Micromechanical sensor device and corresponding manufacturing process

Abstract

Micromechanical sensor device comprising: a sensor substrate (1; 1'; 1''; 1''; 1'''') with a front (VS) and a back (RS); a sensor area (4) provided on the front (VS) which can be brought into contact with an environmental medium; wherein the sensor area (4) is elastically suspended in the sensor substrate (1; 1'; 1''; 1''; 1'''') by means of a spring device (7); wherein a cavity (6) is provided in the sensor substrate (1; 1'; 1''; 1''; 1'''') below the sensor area (4), which extends to the back (RS);and a grid layer (11) applied to the back (RS) which spans the cavity (6), wherein a cap substrate (9) is bonded to the front (VS) which has a media access (10) to the sensor area (4), and wherein the grid layer (11) is sealed media-tight by a pre-closing layer (12), and wherein one or more stabilizing ridges (16) of the sensor substrate (1'') are arranged in the cavity (6) on the grid layer (11), which are separated from the sensor area (4).

Description

The invention relates to a micromechanical sensor device and a corresponding manufacturing process. State of the art Although any micromechanical components can be used, the present invention and the underlying problem are explained using micromechanical pressure sensor devices. US patent 2015 / 0001651 A1 discloses a micromechanical pressure sensor device. Further prior art is also disclosed in the patent applications US 5 452 268 A , JP 2008 - 271 426 A , DE 10 2017 221 082 A1 , DE 10 2013 213 065 A1 and US 2010 / 0 052 082 A1. Fig. 6 is a schematic cross-sectional representation to illustrate the micromechanical pressure sensor device known from US 2015 / 0 001 651 A1. In Fig. 6, reference numeral 1 denotes a sensor substrate, for example a silicon sensor substrate, in which a thin membrane 3 is formed over a cavity 2. A sensor area 4 is provided in the membrane 3, which, for example, has piezoresistive resistors for pressure measurement. When external pressure is applied to the membrane 3, it is bent, whereupon the piezoresistive resistors change their resistance values, so that the applied external pressure can be determined by measuring the resistance values. To decouple the known micromechanical pressure sensor device from external forces, a cavity 6 is provided in the sensor substrate 1 below the membrane 3, and the sensor area 4 is additionally mechanically and elastically decoupled laterally from the sensor substrate 1 by a spring device 7. A rear-facing cap wafer 11 is bonded to the sensor substrate 1 from the rear side. A front-facing cap wafer 9 is bonded to the front of the sensor substrate 1, which has a pressure access point 10 for the sensor area 4. The rear-facing cap wafer 11 can, for example, also be designed as an evaluation ASIC. The assembly of the cap wafers 9, 11 and the sensor substrate 1 is bonded to a carrier substrate 8 and simultaneously encapsulated with a molding compound 5a. This is intended to decouple external forces acting on the housing 5 as much as possible from the sensor area 4. Thus, the known micromechanical pressure sensor device requires two cap wafers and the associated bonding processes, which makes the manufacturing process complex, and in addition, the known micromechanical pressure sensor device has a relatively large installation height. Disclosure of the invention The invention provides a micromechanical sensor device according to claim 1 and a corresponding manufacturing method according to claim 5. Preferred further training courses are the subject of the respective sub-claims. Advantages of the invention The micromechanical sensor device according to claim 1 or the corresponding manufacturing method according to claim 5 enables the provision of a stress-decoupled micromechanical sensor device that can be manufactured cost-effectively and has a low overall height. The underlying idea of ​​the present invention is that a grid layer is provided on the back side of the sensor wafer, with the etching process for the cavity located beneath the sensor area being carried out through this grid layer. On the front side, the sensor area is suspended by springs formed in the sensor substrate. The back grid layer can either be sealed after the etching process by a layer deposition or used as a media access point. In the latter case, it is advantageous to provide an evaluation ASIC for capping the front side instead of a capping wafer. According to the invention, a cap substrate is bonded to the front side, which has media access to the sensor area, and the grid layer is sealed media-tight by a sealing layer. This enables a thin rear seal. According to a further preferred embodiment, the sensor area is arranged on a stamp of the sensor substrate, which extends to the back. This further increases stability and protects against vibrations. According to the invention, one or more stabilizing ridges of the sensor substrate are arranged in the cavity on the grid layer, which are separated from the