WO2006013119A1

WO2006013119A1 - Microstructured sensor and method for production thereof

Info

Publication number: WO2006013119A1
Application number: PCT/EP2005/052469
Authority: WO
Inventors: Thorsten Pannek
Original assignee: Robert Bosch Gmbh
Priority date: 2004-07-31
Filing date: 2005-05-31
Publication date: 2006-02-09
Also published as: DE102004037304A1

Abstract

The invention relates to a microstructured sensor comprising at least: a substrate (1), a cavity (6), etched in the substrate, a membrane (7), embodied to extend freely above the cavity and laterally bonded on a membrane edge (11) and a sensor layer (3) with sensor structures, whereby the membrane edge (11) is separated from etch surfaces (10) of the cavity. According to the invention, the membrane edge is separated from the irregular etch edges of the etch surfaces such as to improve the membrane stability. The membrane edge (11) can hence be embodied in a lateral recess (9) of the side wall (10) of the cavity (6) and separated from an upper edge (10a) of the side wall. Alternatively the side wall of the cavity can be embodied by a vertically running annular layer, whereby the annular layer and a lower membrane layer of the membrane are parts of a filler layer embodied on the substrate and the membrane edge in the filler layer is embodied in the transition between the annular layer and the lower membrane layer.

Description

Microstructured sensor and method for its manufacture

The invention relates to a microstructured sensor and a method for its production.

Such microstructured sensors may in particular be thermal sensors or temperature sensors. These can be z. For example, infrared sensors or spectroscopic gas sensors in which a temperature-sensitive measuring structure, e.g. a thermopile structure made of contact printed conductors, and an absorber layer covering this is applied for absorption of infrared radiation. Furthermore, such a temperature sensor can also serve as a side-crash sensor for measuring a temperature increase that occurs in the case of adiabatic compression of a trapped gas volume in a vehicle door. In addition to temperature sensors continue to z. As pressure sensors or mass flow sensors relevant in which are formed on a self-supporting membrane measuring structures.

Surface micromechanical sensors are typically fabricated by a gas phase etching process using a silicon selectively etching gas, e.g. ClF3. In this case, the etching gas is passed through a perforated membrane layer formed on the substrate and a cavity is etched under the membrane in order to achieve thermal decoupling of the membrane from the bulk silicon of the substrate.

During the etching of the cavern, etching fronts are formed in the bottom region of the cavern as well as on their side surfaces, which are defined essentially by the crystal planes. The etching process is largely isotropic. Since several etching openings are required in particular for large-surface membrane with surfaces> 0.5 mm ² for complete undercut, run different, from the individual etching openings emanating etched fronts zu¬ together. At the intersections of the etched fronts corresponding peaks form, at which a preferred tearing of the thin membranes is possible. As a result, the stability and resilience of the membrane is impaired.

In contrast, the sensor according to the invention and the method according to the invention for its production have several advantages:

By means of suitable measures both in construction and in the process guidance, the membrane edge in which the self-supporting membrane is connected laterally is decoupled from the irregular etching flanks of the etching surfaces, so that the membrane stability is improved.

The decoupling can be done in different ways. According to a first embodiment of the invention, by temporarily forming a sacrificial layer on the substrate and below the membrane layer, a lateral decoupling of the membrane edge and the upper edge of the etched side wall of the cavern is achieved. The e.g. SiO2 sacrificial layer is after the cavern etching process step in an additional process step by supplying an etching gas not etching the silicon of the substrate, e.g. HF, removed.

