Method for producing a microelectromechanical (MEMS) layer structure

Abstract

The invention relates to a method for producing a microelectromechanical (MEMS) layer structure, comprising the following steps: S1 - depositing a first layer arrangement comprising at least one first layer of a first semiconductor material on a substrate; S2 - depositing at least one first barrier layer on the first layer arrangement, the first barrier layer comprising silicon-rich nitride (SRN).

Description

[0001] R. 410804

[0002] - 1 -

[0003] Description

[0004] title

[0005] Method for producing a microelectromechanical (MEMS) layer structure

[0006] The invention relates to a method for producing a microelectromechanical (MEMS) layer structure, a MEMS accelerometer and a MEMS gyroscope.

[0007] State of the art

[0008] One aspect of manufacturing inertial MEMS sensors, such as accelerometers and gyroscopes, involves the layer-by-layer deposition and structuring of silicon and silicon oxide layers within a layered structure. At the end of these steps, the silicon oxide is removed, exposing the mechanical structures embedded in the silicon layers. These mechanical structures can consist of one or more functional layers. The number and thickness of these layers define the available space for structures such as inertial sensors. Individual functional layers within this layered structure can be built up epitaxially and structured, for example, using etching techniques. A disadvantage of this approach is that such structuring always extends across the entire depth of the functional layer, which imposes limitations on the design of the structures to be manufactured.

[0009] It is an object of the invention to provide an alternative or improved method for producing a microelectromechanical (MEMS) layer structure, as well as a MEMS accelerometer and a MEMS gyroscope. R. 410804

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[0011] Disclosure of the invention

[0012] The object of the invention is achieved by means of a method for producing a microelectromechanical (MEMS) layer structure according to claim 1, by means of an accelerometer according to claim 10, and by means of a gyroscope according to claim 11. Advantageous further developments, additional features and / or advantages of the invention will become apparent from the dependent claims and the following description.

[0013] It should be noted that all features mentioned in connection with the disclosed process can also be configurations of the disclosed system, and vice versa.

[0014] According to a first aspect, the following is disclosed:

[0015] A method for fabricating a microelectromechanical (MEMS) layer structure, comprising the following steps:

[0016] 51 - Deposition of a first layer structure comprising at least a first layer of a first semiconductor material on a substrate;

[0017] 52 - Deposition of at least one first barrier layer on the first layer structure, wherein the first barrier layer comprises silicon-rich nitride.

[0018] The first barrier layer can be deposited at least partially horizontally above the first layer, and the process can include the following steps:

[0019] 53 - Deposition of a second layer structure on the first barrier layer, the second layer structure comprising at least a second layer of a second semiconductor material;

[0020] 54 - Execution of an etching process to generate a structure in the second layer structure, wherein boundary conditions of the etching process are set such that the etching process does not attack at least one first barrier layer.

[0021] The procedure may include the following step: R. 410804

[0022] - 3 -

[0023] S1a - Execution of an etching process to create a vertical trench structure in the first layer structure; wherein step S2 comprises depositing the first barrier layer into the vertical trench structure.

[0024] The first semiconductor material and / or the second semiconductor material may include silicon, preferably polycrystalline silicon.

[0025] Step S1 may include producing the first layer as an epitaxially grown layer.

[0026] Step S3 may include producing the second layer as an epitaxially grown layer.

[0027] In step S4, the execution of the etching process may include the execution of reactive ionic deep etching.

[0028] At least one barrier layer can act as an electrical insulator.

[0029] The MEMS structure can be part of a MEMS accelerometer or a gyroscope.

[0030] According to another aspect, the following is disclosed:

[0031] A MEMS accelerometer comprising: a sensor substrate; a seismic mass extending between a first end and a second end, which is spring-elastically mounted on the sensor substrate about a first axis running geometrically midway between the first end and the second end; at least one movable electrode arranged on the seismic mass and movable together with it; at least one first stationary electrode arranged on the sensor substrate; sensor electronics and an evaluation circuit for measuring a capacitance between the at least one stationary electrode and the movable electrode. R. 410804

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[0033] Electrode, wherein a change in the distance between the at least one stationary electrode and the movable electrode is detected by the evaluation circuit as a change in capacitance; wherein the at least one stationary electrode has an armature region projecting vertically from and connected to the sensor substrate and a projecting region extending substantially perpendicularly from an end of the armature region located away from the sensor substrate, and wherein the projecting region has a layered structure produced according to a method disclosed above.

