The present application claims priority to European Patent Application No. 14 307 127.2, entitled “MEMS STRUCTURE WITH MULTILAYER MEMBRANE,” filed on Dec. 22, 2014, the entire contents of which are hereby incorporated by reference for all purposes.
The present invention relates to Micro-Electromechanical Systems (MEMS), particularly, to RF MEMS switches comprising multilayer membranes.
Micro Electromechanical Systems (MEMS) are used in a very broad diversity of applications, as sensors, actuators or passive devices. RF MEMS in particular are used in radio frequency applications such as radio transmission, and in various industries such as mobile telecommunications. In mobile phones for example, RF MEMS can be used in the RF front-end for antenna tuning or antenna switching. MEMS components offer for those applications strong technical advantages such as very low insertion losses, high isolation in the case of switches, and very high linearity (over 70 dB in IIP3) compared to other technologies such as solid state devices.
More specifically regarding RF MEMS switches, the actuation principle remains the same in all existing art: a conducting transmission line is either opened or closed by means of an electromechanical actuation. The closing of the transmission line can be made for example using a conducting element featured on a beam, a bridge or a membrane located at short distance from two conductive ends of the transmission line separated one from the other in order to avoid electric conduction in the transmission line. The beam, the bridge or the membrane can be then electromechanically actuated so that the featured conductive element shorts the circuit between the two conductive ends of the transmission line through Ohmic contact for example, thus creating a conductive line and therefore closing the switch.
As mentioned before, the part supporting the conductive element can be either a beam (anchored at one end), a bridge (anchored on two ends) or a membrane (either free or featuring several anchors). According to an embodiment, the membrane can be completely free and maintained by pillars and stoppers (cf. EP 1 705 676 A1 and EP 2 230 679 A1).
An example of a known MEMS switch structure comprising a freely supported membrane is illustrated in
By the bent membrane Ohmic contact to a transmission line 4 is realized thanks to the contact featured under the membrane. The transmission line 4 may be adapted to transmit an RF signal. In the shown example the electrodes 2, posts 3 and the transmission line 4 are formed on the same substrate 5. In order to come back to the rest position or to go in isolation position (a position where the gap between the conductive element and the conductive plots is maximal), actuation voltage of electrodes 2A goes back to 0 and actuation voltage is applied to external electrodes 2B which will perform a lever effect with the posts 3 thus bending the membrane the other way. It is noted that the freely supported membrane switch can also function as a capacitive contact switch rather than an Ohmic contact one.
One of the most important specifications of an MEMS switch as described with reference to
The above-mentioned object is addressed by a method of manufacturing an MEMS device, in particular, an MEMS switch, comprising the steps of forming posts and a conduction (transmission) line over a substrate and forming a membrane over the posts and the conduction line. The step of forming the membrane comprises forming a first membrane layer and forming a second membrane layer over the first membrane layer in a region over one of the posts (in particular, over all posts) and/or a region over the conduction line such that the second membrane layer only partially overlaps the first membrane layer. The step of forming the membrane may comprise forming a first membrane layer and forming a second membrane layer over the first membrane layer in a region over one of the posts (in particular, over all posts) and/or a region over the conduction line such that the first membrane layer has a region where the second membrane layer is not formed adjacent to the region where the second membrane layer is formed. Thus, the second membrane layer is locally formed over the first membrane layer in order to stiffen regions of the membrane over the posts/conduction line. Particularly, the step of forming the membrane may comprise forming a first membrane layer and a second membrane layer over the first membrane layer only in regions over the posts and/or the conduction line (particularly, when the membrane is at rest without being bowed by any actuators).
The second membrane layer may be formed in the shape of one or more stripes. The second membrane layer may be formed locally over the first membrane layer, i.e., the second membrane layer may not fully cover the first membrane layer but may cover it only over dedicated portions related to the posts and/or conduction line. The second membrane layer may be formed of the same material as the first one and may have the same thickness as the first one or a different thickness. The locally formed second membrane layer provides an enhanced stiffness/thickness of the membrane at the regions of the posts and/or conduction line. Thereby, insertion losses can be reduced due to an improved transformation of electrostatic force into contact force and switching speed can be increased due to improved transfer of mechanical forces thereby improving the overall performance of the MEMS device.