According to a further embodiment of the invention, the sidewall of the cavern is formed by a previously formed ring layer extending vertically into the substrate, so that the membrane edge is formed between this ring layer and a lower membrane layer. The ring layer is not attacked by the etching gas during the etching process, so that no etching fronts are formed in the region of the membrane edge. In order to form the annular layer, an annular trench or ring trench is first formed in the substrate, which is subsequently filled with a filling layer filling the same, wherein the filling layer furthermore keeps the substrate upper surface. surface covered between the ring layer. This region of the fill layer may subsequently be used as the lower membrane layer or removed to form other membrane layers. The cavern below the membrane can be completely covered by the ring layer or undercut it by stopping the etching process later. The etching openings or passages in the membrane can be filled by subsequent application of a cover layer, if this is desired in the relevant sensor. In accordance with the invention, basically any desired micromechanical sensors can be provided with a membrane, eg. As temperature sensors, in particular infrared sensors or spectroscopic gas sensors or side impact sensors, pressure sensors or mass flow sensors continue to be produced.

The invention will be explained below with reference to the accompanying Zeichnun¬ gene on some embodiments. Show it:

1 to 6 an inventive manufacturing method of a sensor according to the invention according to one embodiment:

1 shows a first process step after application of a sacrificial layer on a substrate;

FIG. 2 shows a subsequent process step after formation of the membrane layer; FIG.

3 shows a subsequent process step after applying a

Lacquer coating and photolithographic structuring;

4 shows a subsequent process step after undercut the

Membrane;

5 shows a subsequent process step after removal of the sacrificial layer; FIG. 6 shows a sensor according to the invention produced by closing the membrane according to this embodiment; FIG.

FIGS. 7 to 13 describe a manufacturing method for a sensor according to a further embodiment with a ring trench:

FIG. 7 shows a first process step of forming an annular etching pit in the substrate; FIG.

8 shows a subsequent process step of applying a filling layer;

9 a subsequent process step according to an alternative with removal of the filling layer on the surface within the ring trench;

FIG. 10 shows a process document, which is provided as an alternative to FIG. 9 and follows FIG. 8, of the application of sensor structures on the filling layer;

FIG. 11 shows a process step following the perforation of the layer system above the substrate following FIG. 10; FIG.

FIG. 12 shows a subsequent process step of undercutting the membrane within the ring trench; FIG.

FIG. 13 shows a state optionally following FIG. 12 on continuation of the etching process. On a substrate 1 made of silicon, a sacrificial layer 2 of, for example, SiO 2 is deposited by means of, for example, CVD and structurally structured (in plan view). The annularly structured sacrificial layer 2 in this case advantageously has the shape and approximate dimensions of the later membrane. The etching flanks 2a or edges of the annular sacrificial layer 2 are preferably made flat, since the membrane to be formed will extend over these edges. The annular sacrificial layer 2 may also be produced in a Local Oxidation of Silicon (LOCOS) process that produces a bird's beak profile correspondingly having very shallow flanks. The thickness of the sacrificial layer 2 is preferably thin, for example in the range of 100 nm, but may also be listed up to several μm thick, in particular when a LOCOS method is used.

From the substrate 1 and the sacrificial layer 2, a sensor layer 3 with sensitive structures is produced as shown in FIG. It may be formed in particular of Si3N4 or SiO2; it is made of a material different from the material of the sacrificial layer 2

The sensor layer 3 is subsequently photolithographically patterned according to FIG. 3 both in the region on the sacrificial layer 2 and on the substrate 1. Here, a lacquer layer 4 is applied and openings 5 are formed by known photolithographic structuring, which extend through the lacquer layer 4 and the sensor layer 3 both to the sacrificial layer 2 and to the substrate 1.

Subsequently, the sensor layer 3 is undercut with a silicon etching medium, such as CIF 3 or XeF 2, by etching, by supplying the etching gas through the openings 5. The sacrificial layer 2 made of SiO.sub.2 or another material resistant to the etching gas, eg Si.sub.3N.sub.4, is not attacked in the case of the selectively etch-free etching process. In the next step, according to FIG. 5, the annular sacrificial layer 2 is removed. For this purpose, for example, an etching gas, eg HF, which etches the material of the sacrificial layer 2 through the openings 5, does not substantially etch the material of the sensor layer 3. By means of this HF vapor etching, instead of the earlier sacrificial layer 2, an annular lateral recess 9 is formed above the upper edge 10a of the side walls 10 of the cavity 6.

