Stress-decoupled micromechanical pressure sensor and method for the production thereof

Abstract

The invention relates to a micromechanical pressure sensor (100), comprising: - a substrate (110) with a layer system (120) located thereon. The micromechanical pressure sensor (100) further comprises a sensor structure (200) which is self-supportingly fastened to the layer system (120) by means of at least one suspension structure (140) and which has a first membrane structure (210), a second membrane structure (220) and a cavity region (240) that is located between the two membrane structures (210, 220) and closed by a side wall (230) extending in an edge region (212, 222) of the two membrane structures (210, 220). The sensor structure (200) also has a useful capacitor (250) located in the cavity region (240) between the two membrane structures (210, 220), having a first electrode (251) fastened to the first membrane structure (210) and a second electrode (253) located opposite the first electrode (251) and fastened to the second membrane structure (220).

Description

[0001] Description

[0002] title

[0003] Stress-decoupled micromechanical pressure sensor and method for its manufacture

[0004] The invention relates to a stress-decoupled micromechanical pressure sensor in the form of a self-supporting double-membrane structure. Furthermore, the invention also relates to a manufacturing method for such a stress-decoupled micromechanical pressure sensor.

[0005] Micromechanical pressure sensors are used in a variety of applications. Among other things, capacitive pressure sensors are known in which a center electrode is arranged between two diaphragms. This arrangement is cantilevered onto a silicon substrate by at least one spring / armature structure. When pressure is applied to such an arrangement, the diaphragms simultaneously bend toward the center electrode. This causes a measurable change in capacitance, which can be used to measure or determine the applied pressure. In such an arrangement, the movable electrodes of the capacitive measuring system consist of the two diaphragms themselves. Due to the warping of the diaphragms when pressure is applied, a nonlinear capacitive sensor signal is generated. As a result, the pressure can only be measured linearly and with a relatively high resolution over a narrow pressure range.

[0006] Furthermore, the production of such a membrane arrangement requires the sacrificial layer between the membranes and the center electrode to be removed through holes in the uppermost membrane structure, which must then be subsequently sealed. Since this requires a certain thickness of sealing material, the membrane thickness cannot be made thinner than this sealing layer thickness, and / or the sealing material alters the flexural rigidity of the membrane structure, at least locally.

[0007] The object underlying the invention can therefore be seen as providing a possibility for implementing a stress-decoupled capacitive pressure sensor that has high sensitivity, a linear pressure sensor signal, and optional reference capacitances, which can advantageously be arranged in a space-saving manner within a cavern area. This object is achieved by means of the respective subject matter of the independent claims. Advantageous embodiments of the invention are the subject matter of respective dependent subclaims.

[0008] According to the invention, a micromechanical pressure sensor is provided which comprises a substrate with a layer system arranged thereon and a sensor structure which is self-supporting and fastened to the layer system by means of at least one suspension structure and which has a first membrane structure, a second membrane structure, and a cavern region arranged between the two membrane structures and enclosed by a side wall running in an edge region of the two membrane structures. The sensor structure comprises a useful capacitor structure arranged in the cavern region and having a first electrode preferably fastened to / in a central region of the first membrane structure and a second electrode arranged between the first electrode and the second membrane structure and preferably fastened to / in a central region of the second membrane structure.

[0009] By designing the sensor structure as a self-supporting double membrane structure, stress decoupling from the surrounding layer system and the silicon substrate is achieved. This ensures that mechanical bending (e.g. due to thermal expansion) of the sensor structure no longer has any influence on the resulting sensor signal. Furthermore, the self-supporting design means that applied external pressure can deform both the first membrane structure and the second membrane structure. In doing so, for example, the first and second electrodes, which are designed to be movable and attached / anchored to the first and second membrane structures, are moved towards one another. This makes it possible to achieve a change in the distance between the movable electrodes when the pressure changes, which is twice as large (with identical membrane structures) than with a sensor structure with only one movable electrode.In this way, in addition to stress decoupling, an increase in measurement sensitivity can also be achieved. While sensor structures with only one movable membrane require a larger and / or thinner membrane to achieve the same measurement sensitivity, or at least two smaller membrane structures must be electrically connected in a Wheatston half-bridge configuration, the same measurement sensitivity can be achieved with the self-supporting double membrane structure on a significantly smaller area. This has the advantage that the membrane structures can be designed to be more stable for a defined pressure range and are therefore mechanically more stable against applied overpressure. This can be further supported by the special stop structures that are attached to the membrane structures and can prevent excessive local deformation of the membrane structures.