[0034] According to another aspect, the following is disclosed:

[0035] A MEMS gyroscope comprising: a sensor substrate with a first surface; a first electrode fixed to the sensor substrate and oriented substantially parallel to the first surface; a seismic mass movably mounted along the first surface of the sensor substrate and substantially trough-shaped, comprising: a bottom section oriented substantially parallel to the first surface; a wall section extending substantially perpendicular to the bottom section and originating from an edge region of the bottom section; wherein the bottom section comprises a layered structure produced according to a method according to any one of claims 3 to 9, with a first and a second electrically differently polarized electrode regions separated by a vertical barrier layer.

[0036] Brief description of the characters

[0037] The invention is explained in more detail below with reference to exemplary embodiments and the accompanying schematic drawing, which is not to scale. The figures (Fig.) in the drawing are merely examples: R. 410804

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[0039] Fig. 1 schematically shows an embodiment of a MEMS layer structure;

[0040] Fig. 2 schematically shows an embodiment of a MEMS layer structure; Fig. 3 schematically shows components of an embodiment of a MEMS accelerometer; and

[0041] Fig. 4 schematically shows components of an embodiment of a MEMS gyroscope.

[0042] The following schematic description of the structure and function of a method for producing a microelectromechanical (MEMS) layer structure, as well as a MEMS accelerometer and a MEMS gyroscope, is based on Figures 1 to 4. Corresponding reference symbols are used for corresponding features.

[0043] According to step S1 in a proposed method for producing a MEMS layer structure 100, which is schematically depicted in Figure 1, a first layer structure 10 of a first semiconductor material is deposited on a substrate. As shown in Figure 1, the first layer structure comprises at least one first layer 12 of the semiconductor substrate, but it can also comprise several layers.

[0044] The first semiconductor material can include silicon, preferably polycrystalline silicon.

[0045] In step S2, at least one first barrier layer 30 is deposited on the first layer structure 10. The first barrier layer 30 comprises silicon-rich nitride (SiRiN) and, in the embodiment of the process shown in Figure 1, is a barrier layer 30 deposited horizontally above the first layer 12. When applied to a silicon layer, the silicon-rich nitride protects this silicon layer during subsequent structuring processes.

[0046] In the present disclosure, “horizontal” means an orientation that is parallel to an orientation of a contact plane 102 between a substrate not shown and the first layer 12, shown in Figure 1a as a dashed line R. 410804

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[0048] The line is indicated. In Figures 1 to 4, the horizontal direction always corresponds to the x or y direction of the Cartesian coordinate system shown in the figures. A "vertical" orientation runs perpendicular to the horizontal direction and is indicated in the figures by the z direction. In the context of layer structures as shown in Figures 1 and 2, the vertical direction is also referred to as the thickness direction of the layer structure; the horizontal direction can therefore also be described as the direction perpendicular to the thickness direction of the layer structure.

[0049] In embodiments of the method, the first layer structure 10 and the second layer structure 20 form an epitaxy. Epitaxy is understood to mean that at least one crystallographic orientation of the crystals of the individual layers corresponds to the orientation of the layer below. In multi-stage epitaxy processes, the method disclosed herein makes it possible to deposit the SiRiN barrier layer 30 between two epitaxy steps. In embodiments, the first layer 12 is produced as an epitaxially grown layer in step S1; in embodiments, this occurs in several epitaxy steps, i.e., in several layers. The first barrier layer 30 is then deposited horizontally above the first layer 12 in embodiments of the method, as already mentioned, in step S1.In step S3, a second layer structure 20 is deposited on the barrier layer 30, wherein the second layer structure 20 comprises at least a second layer 22 of a second semiconductor material. In Figure 1, the second layer structure also comprises a third layer 23 and a fourth layer 24. In the embodiment shown in Figure 1, the first layer 12, the second layer 22, the third layer 23, and the fourth layer 24 are arranged as epitaxially grown layers on a substrate (not shown) and together form a functional layer. The second semiconductor material can comprise silicon, preferably polycrystalline silicon.

