The present application claims priority to European Patent Application No. 14 307 128.0, entitled “MEMS STRUCTURE WITH THICK MOVABLE MEMBRANE,” filed on Dec. 22, 2014, the entire contents of which are hereby incorporated by reference for all purposes.
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 the 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
As can be seen from
The conventional method of manufacturing the switch structure shown in
However, for reliable operation of an MEMS device, for example, an MEMS switch with freely supported membrane 1, a thickness of the membrane 1 in the central part (for mechanical reasons) in the presented art is typically of at least 3 μm. It turned out that in conventional processing as shown in
Thus, there is a need for an improved MEMS device and a method of manufacturing of the same wherein the reliability of operation is enhanced as compared to the art.
The above-mentioned object is addressed by a method of manufacturing an MEMS device according to claim 1. The method comprises the steps of forming a first membrane layer over (for example, on) a sacrificial base layer, forming a second membrane layer over (for example, on) the first membrane layer), wherein the second membrane layer comprises lateral recesses that expose lateral portions of the first membrane layer and forming stoppers to restrict movement of the first membrane layer when operating the MEMS device. The lateral portions of the first membrane layer exposed by the second membrane layer may be covered by any other material layer, in principle. According to the invention, a membrane is provided as a moving element of an MEMS device that comprises a first membrane layer covered by a second membrane layer except at some lateral regions where recesses are formed in the second membrane layer. The recesses may be arranged in the longitudinal direction of the membrane at edges of the second membrane layer (see also detailed description below). Herein, the term “longitudinal” refers to the longitudinal axis of the membrane whereas the term “transverse” refers to the transverse axis of the same.
Movement of the membrane comprising the first and second membrane layers is restricted by mechanical contact of the first membrane layer with the stoppers. In operation, the first membrane layer may contact the stoppers that partly may extend into the recesses of the second membrane layer without contacting the second 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 is thinner than a conventionally used membrane of uniform thickness the free moving distance of the membrane can be reduced as compared to the art as a sacrificial layer with a thinner thickness has to be applied over the membrane during the manufacturing process, thereby improving reliability of operation.
In particular, the second membrane layer may be formed thicker than the first membrane layer. The first membrane layer may be formed at a thickness of at most 2 μm, particularly, at most 1 μm, and the second membrane layer may be formed at a thickness of at least 2 μm.
According to an embodiment, a sacrificial layer (for example, made of a polymeric or other dielectric material) is formed over (for example, on) the first membrane layer and over parts of the sacrificial base layer that are not covered by the first membrane layer and the forming of the stoppers comprises forming a stopper layer over the sacrificial layer and patterning, for example, by etching, the stopper layer to form the stoppers provided for restricting movement of the first membrane layer. The thickness of the first membrane layer determines the thickness of the sacrificial layer (see detailed description below) and thereby the free moving distance of the membrane comprising the first and second membrane layers during operation of the MEMS device.
Furthermore, the inventive method may comprise removing, for example, etching, the sacrificial layer over a portion of the first membrane layer and forming the second membrane layer over the portion of the first membrane layer that is exposed by the process of removing the sacrificial layer. In this embodiment, the above-mentioned recesses of the second membrane layer are formed by not completely covering the first membrane layer by the second membrane layer due to the parts of the sacrificial layer that are not removed before the formation of the second membrane layer.
The stoppers and the second membrane layer can be formed in the same processing step thereby accelerating and simplifying the overall processing. Moreover, the stopper layer and the second membrane layer can be formed of the same material, for example, a metal or electrically conductive alloy. According to an alternative embodiment, the stopper layer as well as the stoppers are formed before removing the sacrificial layer over (for example, on) the portion of the first membrane layer.
The MEMS device may comprise electrodes as actuation means that are formed over a substrate. The substrate can be a wafer of silicon, silicon-on-insulator, silicon-on-sapphire, gallium-arsenide, gallium-nitride, glass, quartz, aluminum or any other material used for the manufacturing of semiconductor devices. The substrate can, furthermore, be covered by a thin layer of insulating material, for example made of silicon nitride, silicon dioxide, aluminum oxide or any other dielectric layer used for the manufacturing of microelectronics devices. Additionally, functional layer of materials and functional structures may be featured on said wafer, such as electrical connections, protective bumps or support pillars prior to the manufacturing of the membrane layers.
The stoppers may be formed in mechanical contact with the substrate and the method may further comprise forming posts and a transmission line (e.g., made of aluminum or gold) over the substrate and it may comprise removing the sacrificial base layer that may be formed on or over the posts and arranging the first membrane layer (and thereby the second membrane layer formed over the first one) on the posts allowing free movement of the membrane comprising the first and second membrane layers.
The steps of the above-mentioned examples of the method of manufacturing an MEMS device may be carried out in order to achieve a capacitive or Ohmic contact MEMS switch. Alternatively, the steps of the above-mentioned examples of the method of manufacturing an MEMS device may be carried out in order to achieve an MEMS capacitor. In the case of an MEMS capacitor the membrane may represent one of the electrodes of the capacitor.