The second membrane layer may fully or partly extend over the entire transverse dimensions of the posts and/or the conduction line and/or may partly or fully cover the entire longitudinal widths of the posts and/or the conduction line. Herein, the term “longitudinal” refers to the longitudinal axis of the membrane whereas the term “transverse” refers to the transverse axis of the same. The second membrane layer may overlap the posts and/or transmission line (conduction line) in the width directions of the same by some amount, for example, the overlap may be below 2 widths or 1 widths of the posts/transmission line. The second membrane layer may be formed in a region extending along the entire length of a post or only partly along the post and may cover the post completely or partly and have a width in the range of ¼ to 4 times the width of the post. The second membrane layer may be formed in a region extending along the entire (transverse) length of a transmission line or only partly along the transmission line and may cover the transmission line completely or partly and have a width in the range of ¼ to 4 times the width of the transmission line or larger.
According to an embodiment, the method may further comprise forming a third membrane layer between the first membrane layer and the second membrane layer, wherein the third membrane layer comprises lateral recesses exposing lateral portions of the first membrane layer, and forming stoppers to restrict movement of the first membrane layer. The lateral portions of the first membrane layer exposed by the third membrane layer may be covered by any other material layer, in principle. The recesses may be arranged in the longitudinal direction of the membrane at edges of the third membrane layer (see also detailed description below). The stoppers may be arranged to contact edges of the first membrane layer in the recesses of the third membrane layer during operation of the MEMS device.
In operation, the first membrane layer may contact the stoppers that partly may extend into the recesses of the third membrane layer without contacting the third membrane layer when in contact with the first membrane layer. The free moving distance of the membrane is given by the gap between the stoppers and the edges of the first membrane layer. Since the first membrane layer can be provided thinner than a conventionally used membrane of uniform thickness the free moving distance of the membrane can be reduced as compared to the art thereby improving reliability of operation.
According to an embodiment, the first membrane layer is formed over a sacrificial base layer and the method further comprises forming a sacrificial layer over the first membrane layer and over parts of the sacrificial base layer not covered by the first membrane layer and in this embodiment the method comprises the forming of the stoppers comprises forming a stopper layer over the sacrificial layer and patterning the stopper layer to form the stoppers. The sacrificial base layer is also removed after or before or simultaneously to removal of the sacrificial layer. After removal of the sacrificial base layer the membrane may fall on posts formed over the substrate.
Examples of the inventive method may further comprise removing the sacrificial layer from a portion of the first membrane layer and forming the third membrane layer over the portion of the first membrane layer exposed by the removing process. The stopper layer and the stoppers may be formed before removing the sacrificial layer over the portion of the first membrane layer.
Herein, it is also considered the problem of undesirably attacking the first membrane layer during the process of patterning the third membrane layer or removing an electroplating seed layer from the sacrificial layer in the context of an electroplating process used for the formation of the stoppers and third membrane layer (see also detailed description below). Due to some mismatch between the sacrificial layer and the first membrane layer there is a risk that the first membrane layer is attacked by an etching process needed before removal of the sacrificial layer. In order to avoid this risk overhanging portions of the third membrane layer overhanging the recesses of the third membrane layer are formed.
The above-described examples of the inventive method may comprise accordingly, forming an overhanging portion of the third membrane layer over a recess of the lateral recesses of the third membrane layer. In particular, the method may comprise the steps of forming the first membrane layer over a sacrificial base layer, forming a sacrificial layer over the first membrane layer and over parts of the sacrificial base layer not covered by the first membrane layer, removing the sacrificial layer from a portion of the first membrane layer and forming an electroplating seed layer over the portion of the first membrane layer and the remaining sacrificial layer. Subsequently, a mold is formed over the electroplating seed layer exposing the electroplating seed layer over the portion of the first membrane layer and portions of the electroplating seed layer adjacent to the portion of the first membrane layer and electrodeposition of a material layer (providing the stoppers and third membrane layer) on the electroplating seed layer by electroplating using the mold is performed. Subsequently, the mold is removed, the electroplating seed layer over the portion of the first membrane layer is removed and the sacrificial layer is removed.
Alternatively, the examples of the inventive method may comprise the steps of forming the first membrane layer over a sacrificial base layer, forming a sacrificial layer over the first membrane layer and over parts of the sacrificial base layer not covered by the first membrane layer, removing the sacrificial layer from a portion of the first membrane layer and forming the third membrane layer over the portion of the first membrane layer and portions of the sacrificial layer adjacent to the portion of the first membrane layer.