[0050] Figure 1b schematically shows the MEMS layer structure 100 after step S4, in which a first trench 40 and a second trench 40' are arranged in the second layer structure 20. This can be done by means of an etching process, for example plasma-assisted etching as described in R. 410804.

[0051] - 7 - reactive ionic deep etching. In this process, an etch front penetrates the second layer structure 20 from a first surface 101 of the second layer structure 20, perpendicular to the first surface 101, in the area of ​​the first trench 40 and the second trench 40'. The areas of the first surface 101 that are not to be structured can be masked, for example, by photoresist or another mask in a manner known per se. Because SiRiN is not attacked by the etching process, the etch front can only penetrate as far as the first barrier layer 30. The areas of the first surface 101 that are no longer present after etching are shown by dashed lines in Figure 1b.

[0052] As can be seen in Figure 1b, the method disclosed herein thus allows structures such as the trenches 40, 40' to be produced in a single, epitaxially grown functional layer without the epitaxy being penetrated by the structure in its entire layer thickness.

[0053] Since the first barrier layer 30' comprises SiRiN, it can also function as an electrical insulator. Figure 2b shows an embodiment of a MEMS layer structure 100' in which, in addition to the horizontally oriented first barrier layer 30, a second, vertical barrier layer 30' is arranged perpendicular to the first barrier layer 30 in the first layer structure 10, thus dividing the first layer 12 into a first section 12a and a second section 12b, which are electrically separated from each other. Another application example for such an arrangement will be described in more detail later. The MEMS layer structure 100' shown in Figure 2b also comprises a third barrier layer 30", which is likewise vertically oriented and thus arranged perpendicular to the first barrier layer 30 in the first layer structure 10.The third barrier layer 30” separates the second section 12b from a third section 12c and causes electrical separation of these two sections.

[0054] In one embodiment, a method for producing the MEMS layer structure 100' of Figure 2b comprises a step S1a, indicated in Figure 2a, which involves performing an etching process to generate the R. 410804

[0055] - 8 - vertical barrier layers 30', 30” corresponding vertical trench structures 50, 50' in the first layer structure 10. The etching process can include the aforementioned plasma-assisted etching and, in particular, reactive ionic deep etching. In this embodiment, step S2 comprises depositing the barrier layers 30', 30” into the vertical trench structures 50, 50'.

[0056] The person skilled in the art will recognize that for the formation of the vertical barrier layers 30' and 30" it is not necessary to also form the horizontal barrier layer 30, and the disclosed method therefore includes embodiments in which only one or more vertically oriented barrier layers are formed in the MEMS layer structure. Likewise, the method disclosed herein includes embodiments in which one or more horizontal barrier layers, optionally in combination with one or more vertical barrier layers, are formed, optionally arranged vertically one above the other and / or horizontally next to each other.

[0057] The method described above advantageously enables the fabrication of MEMS sensors. In this context, Figure 3 shows components of a MEMS accelerometer 200. The MEMS accelerometer 200 is a so-called out-of-plane accelerometer comprising a sensor substrate 231 and a seismic mass 210 extending between a first end 212 and a second end 214. The seismic mass 210 is spring-loaded and movable about a first axis 216, which is geometrically centered between the first end 212 and the second end 214. The seismic mass 210 can perform a tilting or rocking motion about the first axis 211. As can be seen in Figure 3a, the seismic mass 210 has a cavity 216 at the second end 214, while it is solid at the first end 212.Due to this arrangement, the seismic mass 212 reacts to an acceleration perpendicular to the plane of the drawing, i.e., in the z-direction with respect to the Cartesian coordinate system shown in Figure 3, by tilting about the first axis 211, i.e., by moving the seismic mass 210 out of its plane (English: "out of plane"). R. 410804.

