WO2022189056A1

WO2022189056A1 - Electrically actuatable mems switch

Info

Publication number: WO2022189056A1
Application number: PCT/EP2022/051537
Authority: WO
Inventors: Matthew Lewis; Jochen Reinmuth
Original assignee: Robert Bosch Gmbh
Priority date: 2021-03-09
Filing date: 2022-01-25
Publication date: 2022-09-15
Also published as: TW202239697A; FR3120622A1; DE102021202238A1

Abstract

The invention relates to an electrically actuatable MEMS switch, comprising a substrate (1) on which a first insulation layer (100), a silicon layer (110), a second insulation layer (14) and a metal layer (16) are arranged one on top of the other, wherein the silicon layer, the second insulation layer and the metal layer form a micromechanical functional layer (120) in which there is a fixed part (121) and an electrically actuatable, deflectable switch element (122), wherein the switch element is designed to abut the fixed part in an operating state of the switch and thus form a mechanical and electrical contact between the switch element and the fixed part. The invention also relates to a method for producing an electrically actuatable MEMS switch.

Description

title

Electrically operable MEMS switch

State of the art

A wide variety of types of electrically actuated switches are known. Most relays are electromagnetically driven and require a solenoid coil to operate. Large forces can thus be generated, but such relays cannot be miniaturized because of the wound coil required. Nor can they be manufactured in a cheap wafer process. They also have a very high power consumption.

The ADGM1304 MEMS switch produced in a wafer process by Analog Devices (FIG. 1) is also known. This relay is capacitive driven. In this arrangement, the material from which the lever element is made must have good mechanical properties as well as good electrical conductivity. The gold used is one of the few materials that are suitable. However, gold is expensive and complex to process. In addition, the material for the lever element must be able to be processed selectively with respect to the underlying sacrificial layer. Gold over an oxide layer etched with HF is one of the few systems that has been able to do this. If a high force is to be generated in order, for example, to enable low contact resistances, the area of the electrode and the corresponding lever element must be enlarged. However, this also increases the size of the component and thus its costs. The electrical potential of the lever element is also a connection of the switching line at the same time. In this arrangement, there is therefore no galvanic isolation between the switching lines and the control line.

object of the invention

The search is for a method and an arrangement that, based on a wafer process, allows an electrically actuated MEMS switch to be built without expensive Lever structure made of gold and with which one can build up high contact forces, especially capacitively driven.

Advantages of the Invention

The invention relates to an electrically actuable MEMS switch with a substrate on which a first insulation layer, a silicon layer, a second insulation layer and a metal layer are arranged one above the other, the silicon layer, the second insulation layer and the metal layer forming a micromechanical functional layer in which a fixed part and an electrically actuable, deflectable switching element are formed, wherein the switching element is designed to rest on the fixed part in an operating state of the switch and thus to form a mechanical and electrical contact between the switching element and the fixed part.

Advantageous configurations of the invention can be found in the dependent claims.

A core idea of the invention is to carry out a functional separation in the MEMS element. The relatively poorly conducting silicon layer has very good mechanical properties and is very well suited to form a capacitive drive. For this purpose, this layer can be made relatively thick, so that it characterizes the mechanical properties of the relay. This layer is supplemented by a metal layer applied vertically above an insulating layer, which has very good conductivity and whose task is to create good electrical contact.

Due to the separation of the two functions and the special overhang of the metal layer, it is also possible to create an electrical contact via an in-plane movement. The electrostatic force that can be generated by a capacitive structure can be adjusted with the height of the silicon layer and to a certain extent also with the aspect ratio (trench width to trench depth) with which trenches can be created in the silicon layer.

A further advantage of the invention lies in the fact that, by separating the two functions, it is also possible to implement MEMS switches that are switched capacitively, there being complete galvanic isolation between the switching lines and the control line. The invention also relates to a method for manufacturing an electrically actuable MEMS switch. The process enables MEMS relays to be manufactured at low cost. It enables the production of relays with low electrical resistance. In addition, it enables the production of very compact relays with a low control voltage and with galvanic isolation between the switched lines on the one hand and the control line on the other.

drawing

FIG. 1 schematically shows an electrically actuable MEMS switch in the prior art.

FIGS. 2a to h show a method according to the invention for producing an electrically actuable MEMS switch in individual stages of the device.

FIG. 3a shows an electrically actuable MEMS switch according to the invention.

FIG. 3b shows the electrically actuable MEMS switch according to the invention in a switched operating state.

FIG. 4 schematically shows the method according to the invention for producing an electrically actuable MEMS switch.

description

FIG. 1 schematically shows an electrically actuable MEMS switch in the prior art. A first electrode 2 and a first contact area 3 are provided on a substrate 1 . Lever structure 4 is arranged above both structures, separated by a distance. If a voltage is applied between the lever and the first electrode, there is a movement out of the substrate plane. The cantilever is deflected perpendicularly to the substrate and contact is created between the cantilever and the contact surface. This arrangement can be made by looking at the electrode and the contact area first a sacrificial layer 5 is applied, then a gold layer is applied and structured, and then the sacrificial layer is removed.

