Abstract
A method for producing a microelectromechanical sensor. The microelectromechanical sensor is produced by connecting a cap wafer to a sensor wafer. The cap wafer has a bonding structure for connecting the cap wafer to the sensor wafer. The sensor wafer has a sensor core having a movable structure. The cap wafer has a stop structure for limiting an excursion of the movable structure. The method includes a first step and a second step following the first step, the stop surface of the stop structure being situated at the level of the original surface of the unprocessed cap wafer.
Claims
1-9. (canceled)
10. A method for producing a microelectromechanical sensor, the method comprising: connecting a cap wafer with a sensor wafer, the cap wafer having a bonding structure for connecting the cap wafer with the sensor wafer, the sensor wafer having a sensor core having a movable structure, and the cap wafer having a stop structure configured to limit an excursion of the movable structure; wherein the method has a first step and a second step following the first step, a hard mask being applied onto a subregion of the cap wafer in the first step, the masked subregion of the cap wafer defining a stop surface of the stop structure, a bonding layer being applied onto the cap wafer in the second step, and the bonding structure being produced by etching the bonding layer.
11. The method as recited in claim 10, wherein the movable structure of the sensor core includes two substructures connected to one another by a polysilicon bridge, such that, in a rest state of the movable structure, the polysilicon bridge is at a distance, in a direction of excursion, from an immovable structure of the sensor core.
12. The method as recited in claim 10, wherein, in a third step, preceding the first step, an oxide layer is applied onto the cap wafer and a protective structure is produced by etching the oxide layer, the protective structure protecting the sensor core against penetration of material from the bonding structure during the connection of the cap wafer with the sensor wafer.
13. The method as recited in claim 10, in which, in a fourth step, preceding the first step, a recess is produced on a surface of the cap wafer by etching, and, in the first step, a further hard mask is applied onto a further subregion of the cap wafer, the further subregion being situated inside the recess, and the masked further subregion of the cap wafer defining a surface of a capping electrode.
14. The method as recited in claim 10, wherein, in a fifth step, following the second step, a recess for the formation of a cavern is produced by etching.
15. The method as recited in claim 12, wherein, in a sixth step, following the second step, the hard mask is removed.
16. The method as recited in claim 15, in which, in a seventh step, following the sixth step, the bonding structure and/or the protective structure is provided with a protective lacquer that protects the bonding structure and/or the protective structure from removal of material during the removal of the hard mask.
17. A microelectromechanical sensor, comprising: a sensor wafer having a sensor core that has a movable structure; and a cap wafer that has a stop structure, the cap wafer having a bonding structure for connecting the cap wafer with the sensor wafer, the sensor wafer and the cap wafer being bonded to one another by a eutectic alloy, the stop structure of the cap wafer configured to limit an excursion of the movable structure of the sensor core, the movable structure having two substructures bonded to one another by a polysilicon bridge, such that, in a rest state of the movable structure, the polysilicon bridge is at a first distance, in a direction of excursion, from an immovable structure of the sensor core, and, in the rest state, the movable structure being at a second distance, in a direction of excursion, from a stop surface of the stop structure, wherein the first distance is greater than the second distance.
18. The micromechanical sensor as recited in claim 17, wherein a hard mask is applied onto a subregion of the cap wafer, the masked subregion of the cap wafer defining the stop surface of the stop structure, and a bonding layer is applied onto the cap wafer, and the bonding structure is produced by etching the bonding layer
19. The microelectromechanical sensor as recited in claim 17, wherein the cap wafer has a capping electrode that is situated in a recess of the cap wafer, such that, in the rest state, the movable structure is at a third distance, in the direction of excursion, from a surface of the capping electrode, the third distance being greater than or equal to the second distance.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIGS. 1a, 2a, 3a, 4a, 5a, 6a, 7a, and 8a schematically show a cap wafer in various successive stages of a production method according to the related art.
[0021] FIGS. 1b, 2b, 3b, 4b, 5b, 6b, 7b, and 8b schematically show a cap wafer in various stages of a specific embodiment of the production method according to the present invention.
[0022] FIGS. 1c, 2c, 3c, 4c, 5c, 6c, 7c, and 8c schematically show a cap wafer in various stages of a further specific embodiment of the production method according to the present invention.
[0023] FIG. 9a schematically shows a sensor according to the related art, formed by joining a sensor wafer and a cap wafer.
