Method for producing a micromechanical layer structure with high aspect ratio and micromechanical layer structure

Abstract

The invention relates to a method for producing a micromechanical layer structure with a high aspect ratio of a layer thickness (33) to a distance (34) of a first structural element (31) from an adjacent second structural element (32) in a main direction of extent (35) of the layer structure, said method having the steps of: (A) providing a substrate (10) with etching stop layer (20) arranged thereon and a micromechanical functional layer (30) arranged above the etching stop layer; (B) forming at least one recess (36) in the functional layer (30) by etching as far as the etching stop layer (20); (C) depositing an intermediate layer sequence (40) comprising at least a first insulation layer (42), an intermediate layer (44) and a second insulation layer (46); (D) filling the recess (36) by depositing a filling layer (50); (E) planarising the surface of the filling layer (50); (F) etching the intermediate layer (44) by etching access points (70) through the intermediate layer sequence (40) by a first etching process; and (G) exposing the first structural element and the second structural element by etching the first insulation layer (42), and the second insulation layer (46) by a second etching process. The invention also relates to a micromechanical layer structure with a high aspect ratio and to a capacitive sensor and a capacitive actuator having such a layer structure.

Description

[0001] Description

[0002] title

[0003] Method for producing a micromechanical layer structure with a high aspect ratio and micromechanical layer structure

[0004] State of the art

[0005] Conventional capacitive MEMS actuators and sensors have the disadvantage of being very area-intensive. Achieving high forces, deflections, or even measurement signals in the substrate plane requires comparatively large areas. MEMS electrode combs are currently manufactured by directly etching a conductive electrode layer. The maximum capacitance density is therefore limited by the achievable aspect ratio of the electrode gap with DRI E-etching of approximately < 30:1 (etch depth : etch trench width).

[0006] However, MEMS loudspeakers as described in document DE102019203914 require energy-efficient actuators with a very high aspect ratio (HAR) and thus high force density. The force density can be increased by increasing the aspect ratio of the electrode height to the width of the electrode gap.

[0007] Document W020078541 describes a nanoelectric drive (NED) actuator, but its fabrication is very challenging. Due to the achievable aspect ratios in DRI e-etching, such NED actuators with electrode spacings of ~2.5 pm can conventionally only be fabricated to a height of less than approximately 75 pm, as disclosed in the article by B. Kaiser, S. Langa, L. Ehrig, et al., "Concept and proof for an all-silicon MEMS micro speaker utilizing air chambers." Microsystems and Nanoengineering 5, 43 (2019).

[0008] Object of the invention

[0009] The object of the invention is to produce electrostatic MEMS electrode gaps with an aspect ratio of >30:1 (etch depth : etch trench width), which exceeds the current state of the art. Core and advantages of the invention

[0010] The invention relates to a method for producing a micromechanical layer structure with a high aspect ratio of a layer thickness to a distance of a first structural element from an adjacent second structural element in a main extension direction of the layer structure, comprising the steps:

[0011] (A) providing a substrate having an etch stop layer arranged thereon and a micromechanical functional layer arranged above the etch stop layer;

[0012] (B) producing at least one recess in the functional layer by etching up to the etch stop layer;

[0013] (C) depositing an intermediate layer sequence comprising at least a first insulation layer, an intermediate layer and a second insulation layer;

[0014] (D) filling the recess by depositing a backfill layer;

[0015] (E) planarizing the surface of the backfill layer;

[0016] (F) etching the intermediate layer by etching accesses through the intermediate layer sequence using a first etching process; and

[0017] (G) exposing the first structural element and the second structural element by etching the first insulation layer and the second insulation layer with a second etching process.

[0018] The above-mentioned object is achieved by the method according to the invention, since it enables the production of, in particular freely movable, micromechanical structures with a very large aspect ratio.

[0019] The method can be advantageously used, for example, for the production of NED actuators, for example in MEMS loudspeakers. The invention thus also relates to a micromechanical layer structure with a high aspect ratio, as well as to a capacitive sensor and a capacitive actuator with such a layer structure.