FIGS. 2a to h show a method according to the invention for producing an electrically actuable MEMS switch.

In a first variant of the method, an SOI substrate 13 is first provided (FIG. 2a). The SOI substrate consists of a substrate 1, a first insulation layer 100 and a silicon layer 110.

A second insulation layer 14 is deposited and patterned on the silicon layer 110 of the SOI substrate 13 (FIG. 2b). A silicon-rich nitride layer is preferably deposited.

Optionally, a further silicon or germanium layer 15 is then deposited and polished back up to the level of the second insulation layer (FIG. 2c). This is particularly advantageous when using relatively thick insulation layers (>300 nm).

A metal layer 16 is then deposited and structured (FIG. 2d). In the first and second contact areas 1210, 1220, that is to say in the area in which the electrical contact is later established by the movement of a switching element, the metal layer covers the opening in the insulation layer. The coverage is preferably more than 20 nm and less than 20% of the thickness of the silicon layer. In particular, a metal layer that passivates itself, such as Al or W, can be used for the metal layer.

Optionally, further insulation layers or also metal layers or also auxiliary bonding layers 17 can be deposited and structured (FIGS. 3a, b).

A trenching process is carried out, in which trenches are advanced through the silicon layer 110 to the insulating layer 100 . In particular, the trench process is carried out in such a way that the connection between the metal layer and the silicon layer is interrupted in partial areas, in particular in the contact area, and the metal edge completely protrudes beyond the silicon edge in the horizontal direction. After a first process step with isotropic SF6 etching 18 (FIG. 2e), it is favorable to carry out a trench process use, which produces a horizontal undercutting of the metal layer 16 of at least 50 nm (Figure 2f).

In a final process step, the first insulation layer 100, here the oxide layer of the SOI wafer, is locally removed using an etching process 20 (FIG. 2g). An etching process with gaseous HF is preferably used.

Tempering can optionally be carried out in a further step. In particular, annealing in hydrogen or in a hydrogen-containing atmosphere at a temperature above 850°C. Metal oxides 21, which have formed on the surface of the metal 16 and can lead to poor contacts, are thus reduced. This also results in a smoothing of the silicon trenches. Caused by the trenching process, the silicon trenches have a slightly rough surface with the so-called etched grooves 22. Due to the high temperature and the reducing effect of the hydrogen, the surface can be restructured and become smooth (FIG. 2h).

Optionally, the surface of the metal layer can be activated via an etching step. In particular, metallic oxide compounds that have formed on the surface of the metal layer, preferably during oxide etching, in particular in the contact area, can be removed using a back-sputtering method. The insulating layer can optionally be removed in partial areas (26) (FIG. 3a). Optionally, the movable relay structure can be protected with a cap wafer (23) that is applied using a bonding process (24) (FIG. 3a). A eutectic bonding method such as AlGe or CuSn or a seal-glass bonding method can preferably be used.

In a second variant of the method, an oxide layer 27 is first deposited on a silicon substrate 1 . The oxide layer can optionally be structured in order to define contact areas that make it possible to apply a defined electrical potential to the substrate later in the process.

Optionally, polysilicon layers 28 and further oxide layers can now be deposited and structured. In this way, buried conductor tracks can be produced, which make it possible to provide individual electrodes that are produced in the polysilicon functional layer with a defined electrical potential. Furthermore, the polysilicon layers can also be used as a very small but electrically isolated suspension for the movable relay structures or for the fixed drive electrodes (see Figures 3a and 3b). Finally, a silicon layer 110 is deposited, in particular as epitaxial polysilicon. As a result, an SOI substrate 13 is created again and the further course of the process is the same as in the first variant of the method.

With the manufacturing method described above, it is possible to implement favorable design parameters for a relay and to optimize them in a wide range. For the electrically actuable MEMS switch to be produced, it is advantageous for the silicon layer to be at least three times thicker than the metal layer. The mechanical properties are essentially shaped by the silicon layer. It is also advantageous to remove the metal layer in partial areas of the movable structure. It is favorable to remove at least half of the area of the metal layer on the movable structure.

Depending on the insulation layer, particularly in the case of insulation layers that generate high intrinsic stress on silicon, it can also be advantageous to remove the insulation layer in partial areas of the movable structure. It is favorable to remove at least half of the area of the insulating layer on the movable structure.

It is favorable to use thick silicon layers and to etch very narrow trenches into the silicon layer in some areas in order to achieve high capacitances. In particular, it is favorable to produce trenches in partial areas that are narrower than 15% of the silicon thickness.