[0024] FIG. 9b schematically shows a specific embodiment of the microelectromechanical sensor according to the present invention formed by joining a sensor wafer and a cap wafer.
[0025] FIG. 9c schematically shows a further specific embodiment of the microelectromechanical sensor according to the present invention formed by joining a sensor wafer and a cap wafer.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
[0026] In the figures, FIGS. 1c, 2c, 3c, 4c, 5c, 6c, 7c, 8c, and 9c correspond to various stages in the production process of a cap wafer 2, or of the sensor 1 formed thereby, according to a specific embodiment of the method according to the present invention. FIGS. 1b, 2b, 3b, 4b, 5b, 6b, 7b, 8b, and 9b show a variant of the method according to the present invention in which, in addition to stop structure 7, a capping electrode 17 is formed as part of cap wafer 2. FIGS. 1a, 2a, 3a, 4a, 5a, 6a, 7a, 8a, and 9a show a production method according to the related art. The respectively corresponding stages of the various production processes are shown one over the other (see for example the sequence of FIGS. 1a, 1b, 1c, which show the respective first substep at the beginning of the processing of the cap wafer), so that the differences, in particular missing or additional substeps, can easily be worked out. In the various Figures, identical or equivalent parts are always provided with the same reference characters, and are therefore as a rule also each named or mentioned only once.
[0027] As is shown in FIGS. 1a through 1c, at the beginning of the processing of cap wafer 2 a protective structure 15, having a typical thickness 15′ between 1.1 μm and 1.7 μm, is produced by deposition and structuring of an oxide. As is described in more detail below in relation to FIGS. 9a through 9c, this protective structure 15 is used to prevent, during heating of bonding material 4′, 25, bonding material 4′, 25 from penetrating into sensor core 5 and impairing or damaging the functional components of the microelectromechanical structure.
[0028] As is shown in FIG. 2b, a recess 10 of cap wafer 2 is formed in a substep that follows the deposition and structuring of protective oxide 15. In the level defined by floor surface 10′ of recess 10, a capping electrode 17 of cap wafer 2 is formed in the subsequent substeps. This substep is not present in the production methods shown in FIGS. 2a and 2c, in which no such formation of a capping electrode 17 takes place.
[0029] In the following, as is shown in FIGS. 3b and 3c, a hard mask oxide 8″ is deposited on the surface of cap wafer 2. As can be seen in FIGS. 4b and 4c, through structuring, a hard mask 8 or 8a′ is subsequently formed from hard oxide layer 8″, each mask covering particular subregions 9, 9′ of the wafer surface and protecting them against removal of material during subsequent etching. In the variant shown in FIG. 4c, not having capping electrode 17, mask 8 covers a subregion 9 that protects a subsurface of wafer 2 against being material removal, which subsurface will later form stop surface 7′ of stop structure 7. In the variant shown in FIG. 4b, hard masks 8, 8′ cover two subregions 9, 9′, of which the first subregion 9 again defines stop surface 7′ of stop structure 7, while further subregion 9′ in recess 10 defines a surface 17′ of capping electrode 17. The production of hard mask 8 according to FIGS. 3b, 3c, and 4b, 4c corresponds to the first step of the method according to the present invention, and is not present in FIGS. 3a and 4a, which show the method according to the related art.
[0030] In the next substep, bonding structure 4 is produced, with which cap wafer 2 is later connected to sensor wafer 3. For this purpose, a germanium layer is deposited and structured by etching. This corresponds to a specific embodiment of the second step of the method according to the present invention. As can be seen in FIGS. 5a through 5c, here overetching results in an additional height difference 16; i.e., the etching not only structures the germanium layer but also removes material from the rest of the surface of cap wafer 2. Through the resulting surface 16′, in the related art as shown in FIG. 5a the level is defined that forms the starting point of all subsequent structuring processes and that therefore defines the maximum height of the subsequently formed structures. In the variants of the method according to the present invention shown in FIGS. 5b and 5c, in contrast, the areas covered by masks 8 or 8′ are protected against the overetching, so that the covered surfaces remain at the original height level, and are thus in particular situated higher than surface 16′ in FIG. 5a, which was formed using the conventional method from the related art.
[0031] In the next substep, protective structure 15 and bonding structure 4, as shown in FIGS. 6b and 6c, are provided with a protective lacquer 18 and hard mask 8, 8′ is removed. Here, protective lacquer 18 is used to protect protective structure 15 and bonding structure 4 when hard mask 8, 8′ is removed.