[0020] Drawing Figure 1 shows schematically a micromechanical device with anisotropically etched structures in the prior art.

[0021] Figures 2a to j show the method according to the invention for producing a device with a micromechanical structure with a high aspect ratio using a device in various stages of production in a first embodiment.

[0022] Figures 3a to c show the method according to the invention in a second embodiment.

[0023] Figure 4 shows schematically the method according to the invention for producing a device with a micromechanical structure with a high aspect ratio.

[0024] Figure 5 shows an alternative embodiment of the process step of Figure 2c.

[0025] Description of the figures and further embodiments

[0026] Figure 1 schematically shows a micromechanical device with anisotropically etched structures according to the prior art. An etch stop layer 20 is deposited over a substrate 10, and a micromechanical functional layer 30 is deposited above this. The micromechanical functional layer has a height of approximately 75 pm. Using an anisotropic DRI E etching process (DRIE - deep reactive ion etch), narrow trenches approximately 2.5 pm wide are introduced into the micromechanical functional layer. The anisotropic etching thus enables an aspect ratio of 30:1 (height to width). The device thus has electrodes 32 with a height of 75 pm and electrode gaps 35 with a width of 2.5 pm.

[0027] Figures 2a to j illustrate the method according to the invention for producing a device with a micromechanical structure with a high aspect ratio, using a device in various stages of production in a first exemplary embodiment. They show the production of a MEMS electrode pair, for example, for a MEMS actuator.

[0028] Figure 2a shows the provision of a substrate 10 with at least one buried, preferably insulating etch stop layer 20 or sacrificial layer with a thickness of 0.5-2.5 μm and a micromechanical functional layer 30 with a thickness of 20-800 μm. The etch stop layer helps to precisely define the electrode height of the MEMS actuator. At the same time, the etch stop layer 20 can serve as a sacrificial layer to expose the functional layer 30. Furthermore, in the areas remaining after the time-controlled sacrificial layer etching, it serves to anchor and insulate the functional regions from the substrate.

[0029] A recess 36 is etched into the functional layer at the locations intended for at least one filling or electrode, extending to the etch stop layer (Figure 2b). The functional layer has a thickness of > 75 pm, and the recess has a width of < 8 pm. The width of the recess corresponds to the intended width of an electrode plus twice the desired electrode spacing.

[0030] Subsequently, an intermediate layer sequence 40 is deposited, consisting of a first insulating layer 42 with a layer thickness of 10–2000 nm, an intermediate layer 44 with a thickness of 50–5000 nm, and a second insulating layer 46 with a thickness of 10–2000 nm (Figure 2c). The two insulating layers can be formed, for example, from SiO2 and deposited using LPCVD-TEOS or by thermal oxidation. For the intermediate layer 44, a suitable material is polysilicon deposited by LPCVD, for example.

[0031] Subsequently, a filler layer 50 is deposited, which now fills the originally etched recess 36 as completely and void-free as possible (Figure 2d). Conductive polysilicon, tungsten, or similar materials can be used as a filler layer for an electrode. Alternatively, silicon-rich nitride is also conceivable for purely mechanical functions.

[0032] Figure 2e shows the planarization of the surface and the partial exposure of the buried intermediate layer sequence 40 by chemical mechanical polishing (CMP).

[0033] Optionally, additional important (auxiliary or functional) layers 60 for the functionality of the component, such as a hard mask, electrical contact layers, or bonding layers, can be applied and structured in this state (Figure 2f). The previously applied layers of the intermediate layer sequence can also be structured, for example, to define sacrificial layer areas, enable electrical contact, etc.

[0034] Figure 2g shows that, if not already present, etching accesses 70 to the defined sacrificial regions 38 in the micromechanical functional layer 30 and the intermediate layer 44 are subsequently created.

[0035] In a first sacrificial layer etching step, the designated sacrificial areas of the mechanical functional layer 30 and the intermediate layer 44 are removed (Figure 2h). This can be done, for example, using SF6 or XeF2 etching. The advantage of this is the large achievable undercut width compared to slow HF gas-phase etching.