[0010] With the proposed pressure measuring structure or pressure measuring arrangement, it is possible to achieve the same comparable measurement sensitivity or measurement accuracy on a significantly smaller area. Since the chip area makes a significant contribution to the overall cost of a sensor element, this approach can also reduce the cost of the sensor element while maintaining at least the same performance, or the smaller space requirement can be used to provide two pressure measuring structures / pressure measuring arrangements on the original area, enabling the measurement of two pressure measuring ranges or the measurement of a larger pressure measuring range. A further advantage of the electrodes of the described sensor structure, which are only suspended locally on the membranes, is that an essentially linear pressure measurement signal is achieved when pressure is applied.This is not the case with a sensor structure with a membrane structure that also serves as an electrode for the measuring capacitance. Since the proposed pressure sensor requires a significantly smaller area for the same measuring sensitivity, it is also possible to provide two pressure sensors with double membrane structures on the area required by a comparable pressure sensor with only a single membrane structure. These can be constructed identically and electrically connected, for example, in a Wheatston half-bridge configuration, whereby four times the measuring sensitivity can be achieved compared to two identically connected membrane structures each with only one movable electrode structure. Furthermore, the area advantage achieved with the double membrane structure can be used to provide another double membrane structure alongside an existing double membrane structure, one designed, for example, for a second pressure range.In this way, the area that a pressure sensor consisting of a membrane structure or two adjacent membrane structures would require to detect / measure one pressure range can be used to detect a second or larger pressure range.

[0011] In one embodiment, the electrodes are each attached to the respective membrane structure by means of at least one fastening structure in an inner region of the respective associated membrane structure. This allows a particularly linear measurement behavior of the pressure sensor to be achieved.

[0012] In one embodiment, at least one reference capacitor with two opposing stationary electrodes is provided in an edge region of the cavity region, with each of the stationary electrodes being arranged between the other stationary electrode and one of the membrane structures. The special design of this sensor structure makes it possible to provide integrated reference capacitors, which can advantageously be arranged in a space-saving manner in a cavity region between the deformable membrane structures.

[0013] In a further embodiment, the at least one reference capacitor is anchored at least partially or in regions in or on the side wall and is arranged between the membrane structures in a self-supporting manner and aligned plane-parallel to the membrane structures. This mechanically decouples the reference capacitor from the membrane structures to such an extent that any bending of the membrane structures due to external pressure has no significant influence on the reference measurement.

[0014] In a further embodiment, a silicon dioxide layer made of the material of the third silicon dioxide layer is arranged between the stationary electrodes of the reference capacitor. This allows the capacitance of the reference capacitor to be increased or, with the same capacitance, the reference capacitance structure to be reduced in area, which in turn allows the movable electrode structures of the useful capacitor to be made larger in area.

[0015] In a further embodiment, it is provided that at least one etching access to the cavern region is formed in the layer system, wherein the etching access has a closure which is formed by at least one material deposited on the surface of the layer system and / or introduced into the etching access and / or by at least one locally melted and re-solidified material of the layer system.

[0016] In a further embodiment, at least one etching access to the cavity region is formed in the layer system outside a deflectable sensing surface of the second membrane structure. The etching access has a closure formed by at least one material deposited on the surface of the layer system and / or introduced into the etching access, or by a locally melted and resolidified material of the layer system. Arranging the etching access outside the deflectable sensing surface of the membrane structure enables precise adjustment of the layer thickness and / or the mechanical properties of the membrane structure in the region of the deflectable sensing surface. This allows for greater measurement accuracy of the pressure sensor.In a further embodiment, the etching access is arranged outside the sensor structure and is connected to the cavity region by means of an etching channel extending through the at least one suspension structure. This arrangement enables a particularly large usable area of ​​the pressure sensor.

[0017] In a further embodiment, the etching access is arranged within the suspension structure. This arrangement allows for a smaller space requirement for the pressure sensor.

[0018] In a further embodiment, the etching access is arranged in an edge region of the sensor structure. This arrangement of the etching access(s) enables better and faster etching of the sacrificial layers within the cavity.

[0019] In a further embodiment, the sensor structure comprises additional anchoring structures for securing the membrane structures, wherein the at least one etching access is arranged between the circumferential sidewall and the additional anchoring structures. This improves the stability of the double membrane structure and reduces or prevents any influence of the closure structure / layer closing the at least one etching access on the measurement signal or measurement behavior.

[0020] In a further embodiment, the circumferential sidewall is formed by outer anchoring structures, inner anchoring structures, and silicon dioxide material remaining between the outer and inner anchoring structures of the silicon dioxide layers of the layer system. The additional silicon dioxide material increases the stability of the sidewall.

[0021] In a further embodiment, at least one mechanical stop structure is attached to at least one membrane structure in the cavern region, which limits the relative movement between the two membrane structures above a predetermined pressure value. Such stop structures prevent excessive local deformation of the membrane structures. This protects the sensor against mechanical overload.