Thus, a self-supporting membrane 7 undercut by the cavern is formed, which is connected to the substrate 1 in a membrane edge 11, which lies in the lateral direction outside the upper edge 10a of the side wall 10. According to FIG. 5, the side wall 10 thus no longer directly adjoins the membrane 7 exposed by the cavity 6.

The side walls 10 and the bottom of the cavity 6 form the etching front during the etching of the cavity 6 in the substrate 1. The diaphragm edge 11, which represents the transition from the substrate 1 to the diaphragm 7, lies in the lateral recess 9 according to the invention Accordingly, it is not part of the etch front.

According to FIG. 6, the lacquer layer 4 can subsequently be removed and a die

Openings 5 closing cover layer 12 are applied when a sensor 14 is to be formed with closed openings.

Whereas in the first embodiment of FIGS. 1 to 6, an annular lateral incision or a laterally retracted membrane edge is formed in the upper wall region of the cavern in order to prevent cracking from the etching front into the membrane, this becomes apparent in the prior art 7 to 13 achieved by formation of a vertically extending from the top down ring wall or annular wall layer in the cavern. According to FIG. 7, the lateral enclosure of the later etching pit is first defined in the substrate 1 by removing silicon from the substrate 1 annularly around its area. Thus, an annular trench 20 is formed in the substrate 1, which is preferably very narrow with a ring thickness of, for example, 0.1 to 10 .mu.m in order to be able to completely fill it again later. The depth of the ring trench 20 is determined by the future depth of the cavern.

According to FIG. 8, the ring trench 20 is subsequently filled with a material which is resistant to the etch medium used later for etching the grains, e.g. SiO2 or Si3N4. In this case, preferably a CVD method with good edge coverage is used to prevent plugging of the trench before it is completely filled. According to FIG. 8, a filling layer 22 is thus applied which has a ring layer 22a within the ring trench 20, a laterally inner surface layer 22b within the ring trench 20 on the surface 1a of the substrate 1, and a laterally outer surface layer 22c laterally outside the ring trench 20 - points.

For further formation, the laterally inner surface view 22b may be included or removed in the formation of the membrane. Fig. 9 first shows the alternative in which the laterally inner surface layer 22b within the ring layer 22a is formed by appropriate structuring, e.g. by etching with HF, is removed. Thus, sensor layers can subsequently be applied to the substrate surface 1a. Alternatively, according to FIG. 10, one sensor layer 24 or even a plurality of sensor layers 24 are applied directly to the laterally inner surface layer 22b to form the sensor structures.

Subsequently, the layer 24, 22 b in the region in which the etching pit is to be created later, is perforated as shown in FIG. 11 with the formation of openings 25 extending to the substrate 1.

Subsequently, the membrane 26 formed from the surface view 22b and the sensor layer 24 is undercut by passing through the openings 25 the etching gas, eg CIF3 or XeF2, is supplied. The undercut can be made to varying degrees. According to the embodiment shown in FIG. 12, a cavity 29 is etched within the ring layer 22a, so that the ring layer 22a completely encloses the cavity 29 or forms its sidewall. In this embodiment of the sensor 32 thus the bottom 28 of the cavity 29 is above or at the same height as the lower end of the ring layer 22a.

As an alternative to this, the etching process can be continued beyond the etching of the cavity 29 so that, as shown in FIG. 13, the cavity 30 extends beyond the depth of the ring layer 22a and the ring layer 22a is undercut. In the course of the etching, the cavity 30 works on the basis of its extensive isotropy in the same way down as well as again toward the surface of the substrate 1 on. The etching is therefore stopped at the latest shortly before reaching the substrate surface. This results in a maximum etching depth of the cavity 30 of approximately twice the depth of the ring trench 20 or the ring layer 22a. In the variant of the sensor 33 of FIG. 13, freestanding columns of the ring layer 22a thus arise which, depending on the etching depth, are more or less strongly connected to the surrounding bulk silicon of the substrate 1.