The process of forming the overhanging portions can be performed independently from the process of providing the locally formed second membrane layer. Thus, it is provided a method of manufacturing an MEMS device, in particular, an MEMS switch or capacitor, comprising the steps of forming a first membrane layer over a sacrificial base layer, forming an additional membrane layer (comparable to the third membrane layer described in the context of the above examples comprising a locally thickened membrane) over the first membrane layer, wherein the additional membrane layer comprises lateral recesses exposing lateral portions of the first membrane layer that, however, may be covered by any other material layer), wherein forming of the additional membrane layer comprises forming an overhanging portion of the additional membrane layer over a recess of the lateral recesses of the additional membrane layer. This method of manufacturing the MEMS device without local thickening/stiffening by means of an additional locally formed membrane layer may comprise the steps of forming the first membrane layer over a sacrificial base layer, forming a sacrificial layer over the first membrane layer and over parts of the sacrificial base layer not covered by the first membrane layer, removing the sacrificial layer from a portion of the first membrane layer and forming an electroplating seed layer over the portion of the first membrane layer and the remaining sacrificial layer.
Subsequently, a mold is formed over the electroplating seed layer exposing the electroplating seed layer over the portion of the first membrane layer and portions of the electroplating seed layer adjacent to the portion of the first membrane layer and electrodeposition of a material layer on the electroplating seed layer by electroplating using the mold is performed thereby providing for the above-described third membrane layer and the stoppers, for example). Subsequently, the mold is removed, the electroplating seed layer over the portion of the first membrane layer is removed and the sacrificial layer is removed.
Alternatively, the examples of the inventive method may comprise the steps of forming the first membrane layer over a sacrificial base layer, forming a sacrificial layer over the first membrane layer and over parts of the sacrificial base layer not covered by the first membrane layer, removing the sacrificial layer from a portion of the first membrane layer and forming the additional membrane layer (comparable to the third membrane layer described in the context of the above examples comprising a locally thickened membrane) over the portion of the first membrane layer and portions of the sacrificial layer adjacent to the portion of the first membrane layer.
According to an exemplary embodiment it is provided an MEMS device, in particular, an MEMS switch, in order to address the above-mentioned object. The MEMS device comprises posts and a conduction (transmission) line formed over a substrate and a membrane over the posts and the conduction line. The membrane comprises a first membrane layer and a second membrane layer formed over the first membrane layer in a region over one of the posts and/or a region over the conduction line such that the second membrane layer only partially overlaps the first membrane layer. In particular, the first membrane layer may have a region where the second membrane layer is not formed adjacent to the region where the second membrane layer is formed. The second membrane layer may comprise one or more stripes or other patches locally formed over the first membrane layer. In particular, the second membrane layer may be formed only in regions over the posts and/or conduction line.
According to an embodiment of the MEMS device the second membrane layer fully or partly extends over the entire transverse dimensions of the posts and/or the conduction line and/or partly or fully covers the entire longitudinal widths of the posts and/or the conduction line. The second membrane layer may overlap the posts and/or transmission line (conduction line) in the width directions of the same by some amount, for example, the overlap may be below 2 widths or 1 widths of the posts/transmission line. The second membrane layer may be formed in a region extending along the entire (transverse) length of a post or only partly along the post and may cover the post completely or partly and have a width in the range of ¼ to 2 times the width of the post. The second membrane layer may be formed in a region extending along the entire (transverse) length of a transmission line or only partly along the transmission and may cover the transmission line completely or partly and have a width in the range of ¼ to 2 times the width of the transmission line or larger.
The MEMS device may further comprise a third membrane layer between the first membrane layer and the second membrane layer, wherein the third membrane layer comprises lateral recesses exposing lateral portions of the first membrane layer; and stoppers configured to restrict movement of the first membrane layer. The lateral portions of the first membrane layer exposed by the third membrane layer may be covered by any other material layer, in principle. Particularly, the stoppers may be arranged to contact edges of the first membrane layer in the recesses of the third membrane layer during operation of the MEMS device.
According to another embodiment, the third membrane layer of the MEMS may comprise an overhanging portion over a recess of the lateral recesses of the third membrane layer. The third membrane layer may comprise overhanging portions extending over all of the recesses, for example.
Additional features and advantages of the present invention will be described with reference to the drawings. In the description, reference is made to the accompanying figures that are meant to illustrate preferred embodiments of the invention. It is understood that such embodiments do not represent the full scope of the invention.
Herein, it is described a method of manufacturing of an MEMS device, in particular, an MEMS switch, comprising a movable membrane with localized thickened portions. The membrane may be formed of a conductive alloy or metal. The MEMS device, in principle, can be a capacitive or Ohmic contact MEMS switch.
Referring to
The two lateral pillars 30, 30′ and the central pillar 40 form a Coplanar Waveguide (CPW), the two lateral pillars 30, 30′ corresponding to the ground lines. The central pillar 40 forms the signal line (transmission line) for the transmission of an RF electric signal within the coplanar waveguide (CPW). In another variant, the RF signal line can be also implemented by means of a microstrip waveguide. The lateral pillars 30, 30′ and the central pillar 40 are for example made of a metal such as gold or gold alloy.