[0032] As is shown in FIGS. 7a through 7c, subsequently a cavern trench is carried out; i.e. by trenching of particular regions, for example using a lacquer mask, a recess 19 is created that later, during the joining with sensor wafer 3, forms the cavern in which sensor core 5 is enclosed. As is shown in FIGS. 7b and 7c, at the same time the remaining part of stop structure 7, or the capping electrode (in FIG. 7b), is formed. In the method according to the related art, in this substep the overall height of stop structure 7″ is formed, whose stop surface 7′″ is situated at the level of surface 16′ shown in FIG. 5a. As can be seen from a comparison of FIG. 7a with FIGS. 7b and 7c, stop surface 7′ of stop structures 7 shown in FIGS. 7b and 7c, which were produced using variants of the method according to the present invention, is situated significantly higher than stop surface 7′″ produced using the conventional method.
[0033] In FIGS. 8a through 8c, the height difference is 27, 27′, 28, produced by the various methods, of the structures of cap wafer 2 are shown. In each case the highest point is formed by upper side 29 of protective structure 15. In the method from the related art shown in FIG. 8a, height difference 27′ between highest level 29 and stop surface 7′″ of stop structure 7″ is typically 2.5 micrometers to 3.5 micrometers. In the two specific embodiments of the method according to the present invention shown in FIGS. 8b, and 8c, in contrast, height difference 27 is determined exclusively by the thickness of protective structure 15, and is typically 1.1 μm to 1.7 μm. Distance 30 between highest level 29 and upward-pointing surface 17′ of capping electrode 17 is selected somewhat larger than distance 27, and, as can be seen in the next FIG. 9b, determines, in assembled sensor 1, a height difference 22 between surface 17′ of the capping electrode and the upper edge of movable structure 6.
[0034] FIGS. 9a through 9c show sensors 1 produced by the various methods and made up of a cap wafer 2 and a sensor wafer 3. Outside protective structure 15, between cap wafer 2 and sensor wafer 3 there is situated a connecting element 25 made of germanium bonding structure 4 and an aluminum bonding structure 4′. For the subsequent eutectic bonding, from alloy partners 4, 4′ a eutectic alloy forms that materially bonds cap wafer 2 and sensor wafer 3 to one another. In the figure, only a part of connecting element 25 can be seen, which in its totality completely surrounds sensor core 5, and, during the eutectic bonding, hermetically separates the cavern from the external space. Protective structure 15 likewise surrounds the entire sensor core 5 and prevents liquid alloy material from moving into the sensor core when connecting element 25 is melted.
[0035] Sensor wafer 3 has a movable structure 6 made of two substructures 12, 12′ that are connected to one another by an FP bridge 13. An immovable structure 14 of the sensor core is situated between substructures 12 and 12′. When movable structure 6 moves in excursion direction 11, the amplitude of the movement is limited by stop structure 7 of cap wafer 2. When there are strong excursions, substructure 12 impacts stop surface 7′ of stop structure 7 and in this way prevents excursions that are greater than distance 21, or 21′, between stop surface 7′ and substructure 12. Preferably, stop structure 7 is formed in such a way that both substructure 12 and substructure 12′ impact stop structure 7 when there are strong excursions, and are stopped in this way. In sensor 1, produced according to the related art, in FIG. 9a, here there results the problem that distance 21′ comes out to be smaller than distance 20 between FP bridge 13 and immovable structure 14, so that when there is a correspondingly large amplitude of the excursion there is the danger that FP bridge 13 will impact immovable structure 14 and will be mechanically damaged or even destroyed.
[0036] In contrast, in sensors 1 shown in FIGS. 9b and 9c, produced using specific embodiments of the method according to the present invention, distance 21 is significantly smaller, and in particular is smaller than distance 20 between FP bridge 13 and immovable structure 14. In this way, movable structure 6 is caught by surface 7′ of stop structure 7 before FP bridge 13 can be damaged. In the variant having capping electrode 17 in FIG. 9b, distance 22 between the upper edge of cap wafer 2 and capping electrode 17 is equal to distance 20 between FP bridge 13 and immovable structure 14. In this way it is ensured that, when there are strong excursions, movable structure 6 is damaged neither by a mechanical contact between FP bridge 13 and immovable structure 14 nor by a contact between movable structure 6 and capping electrode 17.