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[0059] At least one electrode 230, movable with the seismic mass 210, is arranged in a region of the seismic mass 210, preferably at the first end 212 and / or the second end 214. In embodiments, electrodes with opposite polarities are arranged at the first end 212 and the second end 214. Correspondingly, a first stationary electrode 222 and a second stationary electrode 222 are arranged on the sensor substrate 231, the electrical potential of which is opposite to the potential of the at least one movable electrode 230.In an undisplaced configuration of the seismic mass 210, the at least one movable electrode 230 has a defined overlap or distance to the first fixed electrode 222 and / or to the second fixed electrode 222', and a specific capacitance between the at least one movable electrode 230 and the respective fixed electrodes 222, 222' corresponds to this distance. This capacitance can be measured by sensor electronics and a corresponding evaluation circuit. When an acceleration in the z-direction acts on the accelerometer, the seismic mass 210 is converted from an undisplaced configuration to a displaced configuration by a tilting motion, as described above, and the overlap or distance between the at least one movable electrode 230 and the fixed electrodes 222, 222' changes.As a result, the capacitance changes between the at least one movable electrode 230 and the stationary electrodes 222, 222'. The change in capacitance is registered by the evaluation circuit and can be used as a measure of the acting acceleration.

[0060] The two stationary electrodes 222, 222' are essentially identical in construction. The following explanations are based on the first stationary electrode 222, but also apply to the second stationary electrode 222'. As can be seen in Figure 3a in combination with Figure 3b, the first stationary electrode 222 has an anchor region 224 extending vertically from a first end 224a and connected to the sensor substrate 231. In the embodiment shown in Figure 3a, the anchor region 224 has a cell structure in cross-section to provide an R. 410804

[0061] - 10 - to achieve high stiffness with low mass. At a second end 224b located away from the first end 224a, a cantilevered area 226 of the first fixed electrode 222 projects essentially perpendicularly from the armature area 224.

[0062] As can be seen in Figure 3b, the first electrode 222 essentially comprises two functional layers: a first, thicker functional layer 234 and a second, thinner functional layer 233. In some applications, such functional layers of different thicknesses are used to achieve greater freedom in the design of the structures to be produced. This allows for a certain variety of design elements, for example, the shoebox structure of the projecting area with a base and a protruding wall, visible in Figure 3b.

[0063] As can be further seen in Figure 3b, the projecting area 226 additionally has a layer structure 100' comprising a first layer structure 10' and a second layer structure 20'. The first layer structure 10' is separated from the second layer structure 20' by a barrier layer 30'. In a process for manufacturing the MEMS accelerometer 200, the layer structure 100' is produced by the process described in connection with Figure 1. After deposition of the first layer structure 10', the barrier layer 30', and the second layer structure 20', a recess 228 is produced in the second layer structure 20' by an etching process originating from a surface 229 of the second layer structure 20'. Since Figure 3b shows the completed stationary electrode 222, the surface 229 which is no longer present in the completed layer structure 100' is represented by a dashed line.The first layer structure 10', the barrier layer 30', and the second layer structure 20' are produced as epitaxially grown layers in the embodiment shown.

[0064] As already mentioned, the structure of the cantilevered area 226 shown in Figure 3b is also referred to as a shoebox in connection with out-of-plane accelerometers and, as explained above, can be produced in a single step by applying the method disclosed herein. R. 410804

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[0066] A functional layer 234 is produced. This structure can therefore also be described as an epitaxial shoebox. By applying this method, the mass of the projecting area 226 can be kept small, which in turn allows the armature area 224 to be made less bulky. This increases the frequency of the first fixed electrode 222, which has a positive effect on the electrical performance of the sensor. Furthermore, if the armature area 224 is small, the first free-standing electrode 222 can be positioned close to the first axis 216, which also improves the offset performance of the MEMS accelerometer 200.