[0036] Finally, the remaining sacrificial regions of the first and second insulation layers 42, 46 and the etch stop layer 20 intended for removal are removed using a second sacrificial layer etching process (Figure 2i). This can be, for example, an RF gas-phase etching process. The previous removal of the intermediate layer 44 creates a cavity that enables immediate RF gas-phase etching on the entire cavity surface, even with large undercut widths, which significantly simplifies the exposure process. This process therefore allows for the production of electrode pairs approximately >75...800 pm high with an electrode gap of approximately 100 nm - 10 pm, whereas the prior art process only allows for electrode gaps <75 pm high and >2.5 pm wide.

[0037] Alternative embodiments of the manufacturing method according to the invention are feasible.

[0038] After etching the recess in the functional layer (Figure 2b), the surface can be smoothed, for example by tempering (thermal annealing).

[0039] If necessary, the electrode gap can be further narrowed by depositing conductive material in the form of an electrically conductive layer 80 onto the electrodes after sacrificial layer etching. Alternatively or additionally, a surface passivation layer 85 made of, for example, Al2O3 / SiO2 can be deposited using ALD (atomic layer deposition) to protect the surface (Figure 2j).

[0040] After deposition of the layers of the intermediate layer sequence, individual or all of these layers can optionally be anisotropically etched back, whereby they are removed from horizontally lying regions and remain on the sidewalls of the recess. Figure 5 shows an alternative embodiment of the process step of Figure 2c. It shows the deposition of an intermediate layer sequence 40 comprising a first insulating layer 42, an intermediate layer 44, and a second insulating layer 46, for example, a lower oxide, a polysilicon, and an upper oxide, with each of the layers being anisotropically etched back after deposition. The result is a structure on the sidewalls of the recess 36 that defines the width of the electrode gap to be created.

[0041] When depositing the backfill layer, as shown in Figure 2d, voids may be included.

[0042] Figures 3a to c show the method according to the invention in a second embodiment, wherein voids are removed or avoided.

[0043] Figure 3a shows, analogous to Figure 2d, the deposition of a polysilicon fill layer 50. This can include voids 55, usually centrally.

[0044] In a next step, the filling layer 50 is therefore partially etched back, and the cavities 55 are opened in the process (Figure 3b).

[0045] Subsequently, another layer of the filling layer 50 is deposited. The voids 55 are filled and disappear (Figure 3c).

[0046] In summary, Figure 4 schematically shows the method according to the invention for producing a device with a micromechanical structure with a high aspect ratio.

[0047] The method comprises the necessary steps: (A) providing a substrate 10 having an etch stop layer 20 arranged thereon and a micromechanical functional layer 30 arranged above the etch stop layer;

[0048] (B) producing at least one recess 36 in the functional layer 30 by etching up to the etch stop layer 20;

[0049] (C) depositing an intermediate layer sequence 40 comprising at least a first insulation layer 42, an intermediate layer 44 and a second insulation layer 46;

[0050] (D) filling the recess 36 by depositing a filling layer 50;

[0051] (E) planarizing the surface of the backfill layer 50;

[0052] (F) Etching the intermediate layer 44 through etching accesses 70 through the intermediate layer sequence 40 using a first etching process; and

[0053] (G) Exposing the first structural element and the second structural element by etching the first insulation layer 42 and the second insulation layer 46 with a second etching process.

[0054] In step (F), sacrificial regions 38 in the functional layer 30 can also be etched.

[0055] In step (G), the etch stop layer 20 can also be etched at least in regions.

[0056] The method according to the invention enables the production of micromechanical structures and devices with a high aspect ratio.

[0057] The invention thus creates an electrostatic MEMS electrode pair with a vertical electrode gap with an aspect ratio of >30:1. This allows for the creation of a high-capacitance micromechanical capacitor structure. Such a MEMS capacitor can also be assembled from multiple electrostatic MEMS electrode pairs. If one of the electrodes is designed to be movable, a capacitor structure with variable electrical capacitance can be created. This capacitance can be used for both detection and drive purposes.