In the embodiments of FIGS. 9 and 10, the diaphragm edge 27 is thus between the ring layer 22a and the lower membrane layer 22b or, if the lower membrane layer 22b according to FIG. 9 is removed, between the ring layer 22a and a subsequently applied membrane formed ranband. Subsequently, a cover layer can again be applied to the components shown in FIGS. 12 and 13, or the membrane 26 can be left with the openings 25.

In all embodiments shown, to form an infrared sensor or spectroscopic gas sensor prior to the etching of the cavern, for example, a thermopile structure can be made up of conductor tracks which are contacted with one another different materials with different Seeback coefficients auf¬ brought and subsequently applied an absorber layer of, for example, a metal oxide for the absorption of infrared radiation. To form a temperature sensor, furthermore, a bolometer structure with a structural element exposed to the radiation can be formed, the resistance of which changes as a function of its temperature and thus of the absorbed radiation. To form a pressure sensor, a cover cover and, for example, a piezoresistive resistor can be applied and contacted.

In both embodiments of FIGS. 12 and 13, the edges of the etching pit or cavern 29, 30 are no longer determined by the isotropic CIF3 etching with their peak formation, but by the geometry of the ring layer 22a or the previously formed ring trench 20. Thus Membrane geometries can be produced in largely arbitrary and very precise embodiments, even with isotropic etching processes.

Claims

claims

A microstructured sensor comprising at least: a substrate (1), a cavern (6, 29, 30) etched into the substrate (1), a membrane (7, 26) which is located above the cavern (6, 29, 30 ) is cantilevered and laterally connected in a diaphragm edge (11, 27) and has a sensor layer (3, 24) with sensor structure, wherein the diaphragm edge (11, 27) of etching surfaces (10, 28, 31) of the beam (6 , 29, 30) is separated.

2. Microstructured sensor according to claim 1, characterized gekennzeich¬ net, that the side wall (10) of the cavern (6) is formed as an etching surface, and the diaphragm edge (11) in a lateral recess (9) of the side wall (10) and formed by an upper edge (10a) of the side wall (10) is separated.

3. A microstructured sensor according to claim 2, characterized gekennzeich¬ net, that the diaphragm edge (11) on the substrate upper side (1a) and la¬ teral outside the upper edge (10a) of the side wall (10) is arranged.

4. A microstructured sensor according to claim 1, characterized gekennzeich¬ net, that the side wall of the cavern (29, 30) by a vertically erstre¬ ckende ring layer (22a) is formed, and the diaphragm edge (27) between the ring layer (22a) and a lower membrane layer (22b) is formed.

5. Microstructured sensor according to claim 4, characterized net, that the annular layer (22a) and a lower membrane layer (22b) of the membrane (26) are parts of a filling layer (22) formed on the substrate (1), and the membrane edge (27) in the filling layer (22) is trained.

6. A microstructured sensor according to claim 4 or 5, characterized gekenn¬ characterized in that the filling layer (22) extends laterally from the lower Memb¬ ranschicht (22 b) outwardly as the substrate (1) covering the surface layer.

7. A microstructured sensor according to any one of claims 4 to 6, characterized in that the filling layer (22) of a against a Silizi¬ sensitive etching gas (CIF3) resistant material, e.g. SiO 2 or Si 3 N 4.

8. A microstructured sensor according to any one of claims 4 to 7, characterized in that the annular layer (22a) the cavern (29) voll¬ constantly comprises and designed as an etching surface (28) bottom (28) of the cavern (29) above or on the same height of the lower end of the annular layer (22 a) is arranged.

9. Microstructured sensor according to one of claims 4 to 7, characterized in that the cavern (30) extends below the ring layer (22a) and this partially undercut.