[0067] With reference to Figure 4, an application of the method for manufacturing a MEMS gyroscope 300 is disclosed below, in which both the possibilities of electrical separation and structuring are used.

[0068] The MEMS gyroscope 300, of which only selected components are shown in Figure 4, comprises a sensor substrate 302 with a first surface 330, a first stationary electrode 306 attached to the sensor substrate by means of a bearing area 304 and oriented substantially parallel to the first surface 330, and a seismic mass 308 movably mounted along the first surface 330 and shaped substantially like a trough. The seismic mass 308 has a bottom section 311 oriented substantially parallel to the first surface and a wall section 310 enclosing the bottom section 311. The wall section 310 extends substantially perpendicular to the bottom section 311 along an edge region of the bottom section 311. In embodiments, the wall section 310 is substantially closed.The seismic mass 308 was produced according to a procedure as described above, which will be described in more detail later.

[0069] The seismic mass 308 is elastically attached to the sensor substrate 302 by means of spring elements (not shown) and is excited to a drive oscillation parallel to the first surface 330 by means of a suitable drive. The drive oscillation has a drive frequency. Not shown in the figure is a Coriolis element that is elastically attached to the seismic mass 308. R. 410804

[0070] - 12 - is attached and, at a rotation rate perpendicular or parallel to the first surface 330, experiences a periodic Coriolis acceleration perpendicular to the driving oscillation and perpendicular to the rotation rate. The periodic Coriolis acceleration includes the detection oscillation with a corresponding

[0071] Detection frequency. To maximize the signal-to-noise ratio, the gyroscope is operated in full resonance, meaning that the frequencies of the drive mode and the detection mode are aligned. Since, due to manufacturing imperfections, the seismic mass 308 also experiences vibration components perpendicular to the drive direction when the drive oscillation is driven, the gyroscope must be quadrature-compensated to increase the signal-to-noise ratio. This is achieved by appropriately connecting a first electrode range 312 and a second electrode range 312'.

[0072] For this purpose, the seismic mass 308 is designed as a layered structure 100” in which the first electrode region 312 and the second electrode region 312' are integrated. The first 312 and second 312' electrode regions are electrically separated and oppositely polarized by a vertical barrier layer 322. This design allows quadrature compensation to be achieved using a single substrate-side fixed electrode 306, thus enabling more efficient use of space.

[0073] The seismic mass 308 is constructed in some aspects similarly to the cantilevered area of ​​the fixed electrode of the MEMS accelerometer 200 described in connection with Figure 3, namely in the manner described above as a shoebox. As can be seen in Figure 4, the layer structure 100” of the seismic mass 308 has a first layer structure 10” and a second layer structure 20”. The first layer structure 10” is separated from the second layer structure 20” by a barrier layer 30”. In a process for manufacturing the MEMS gyroscope 300, the layer structure 100” is produced by the process described in connection with Figures 1 and 2. After deposition of the first layer structure 10”, an etching process is first carried out in the first layer structure 10” to create a vertical trench structure 321 in the R. 410804

[0074] - 13 - to produce the first layer structure 10”. Subsequently, the vertical barrier layer 322 is deposited into the vertical trench structure 321 to electrically separate the first 312 and second 312' electrode regions formed in the first layer structure 10”. In a next step, the barrier layer 30” is deposited on the first layer structure 10”, and subsequently the second layer structure 20” is deposited on the barrier layer 30”. The recess 316 is then produced in the second layer structure 20” by an etching process originating from a surface 317 of the second layer structure 20”. As in Figure 3b, the surface 317, which is no longer present in the finished product, is represented by a dashed line in Figure 4.

[0075] The method for fabricating a MEMS layer structure has been described above using the examples of a MEMS accelerometer 200 and a MEMS gyroscope 300. In general, however, the method can be applied to any structure created with a single epitaxial layer, where a protective SiRiN layer is introduced between two epitaxial steps to prevent the complete digging or structuring of the epitaxial layer.