[0058] The invention thus also provides a micromechanical sensor with a capacitive measuring element. The micromechanically manufactured variable capacitance consists of at least one fixed first electrode and at least one movable second electrode.

[0059] Furthermore, the invention provides an electrostatic micromechanical actuator with at least one micromechanical electrode pair, in particular a so-called NED actuator.

[0060] List of reference symbols

[0061] 10 Substrat

[0062] 20 Etch stop layer

[0063] 30 micromechanical functional layer

[0064] 31 first micromechanical structural element, electrode

[0065] 32 second micromechanical structural element, electrode

[0066] 33 Layer thickness

[0067] 34 Distance, electrode gap

[0068] 35 Main direction of extension

[0069] 36 recess

[0070] 38 Victims Area

[0071] 40 Intermediate layer sequence

[0072] 42 first insulation layer

[0073] 44 Intermediate layer

[0074] 46 second insulation layer

[0075] 50 backfill layer

[0076] 60 additional layers

[0077] 70 Etching access

[0078] 80 electrically conductive layer

[0079] 85 Passivation layer

Claims

Claims 1. A method for producing a micromechanical layer structure with a high aspect ratio of a layer thickness (33) to a distance (34) of a first structural element (31) from an adjacent second structural element (32) in a main extension direction (35) of the layer structure, comprising the steps: (A) providing a substrate (10) having an etch stop layer (20) arranged thereon and a micromechanical functional layer (30) arranged above the etch stop layer; (B) producing at least one recess (36) in the functional layer (30) by etching up to the etch stop layer (20); (C) depositing an intermediate layer sequence (40) comprising at least a first insulation layer (42), an intermediate layer (44) and a second insulation layer (46); (D) filling the recess (36) by depositing a filling layer (50); (E) planarizing the surface of the backfill layer (50); (F) etching the intermediate layer (44) through etching accesses (70) through the intermediate layer sequence (40) using a first etching process; and (G) exposing the first structural element and the second structural element by etching the first insulation layer (42) and the second insulation layer (46) with a second etching process.

2. A method for producing a micromechanical layer structure with a high aspect ratio according to claim 1, characterized in that after step (D) and before step (F) the filling layer (50) is etched back and a further layer of the filling layer is deposited.

3. A method for producing a micromechanical layer structure with a high aspect ratio according to claim 1 or 2, characterized in that after step (E) and before step (F) further layers (60) are deposited on the filling layer (50) and the intermediate layer sequence (40).

4. A method for producing a micromechanical layer structure with a high aspect ratio according to one of the preceding claims 1 to 3, characterized in that in step (E) or after step (E) and before step (F) etching accesses (70) are produced through the intermediate layer sequence (40) to sacrificial regions (38) in the functional layer (30).

5. A method for producing a micromechanical layer structure with a high aspect ratio according to one of the preceding claims 1 to 4, characterized in that in step (F) sacrificial regions (38) are etched in the functional layer (30).

6. A method for producing a micromechanical layer structure with a high aspect ratio according to one of claims 1 to 5, characterized in that in step (G) or after step (G) the etching stop layer (20) is etched at least in regions and in particular the first structural element (31) and / or the second structural element (32) is made movable.

7. Micromechanical layer structure, with an aspect ratio of a layer thickness (33) to a distance (34) of a first structural element (31) from an adjacent second structural element (32) in a main extension direction (35) of the layer structure of > 30:

1.

8. Micromechanical layer structure according to claim 7, characterized in that a first electrode is formed by the first structural element and a second electrode is formed by the second structural element, between which the distance (34) forms an electrode gap.

9. Micromechanical layer structure according to claim 8, characterized in that the first electrode and / or the second electrode is designed to be movable and that the first and the second electrode form a capacitor with variable electrical capacitance.

10. A micromechanical sensor comprising a capacitive measuring probe having a micromechanical structure according to claim 8 or 9.

11. Micromechanical actuator with a capacitive drive having a micromechanical structure according to claim 8 or 9.

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