10. A microstructured sensor according to one of the preceding claims, da¬ characterized in that on the upper side of the membrane (7, 26) a cover layer (12) is formed, which closes the openings (5, 25) in the membrane.

11. Microstructured sensor according to one of claims 1 to 10, characterized characterized in that it is designed as a pressure sensor with a pressure or Spannungsungsempfindli¬ chen measuring device, for example, a piezoresistive element formed in or on the membrane (7, 26).

12. Microstructured sensor according to one of claims 1 to 10, da¬ characterized in that it is designed as a temperature sensor, infrared sensor, spectroscopic gas sensor or side crash sensor and on the membrane (7, 26) is formed a thermopile structure or bolometer structure.

13. A method for producing a microstructured sensor (14, 32, 33), comprising at least the following steps:

Forming a membrane layer (3, 22b, 24) with sensor structures on a substrate (1),

Forming openings (5, 25) in the membrane layer (3, 22 b, 24), and

Supplying etching gas (Cl F3) through the openings (5, 25) and etching a cavity (6, 29, 30) in the substrate (1) to form a cantilever membrane (7, 26), wherein a membrane edge (11, 27), in which the membrane (7, 26) is laterally connected, is formed separately from the etching surfaces (10, 28, 31) of the cavern (6, 29, 30) in the substrate (1).

14. The method according to claim 13, characterized in that at least the following steps are provided:

Depositing and structuring an annular sacrificial layer (2) on the substrate (1), forming the membrane layer (3) with sensor structures on the substrate (1) and the sacrificial layer (2),

Structuring of openings (5) in the membrane layer (3) which extend to the sacrificial layer (2) and to the substrate (1), Supplying etching gas (CIF3, XeF2) through the openings (5) and etching a cavity (6) in the substrate (1) to form a freitra¬ ing membrane (7) below the membrane layer (3) such that an upper edge (10 a) formed as an etching surface side wall (10) on the underside of the sacrificial layer (2), and

Removing the sacrificial layer (2) by selective etching, wherein the diaphragm edge (11) from the upper edge (10 a) of the Seiten¬ wall (10) is laterally spaced.

15. The method according to claim 14, characterized in that the sacrificial layer is formed of SiO 2 and for etching the sacrificial layer

(2) a SiO 2 selective etchant, silicon non-corrosive etching gas (HF) is used.

16. The method according to claim 14 or 15, characterized in that in the photolithographic structuring of the membrane layer

(3) lacquer layer (4) applied to the membrane layer (3) remains on the membrane layer (3) during the selective etching of the sacrificial layer (2).

17. The method according to claim 13, characterized in that the method comprises at least the following steps: formation of a ring trench (20) in the substrate (1), applying a filling layer (22) on the substrate top (1 a) and in the ring trench (20 ) such that the ring trench (20) is filled with a ring layer (22a) which merges in one piece into a lower membrane layer (22b) which is formed on the substrate surface (1a) within the ring layer (22a) , subsequent application of at least one sensor layer (24), structuring of openings (25) through at least the sensor layer

(24) to the substrate (1), supply of etching gas through the openings (25) and etching of a Ka verne (29, 30) at least partially disposed within the ring layer (22a) to form the membrane (26), wherein the membrane edge (27) is disposed between the ring layer (22a) and the lower membrane layer (22b).

18. Method according to claim 17, characterized in that the at least one sensor layer (24) is applied to the lower membrane layer (22b) and subsequently the openings (25) through the sensor layer (24) and the lower membrane layer (22 b) are formed.

19. The method according to claim 17, characterized in that the lower membrane layer (22b) before application of the sensor layer (24) in the region within the annular layer (22 a) is removed.

20. The method according to any one of claims 17 to 19, characterized gekenn¬ characterized in that the etching process of the etching of the cavern (29) is stopped, while the cavity (29) is completely surrounded by the annular layer (22 a).

21. The method according to any one of claims 17 to 19, characterized gekenn¬ characterized in that the annular layer (22a) of the cavern (30) is partially undercut.