[0076] The invention is not limited to the described and illustrated embodiments. Rather, it also encompasses all further developments by skilled craftsmen within the scope of the invention defined by the claims. In addition to the described and illustrated embodiments, further embodiments are conceivable, which may include further modifications and combinations of features.

Claims

R. 410804 - 14 - Claims 1. Method for fabricating a microelectromechanical (MEMS) layer structure (100), comprising the following steps: 51 - Deposition of a first layer structure (10, 10', 10") comprising at least a first layer (12) of a first semiconductor material on a substrate; 52 - Deposition of at least one first barrier layer (30, 30', 30") on the first layer structure (10, 10', 10"), wherein the first barrier layer (30, 30', 30") comprises silicon-rich nitride (SiRiN).

2. The method according to claim 1, wherein step S2 comprises depositing the first barrier layer (30, 30', 30") at least partially horizontally above the first layer (12), the method comprising the following steps: 53 - Deposition of a second layer structure (20, 20', 20") on the first barrier layer (30, 30', 30"), the second layer structure (20, 20', 20") comprising at least a second layer (22) of a second semiconductor material; 54 - Execution of an etching process to generate a structure (40) in the second layer structure (20, 20', 20"), wherein boundary conditions of the etching process are set such that the etching process does not attack at least one first barrier layer (30, 30', 30").

3. A method according to one of claims 1 and 2, the method comprising the following step: S1a - Execution of an etching process to produce a vertical trench structure (50, 50') in the first layer structure (10, 10', 10"); wherein step S2 comprises depositing the first barrier layer (30, 30', 30") into the vertical trench structure (40, 40'). R. 410804 - 15 - 4. Method according to any one of claims 1 to 3, wherein the first semiconductor material and / or the second semiconductor material comprise silicon, preferably polycrystalline silicon.

5. Method according to any one of claims 1 to 4, wherein step S1 comprises producing the first layer (12) as an epitaxially grown layer.

6. Method according to any one of claims 2 to 5, wherein step S3 comprises producing the second layer (22) as an epitaxially grown layer.

7. Method according to any one of claims 2 to 6, wherein in step S4 the execution of the etching process comprises the execution of reactive ion deep etching.

8. Method according to any one of claims 1 to 7, wherein the at least one barrier layer (30, 30', 30") acts as an electrical insulator.

9. Method according to any one of claims 1 to 8, wherein the MEMS structure (100) is part of a MEMS accelerometer or a gyroscope.

10. MEMS accelerometer (200) comprising: a sensor substrate; a seismic mass (210) extending between a first end (212) and a second end (214), which is spring-elastically mounted on the sensor substrate about a first axis (216) extending geometrically midway between the first end (212) and the second end (214); at least one movable electrode (230) arranged on the seismic mass (210) and movable together with it; at least one first stationary electrode (222) arranged on the sensor substrate; sensor electronics and an evaluation circuit for measuring a capacitance between the at least one stationary electrode (222) and the movable electrode (230), wherein a change in the distance between the at least one stationary electrode (222) and the movable electrode (230) is detected by the evaluation circuit as a change in capacitance; R. 410804 - 16 - wherein the at least one stationary electrode (222) has an anchor region (224) projecting vertically from and connected to the sensor substrate and a projecting region (226) extending substantially perpendicularly from an end of the anchor region (224) away from the sensor substrate, and wherein the projecting region (226) has a layer structure (100) produced according to a method according to any one of claims 1 to 9.

11. MEMS gyroscope (300), comprising: a sensor substrate (302) with a first surface (330); a first electrode (306) fixed to the sensor substrate and oriented substantially parallel to the first surface (330); a seismic mass (308) movably mounted along the first surface (330) of the sensor substrate (302) and substantially trough-shaped, comprising: a ground section (311) oriented substantially parallel to the first surface; a wall section (310) extending substantially perpendicular to the ground section (311) and originating from an edge region of the ground section (311); wherein the bottom section (311) has a layer structure (100) produced according to a method according to one of claims 3 to 9, comprising a first (312) and a second (312') electrically differently polarized electrode regions separated by a vertical barrier layer (322).

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