FIG. 3a shows an electrically actuable MEMS switch according to the invention with a substrate 1 on which a first insulation layer 100, a silicon layer 110, a second insulation layer 14 and a metal layer 16 are arranged one above the other. The silicon layer, the second insulation layer and the metal layer together form a micromechanical functional layer 120 in which a fixed part 121 and an electrically actuable, deflectable switching element 122 are formed.

A first contact area 1210 is formed in the metal layer 16 of the fixed part 121 , and a second contact area 1220 is formed in the metal layer 16 of the switching element 122 . The switching element can be deflected in at least a first direction 7 parallel to a main plane of extension of the substrate. As a result, the first and the second contact area can come into mechanical contact with one another and thus close an electrical contact. For this purpose, the first contact region and the second contact region protrude in the first direction 7 with a projection 12 in relation to the underlying silicon layer 110 . FIG. 3b shows the electrically actuable MEMS switch according to the invention in a switched operating state. A voltage is present between stationary electrodes 10 and opposite electrodes of switching element 122, which causes a capacitive force. The movable switching element 122 is deflected in the first direction 7 towards the fixed part 121 in such a way that the second contact area 1220 bears against the first contact area 1210 . The contact is thus closed.

The procedure has the following essential steps:

Step A - providing an SOI substrate (13) with a substrate wafer (1), a first insulation layer (100) and a silicon layer (110);

Step B - depositing and patterning a second insulating layer (14) on the silicon layer (110);

Step C - depositing and patterning a metal layer (16) over the second insulating layer (14);

Step D - structuring of the silicon layer (110) by anisotropic etching up to the first insulating layer (100), whereby in the silicon layer (110), the second insulating layer (14) and the metal layer (16) a fixed part (121) and a switching element (122 ) be formed;

Step E - etching the first insulating layer (100) thereby at least partially exposing the switching element (122) and making it movable.

Reference List

1 substrate

2 first electrode

3 first contact surface

4 lever structure

5 sacrificial layer

7 first direction parallel to the main extension plane of the substrate

10 fixed electrodes

12 Overhang of the metal layer

13 SOI wafers

14 second insulation layer

15 more silicon or germanium layer

16 metal layer

17 bonding auxiliary layers

18 isotropic SF6 etching

20 local etching (HF, gas phase)

21 metal oxide

22 etched grooves

23 cap wafers

24 bonding material

26 removed sections of the insulation layer

27 oxide layer

28 polysilicon layer

100 first insulation layer

110 silicon layer

120 micromechanical functional layer

121 fixed part

122 electrically actuable, deflectable switching element

1210 first contact area

1220 second contact area

Claims

Expectations

1. Electrically actuatable MEMS switch having a substrate (1) on which a first insulating layer (100), a silicon layer (110), a second insulating layer (14) and a metal layer (16) are arranged one above the other, the silicon layer, the second insulation layer and the metal layer form a micromechanical functional layer (120), in which a fixed part (121) and an electrically actuable, deflectable switching element (122) are formed, the switching element being designed to be on the fixed part when the switch is in an operating state abut and form such a mechanical and electrical contact between the switching element and the fixed part.

2. Electrically actuable MEMS switch according to claim 1, characterized in that in the operating state the contact between a first contact area (1210) of the metal layer (16) of the fixed part (121) and a second contact area (1220) of the metal layer (16) of the switching element (122) is formed.

3. Electrically actuable MEMS switch according to claim 1 or 2, characterized in that the switching element can be deflected in at least a first direction (7) parallel to a main plane of extension of the substrate (1).

4. Electrically actuable MEMS switch according to claim 2 and 3, characterized in that the first contact area (1210) and/or the second contact area (1220) in the first direction (7) has a projection (12) in relation to the silicon layer (110 ) protrudes.

5. Method for manufacturing an electrically actuable MEMS switch with the steps:

A - providing an SOI substrate (13) with a substrate wafer (1), a first insulation layer (100) and a silicon layer (110);

B - depositing and structuring a second insulation layer (14) on the silicon layer

(HO);

C - depositing and patterning a metal layer (16) over the second insulating layer (14); D - structuring of the silicon layer (110) by anisotropic etching up to the first insulating layer (100), whereby in the silicon layer (110), the second insulating layer (14) and the metal layer (16) a fixed part (121) and a switching element (122) be formed;

E - Etching the first insulating layer (100) thereby at least partially exposing the switching element (122) and making it movable.

6. The method for producing an electrically actuable MEMS switch according to claim 5, characterized in that after step B and before step C, a further silicon or germanium layer (15) is deposited and polished back to the level of the second insulation layer.

7. A method for manufacturing an electrically actuable MEMS switch according to claim

5 or 6, characterized in that after step C and before step D one or more further insulating layers and/or metal layers and/or auxiliary bonding layers (17) are deposited and structured, thereby creating a bonding frame.

8. A method for producing an electrically actuable MEMS switch according to claim 7, characterized in that a cap is applied to the bonding frame.