sensor area. This allows a thin, stabilized grid layer to be produced. According to a further preferred embodiment, the front side is bonded to an ASIC substrate, with lateral media access to the sensor area being formed in a space between the sensor substrate and the ASIC substrate, and the grid layer being sealed media-tight by a pre-coating layer. This eliminates the need for a front-side cap wafer and further increases compactness. According to a further preferred embodiment, the front side of the sensor substrate is bonded to an ASIC substrate in a media-tight manner, with the grid layer serving as the media access point. This eliminates the need for a front-side cap wafer and allows the grid layer to be used functionally. Brief description of the drawings Further features and advantages of the present invention are explained below with reference to embodiments and the figures. Figure 1 shows a schematic cross-sectional view to illustrate a micromechanical sensor device according to a first embodiment of the present invention; Figure 2 shows a schematic cross-sectional view to illustrate a micromechanical sensor device according to a second embodiment of the present invention; Figures 3a) and 3b) show schematic cross-sectional views to illustrate a micromechanical sensor device according to a third embodiment of the present invention, wherein Figure 3b) is a detail enlargement of detail A from Figure 3a); Figure 4 shows a schematic cross-sectional view to illustrate a micromechanical sensor device according to a fourth embodiment of the present invention; Figure 5 shows a schematic cross-sectional view to illustrate a micromechanical sensor device according to a fifth embodiment of the present invention; and Figure 6 shows...6 A schematic cross-sectional representation to illustrate a known micromechanical sensor device. Embodiments of the invention In the figures, identical reference symbols denote identical or functionally equivalent elements. Fig. 1 is a schematic cross-sectional representation to illustrate a micromechanical sensor device according to a first embodiment of the present invention. In Fig. 1, reference numeral 1 denotes a silicon sensor substrate with a front side VS and a back side RS. A sensor area 4 is provided on the front side VS, which, for example, has a cavity 2, a membrane 3, and piezoelectric resistors integrated into the membrane 3 (not shown). The sensor area 4 is elastically suspended in the sensor substrate 1 by a laterally arranged spring device 7. A cavity 6 is provided in the sensor substrate 1 below the sensor area 4, extending to the rear surface RS. A grid layer 11 is provided on the rear surface RS, which is made, for example, of a metal, in particular aluminum, or of an oxide layer, and which has small grid openings in the area of ​​the cavity 6, typically on the order of 1 to 10 µm. The grid layer 11 is sealed media-tight by a sealing layer 12, for example, a silicon nitride layer or a silicon oxide layer. On the front face VS, an insulating layer I, for example a silicon oxide layer, is provided, into which a conductor arrangement LB is integrated, connecting an electrical connection 13 of the sensor area to an electrical connection 14 for an external connection. A cap wafer 9 is bonded to the insulating layer I via a bond connection B, which has a media access 10 to the sensor area. The operating principle of the micromechanical sensor device constructed in this manner corresponds to that already described with reference to the known micromechanical sensor device according to claim 6. The thin grid layer significantly reduces the overall height, while maintaining the same level of stress decoupling. Although not shown, a housing can be provided analogously to the known micromechanical sensor device according to Fig. 6. To manufacture the micromechanical sensor device according to Fig. 1, in the present embodiment the sensor substrate 1 is first processed from the front side VS to produce the sensor area 4 and the spring assembly 7. Then the insulating layer I and the conductor track assembly LB embedded therein, as well as the electrical contacts 13, 14, are produced. Finally, the cap wafer 9 is bonded over the bonding area B. In a subsequent process step, if necessary, the sensor substrate 1 on the back side RS can first be ground down to the desired target thickness. Following this, the grid layer 11 is applied to the back side RS. A trenching process is preferably used as the etching process for perforating the lattice layer 11 in the area of ​​the cavity 6 to be etched. In such a trenching process, the lateral undercutting is preferably selected such that all the webs of the lattice layer 11 are completely undercut. The sealing of the lattice layer 11 is then preferably carried out by LPCVD deposition (low-pressure chemical vapor deposition), wherein silicon nitride or silicon oxide or combinations of such layers are preferred as the sealing layer 12. In a preferred embodiment, a nitride / oxide layer stack is