The MEMS switch further comprises a switch element which is constituted by a thin flexible membrane 60. Said flexible membrane comprising membrane layer 60 is moveably positioned above the pillars 30, 30′, 40. The longitudinal axis of the membrane layer 60 is parallel to aforesaid longitudinal direction X and perpendicular to aforesaid transverse direction Y. Both ends 60b, 60c of the membrane layer 60 are not clamped on the substrate 10, and the membrane is thus freely supported at rest (
The MEMS switch further comprises electrostatic lowering actuating means 70 that are used for bending down longitudinally the membrane layer 60 into the down forced state of
The electrostatic lowering actuating means are formed by two internal electrodes 70a, 70b, that are positioned under the functional part of the membrane layer 60. The internal electrodes 70a extends between the lateral pillar 30 and the central pillar 40. The internal electrodes 70b extends between the central pillar 40 and the lateral pillar 30′. The electrostatic raising actuating means comprises two external electrodes 80a, 80b. When the switch is a RF capacitive switch, the top surface of each electrode 70a, 70b 80a, 80b is covered by a dielectric layer 90 (
When no actuation voltage is applied on the electrodes 70a, 70b, 80a and 80b, the membrane layer 60 of the switch is in the rest position of
Thus far, the description has followed the description of the MEMS device of EP 2 230 679 B1. In fact, similar shapes for the membrane can be chosen as disclosed in EP 2 230 679 B1. However, the inventive example illustrated in
In Ohmic MEMS switches, the transmission is made through a metal contact (at pillar 40), and the stronger the contact force, the higher the conductivity and the lower the insertion losses of the switch are. Physically, this is explained by the fact that at nanoscale level, the contact between contact electrodes (for example, transmission line 40) is not uniform but is made by several asperities of a rough contact surface. Due to that, increasing the force on the contacts allows for a larger contact area as more asperities will be in direct physical contact. In Ohmic switches, the contact force is directly linked to the electrostatic force applied by the electrodes. This electrostatic force is intrinsically limited by several aspects of the switch such as the limited electrode size, the contact structure and the limited actuation voltages. Therefore in order to increase the contact forces and to reduce insertion losses, optimization of the membrane structure is proposed herein. By locally increasing the stiffness of the membrane due to the additional membrane layer 66 formed over membrane layer 60 it is possible to optimize the force transfer between the external actuation electrodes 70a, 70b, 80a, 80b and the contact (transmission line) 40 and to increase the mechanical force applied to the contact 40.
As shown in
An example for the manufacture of an MEMS device comprising a three-layer membrane according to the invention is illustrated in
As shown in
A stopper layer is formed over the sacrificial layer 103 and etched in order to form stoppers 104 that are connected to layer 100 (see
As it is illustrated in
Contrary to the art, according to the present invention the membrane comprises a thicker portion and thinner portions at the recesses R of the third membrane layer 105. The thinner portions allow for a thinner sacrificial layer 103 resulting in a smaller lateral free moving space (distance) of the movable (freely supported) membrane as compared to the art. The smaller lateral free moving space allows for a more reliable operation of the resulting MEMS switch.
An additional second membrane layer 106 is formed on the third membrane layer 105. The second membrane layer 106 may be formed over posts and/or a transmission (conduction) line (confer additional membrane layer 66 of
In the example shown in
According to the example shown in
After formation of the stoppers 104 and third membrane layer 105 the sacrificial layer 103 is removed (
A modified version of the example illustrated in
However, the material layer may be over-etched to some degree above the sacrificial layer 103 in order to guarantee that the sacrificial layer 103 can be properly removed in the further proceedings. Due to some mismatch between the third membrane layer 105 and the sacrificial layer 103 there is a risk that the underlying first membrane layer 101 can be attacked (at the lower edges of the third membrane layer 105) during the etching of the material layer. This risk can be avoided by the formation of the overhanging regions 115 over the sacrificial layer 103 as it is shown in
Another example for the manufacture of an MEMS device comprising a membrane with a third membrane layer 105 comprising overhanging regions 115 is shown in
Material layer 109 is formed by electroplating using the mold 108 as illustrated in
In principle, the manufacturing process shown in
A top view of the configuration shown in
All previously discussed embodiments are not intended as limitations but serve as examples illustrating features and advantages of the invention. It is to be understood that some or all of the above described features can also be combined in different ways.
Number | Date | Country | Kind |
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14 307 127.2 | Dec 2014 | EP | regional |