, a conductive back electrode (CL) being one of the backend layers and arranged opposite to the membrane; wherein the integrated circuit is electrically connected to the back electrode and to the membrane by respective through contacts (TC) through the backend layers.

4. Device according to one of the preceding claims, further comprising :

a first depression in the top surface of the stack of the backend layers (BL) , the membrane (MM) covering the depression; a second depression in the semiconductor body (SB) aligned with the first depression, the second depression forming a back volume (BV) ; and

micro-machined back plate holes (VH) guided through the backend layers and connecting the back volume with the volume above the back plate.

5. Method for manufacturing a MEMS device working with a membrane, the method comprising the steps:

- providing a metal foil

laser cutting the foil to produce a flat membrane (MM)

providing a micro-machined bulk body (BB) for the device

- transferring the membrane to the bulk body and fixing it thereto.

6. Method according to Claim 5, wherein

the step of fixing comprises ultrasonic bonding the membrane (MM) to contact pads arranged on the top surface of the bulk body (BB) .

7. Method according to Claim 5 or 6, wherein

the foil is fixed and restrained to a handling frame (HF) before cutting

the cutting is done while the foil is fixed to the handling frame

the membrane (MM) is transferred to the bulk body (BB) with the aid of the handling frame

- the membrane is fixed to the bulk body

8. Method according to one of Claims 6 or 7, wherein the handling frame (HF) comprises a plate of a hard material like glass or metal, the plate having an array of openings (OP) each providing area for one membrane.

9. Method according to one of Claims 6 to 8, wherein the laser is focussed to a spot diameter on the surface of the metal foil of 1-50μπι.

10. Method according to one of claims 6 to 9, wherein a metal foil having a thickness of 1-50 m is used.

11. Device according to one of claims 1 to 4,

designed to be used as a miniature microphone with integrated IC.

12. Device according to one of claims 1 to 4,

designed to be used as a tuneable capacitor.

13. Device according to one of claims 1 to 4,

designed to be used as a switch.