used, which in total is selected such that the lattice layer 11 in combination with all sealing layers forms a layer that is easily under tensile stress and thus prevents bulging of the layer. After the back side RS is sealed and protected, the media access 10 can then be created in the cap wafer 9 in a further process step. Fig. 2 is a schematic cross-sectional representation to illustrate a micromechanical sensor device according to a second embodiment of the present invention. In Fig. 2, the sensor substrate is designated 1'. Unlike the sensor substrate 1 according to the first embodiment, the sensor substrate 1' according to Fig. 2 has a central plunger 15 beneath the sensor area 4. In other words, the sensor area 4 is arranged on a plunger 15 of the sensor substrate 1', which extends to the rear side RS, with the grid layer 11 being closed in the area of ​​the plunger 15. The plunger further stabilizes the sensor area 4, particularly for applications with high vibrations. The spring assembly 7 can optionally be designed to be softer in this embodiment than in the first embodiment. Otherwise, the function and structure of the second embodiment are the same as those of the first embodiment. Figs. 3a) ,b) are schematic cross-sectional views to illustrate a micromechanical sensor device according to a third embodiment of the present invention, wherein Fig. 3b) is a detail enlargement of section A of Fig. 3a). In the third embodiment, the sensor substrate is designated 1'', and the grid layer 11 is closed in strip-shaped areas with webs 16 of the sensor substrate 1''. The webs 16 serve to stabilize the grid layer 11. A two-stage etching process is used to produce the webs 16 with the overlying cavity 6 between sensor area 4 and the webs 16. The first stage of the etching process is carried out as an anisotropic trench etching process such that the grid structures in area 18 are undercut. The second stage of the etching process is carried out as an isotropic etching step in area 18 such that the webs 16 are etched. Fig. 4 is a schematic cross-sectional representation to illustrate a micromechanical sensor device according to a fourth embodiment of the present invention. In the fourth embodiment, the sensor substrate bears reference numeral 1''' and is constructed analogously to the sensor substrate 1 according to the first embodiment described above. In contrast to the first embodiment, however, no cap wafer is provided; instead, the sensor substrate 1''' is bonded to an evaluation ASIC 19 via a bonding area B. A bonding process is used that also enables an electrical contact 20 between the sensor substrate 1 and the evaluation ASIC 19. In this embodiment, the media access 21 is formed laterally to the sensor area 4 in a space between the sensor substrate 1''' and the ASIC substrate 19. Fig. 5 is a schematic cross-sectional representation to illustrate a micromechanical sensor device according to a fifth embodiment of the present invention. In the fifth embodiment, the sensor substrate bears reference numeral 1'''' and differs from the sensor substrate 1 according to the first embodiment in that no closure layer is provided on the grid layer 11. In this fifth embodiment, the front face VS of the sensor substrate 1'''' is bonded to an ASIC substrate 19a in a media-tight manner, with the grid layer 11 forming a media access point 25. Such an arrangement is referred to as a bare-die element 22. As in the fourth embodiment, there is a bonding area and an electrical contact 20 realized via the bonding connection between the sensor substrate 1 and the ASIC substrate 19a. The ASIC signal is routed via vias 23 to the side of the ASIC substrate 19a opposite the sensor substrate 1'''' and can be electrically connected to a corresponding carrier substrate via soldering areas 24. The grid layer 11, which serves as media access 25, is either not closed at all, in which case the grid area acts as particle protection. Additionally, the grid area 11 can be provided, for example, with a water-repellent layer (not shown), which can also act as a protective layer against a liquid medium. If, on the other hand, only a point-like access and not a planar access is desired as media access 25, a media access 25 can also be chosen in the grid layer 11 that is larger than the normal grid constant, whereby this is not closed in the closing process, whereas the smaller grid passages are closed with the closing layer 12. Although the present invention has been described with reference to preferred embodiments, it is not limited thereto. In particular, the materials and topologies mentioned are only examples and are not limited to the examples described. Particularly preferred further applications for the micromechanical sensor device according to the invention are, for example, chemical gas sensors such as metal oxide gas sensors, thermal conductivity sensors, Pirani elements, mass flow sensors such as air mass meters, lambda probes on micromechanical membrane, infrared sensor devices, etc.