According to an embodiment the method may further comprise forming a third membrane layer over (for example, on) the second membrane layer locally in a region over the posts whereon the membrane rests and/or a region over the transmission line. The third membrane layer may be formed over the first membrane layer in a region over one of the posts (in particular, over all posts) and/or a region over the transmission line such that the third membrane layer only partially overlaps the second membrane layer. The third membrane layer may be formed over the first membrane layer in a region over one of the posts (in particular, over all posts) and/or a region over the transmission line such that the second membrane layer has a region where the third membrane layer is not formed adjacent to the region where the third membrane layer is formed. The third 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 third membrane layer may be formed in a region extending along the entire length of a post (in the transverse direction) 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 third membrane layer may be formed in a region extending along the entire length of a transmission line (in the transverse direction) 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 2 times the width of the transmission line or larger. The third membrane layer allows for enhancing reliability of the switching process and may improve the switching speed. In particular, the third membrane layer locally stiffening the membrane may be provided in order to increase contact forces and reduce insertion losses.
In addition, the method may further comprise forming an overhanging portion of the second membrane layer extending over a recess of the lateral recesses of the second membrane layer. By provision of the overhanging portion the first membrane layer can be protected during the process of patterning a material layer for forming the second membrane layer (and the stoppers); see detailed description below. The second membrane layer may comprise overhanging portions extending over all of the recesses, for example.
Furthermore, the above-mentioned object is addressed by an MEMS device (in particular, either a capacitive or Ohmic contact switch or an MEMS capacitor) that comprises a movable membrane comprising a first membrane layer and a second membrane layer formed over (for example, on) the first membrane layer (101), wherein the second membrane layer comprises lateral recesses exposing lateral portions of the first membrane layer. The lateral portions of the first membrane layer exposed by the second membrane layer may be covered by any other material layer, in principle. In operation stoppers may mechanically contact the membrane at edges of the first membrane layer that may have a larger or smaller overall surface area than the second membrane layer. The mechanical contact can be localized within the recesses of the second membrane layer, i.e., the stoppers may partly extend into the recesses without contacting the second membrane layer when contacting the first membrane layer.
In particular, the second membrane layer may be thicker than the first membrane layer. In this case, the first membrane layer can be formed at a thickness of at most 2 μm, particularly, at most 1 μm, and the second membrane layer can be formed at a thickness of at least 2 μm.
According to an embodiment, the MEMS device further comprises posts for supporting the first membrane layer. The first membrane layer is moveable on the posts (together with the second membrane layer formed over the first membrane layer). Stoppers may be configured to contact edges of the first membrane layer within the recesses of the second membrane layer, i.e., the stoppers have parts that extend into the recesses of the second membrane layer when other parts of the stoppers contact the first membrane layer.
The above-described MEMS device may further comprise a substrate, electrodes (for example, made of some refractory metal) and a transmission line and wherein the electrodes, transmission line and the above-mentioned posts are formed over the substrate. As described above the substrate maybe a wafer of silicon, silicon-on-insulator, silicon-on-sapphire, gallium-arsenide, gallium-nitride, glass, quartz, aluminum or any other material used for the manufacturing of semiconductor devices.
The MEMS device may further comprise a third membrane layer formed over the second membrane layer locally in a region over the posts whereon the membrane rests and/or a region over the transmission line. The third membrane layer may be formed over the second membrane layer in a region over one of the posts (in particular, over all posts) and/or a region over the transmission line such that the third membrane layer only partially overlaps the second membrane layer. The third membrane layer may be formed over the second membrane layer in a region over one of the posts (in particular, over all posts) and/or a region over the transmission line such that the second membrane layer has a region where the third membrane layer is not formed adjacent to the region where the third membrane layer is formed. The third membrane layer may overlap the posts and/or transmission 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 third 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 third 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 second membrane layer may comprise an overhanging portion over a recess of the lateral recesses of the second membrane layer. The second membrane layer may comprises overhanging portions extending over all of the recesses.
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.
A method of manufacturing of an MEMS switch comprising a two-layer movable membrane according to an example of the present invention is described with reference to
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 second membrane layer 105. The thinner portions allow for a thinner sacrificial layer 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.
In the example shown in
According to the example shown in
After formation of the stoppers 104 and second 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 second 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 second 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
A further modified version of the example illustrated in
A top view of the configurations shown in
A top view of the configuration shown in
In both configurations shown in
The configurations shown in
Whereas according to the above-described examples an MEMS switch is formed, the same manufacturing steps can be used in the process of forming an MEMS capacitor. In this case, the membrane comprising the first and second membrane layers 101 and 105 can function as an electrode in a configuration similar to the one 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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14 307 128.0 | Dec 2014 | EP | regional |