Claims

Micromechanical sensor device comprising: a sensor substrate (1; 1'; 1''; 1''; 1'''') with a front (VS) and a back (RS); a sensor area (4) provided on the front (VS) which can be brought into contact with an environmental medium; wherein the sensor area (4) is elastically suspended in the sensor substrate (1; 1'; 1''; 1''; 1'''') by means of a spring device (7); wherein a cavity (6) is provided in the sensor substrate (1; 1'; 1''; 1''; 1'''') below the sensor area (4), which extends to the back (RS);and a grid layer (11) applied to the back (RS) which spans the cavity (6), wherein a cap substrate (9) is bonded to the front (VS) which has a media access (10) to the sensor area (4), and wherein the grid layer (11) is sealed media-tight by a pre-closing layer (12), and wherein one or more stabilizing ridges (16) of the sensor substrate (1'') are arranged in the cavity (6) on the grid layer (11), which are separated from the sensor area (4). Micromechanical sensor device according to claim 1, wherein the sensor area (4) is arranged on a punch (15) of the sensor substrate (1') which extends to the rear side (RS). Micromechanical sensor device according to claim 1, wherein the front side (VS) is bonded to an ASIC substrate (19), wherein a lateral media access (21) to the sensor area (4) is formed in an intermediate space between the sensor substrate (1''') and the ASIC substrate (19), and wherein the grid layer (11) is sealed media-tight by a pre-seal layer (12). Micromechanical sensor device according to claim 1, wherein the front side (VS) of the sensor substrate (1'''') is bonded to an ASIC substrate (19a) in a media-tight manner and wherein the grid layer (11) is designed as a media access (25). Method for manufacturing a micromechanical sensor device comprising the steps of: providing a sensor substrate (1; 1'; 1''; 1''; 1'''') with a front (VS) and a back (RS); forming a sensor area (4) provided on the front (VS) which is capable of contacting an environmental medium; forming a spring device (7) by means of which the sensor area (4) is suspended in the sensor substrate (1; 1'; 1''; 1''; 1''''); forming a grid layer (11) on the back (RS); forming a cavity in the sensor substrate (1; 1'; 1''; 1''; 1'''') below the sensor area (4), extending to the back (RS), by an etching process in which an etching medium is passed through the grid layer (11) into the sensor substrate (1; 1'; 1''; 1''; 1''''). wherein the spring device (7) is released so that the sensor area (4) in the sensor substrate (1; 1'; 1''; 1'';1'''') is elastically suspended, wherein a cap substrate (9) is bonded to the front side (VS), which has a media access (10) to the sensor area (4), and wherein the grid layer (11) is sealed media-tight by a pre-closing layer (12), and wherein one or more stabilizing ridges (16) of the sensor substrate (1) are formed in the cavity (6) on the grid layer (11) in a two-stage etching process, which are separated from the sensor area (4), wherein the etching process comprises a first anisotropic etching step and a second isotropic etching step.; Method according to claim 5, wherein when forming the cavity (6) a punch (15) of the sensor substrate (1'9) is formed which extends to the rear side (RS) so that the sensor area (4) is arranged on a punch (15) of the sensor substrate (1'). Method according to claim 5, wherein the front side (VS) is bonded to an ASIC substrate (19), wherein a lateral media access (21) to the sensor area (4) is formed in an intermediate space between the sensor substrate (1''') and the ASIC substrate (19), and wherein the grid layer (11) is sealed media-tight by a pre-closing layer (12). Method according to claim 5, wherein the front side (VS) of the sensor substrate (1'''') is bonded media-tight to an ASIC substrate (19a) and wherein the grid layer (11) is designed as a media access (25).

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