[0022] According to a further aspect, a method for producing the aforementioned micromechanical pressure sensor is further provided, in which a semiconductor substrate is provided and a layer structure is created on the semiconductor substrate by depositing and structuring layers. In this case, a sensor structure is created in a functional region of the layer structure, which sensor structure is connected to the remaining layer structure via a suspension structure and comprises a first membrane structure structured from a first polysilicon layer, a second membrane structure structured from a fourth polysilicon layer, and a cavity region delimited by a side wall running in an edge region of the membrane structures.Furthermore, a useful capacitor is created in the cavity region, comprising a first electrode structured from a second polysilicon layer and attached to the first membrane structure, and a second electrode structured from a third polysilicon layer and attached to the second membrane structure. Subsequently, an etching process is used to remove silicon dioxide layers arranged between the polysilicon layers in the cavity region via at least one etching access having an opening in the fourth polysilicon layer. Subsequently, an etching process is used to remove a first silicon dioxide layer arranged between the first membrane structure and the surface of the semiconductor substrate. The advantages already mentioned in connection with the micromechanical pressure sensor result from this method.

[0023] In one embodiment, it is provided that the etching access is closed by depositing at least one material on the surface of the layer system and / or by melting at least the material of the upper layers of the layer system in the region of the etching access.

[0024] In one embodiment, the removal of the silicon dioxide layers in the cavity region and the removal of the first silicon dioxide layer beneath the first membrane structure are performed in a single etching process. This simplifies the manufacturing process.

[0025] In a further embodiment, trench structures are created in the surface of the semiconductor substrate beneath the sensor structure, which are not completely filled during the deposition of the first silicon dioxide layer. The trench structures cause the etching medium to be rapidly distributed beneath the first silicon dioxide layer during the etching of the first silicon dioxide layer beneath the first membrane structure. This can improve or accelerate the removal of the sacrificial layer beneath the sensor structure.

[0026] The invention is described in more detail below with reference to the figures, in which:

[0027] Fig. 1 schematically shows a cross section through a micromechanical pressure sensor according to a first embodiment with a self-supporting double membrane sensor structure and an etching access arranged outside the sensor structure,

[0028] Fig. 2 shows a schematic plan view of a micromechanical pressure sensor realized according to Figure 1,

[0029] Fig. 3 to Fig. 7 schematically show different stages of a manufacturing process for the micromechanical pressure sensor from Figure 1,

[0030] Fig. 8 schematically shows an alternative embodiment of the micromechanical pressure sensor in which the reference capacitor is filled with a dielectric,

[0031] Fig. 9 schematically shows an alternative embodiment of the unfinished pressure sensor, in which special trench structures are provided on the substrate to support the undercutting of the sensor structure, Fig. 10 schematically shows a cross-section of a micromechanical pressure sensor according to a second embodiment with a self-supporting double-membrane sensor structure and an etching access arranged in the edge region of the sensor structure,

[0032] Fig. 11 shows a schematic cross-section through the still unfinished micromechanical pressure sensor from Figure 10 before undercutting the sensor structure,

[0033] Fig. 12 schematically shows a plan view of the micromechanical pressure sensor from Figure 10,

[0034] Fig. 13 schematically shows an alternative embodiment of the micromechanical pressure sensor of Figure 10 with a reinforced side wall, and

[0035] Fig. 14 a simplified flow diagram of the manufacturing process.

[0036] The sensor concept explained in more detail below is based on a sensing area / sensor structure suspended cantilevered above a silicon substrate on at least one spring or anchoring structure, in which, however, no central electrode is provided between two deformable membranes, but rather electrode structures are provided locally suspended on the membrane structures, which move towards each other when the membranes are deformed and do not warp or bend themselves in the process.

[0037] Figure 1 shows, by way of example, a micromechanical pressure sensor 100 with a sensor structure 200 suspended in a self-supporting manner via at least one spring or suspension structure 140. The sensor structure 200 is produced in a layer system 120 arranged on a semiconductor substrate 110 and is separated from the remaining regions of the layer system 110 by one or more separation regions 161 extending along its circumference. The separation regions are separated from the surrounding regions of the layer structure 120 by etch stop structures 286, which in the present example are formed, among other things, by vertical regions of the layers 131, 124, 132, 133. The sensor structure 200, also referred to below as the sensing region, is a double-membrane structure with a first and second membrane 210, 220 and a cavern region 240 arranged between the two membranes 210, 220.The cavern region 240 is sealed gas-tight from the environment 300 by a side wall 230 arranged between the membrane structures 210, 220 and extending along the circumference of the sensor structure 200. The side wall 230 is formed by etched top structures, which in the present example also serve as anchoring structures for the membranes 210, 220 and the stationary electrodes 261, 262.

[0038] In a central region 241 of the cavern region 240, a useful capacitance 250 is arranged, which comprises two movable electrodes 251, 253 arranged opposite one another at a defined distance from one another and each fastened to one of the two membrane structures 210, 220, wherein the first electrode 251 is arranged between the second electrode 253 and the first membrane structure 210 and the second electrode 253 is arranged between the first electrode 251 and the second membrane structure 220. The first electrode 251 is fastened to / in an inner region 211 (such as the central region) of the first membrane structure 210 by means of at least one first fastening structure 252, while the second electrode 251 is fastened to or in an inner or central region 221 of the second membrane structure 220 by means of at least one second fastening structure 254.As an alternative to the embodiment shown here, in which each electrode 251, 253 is fastened by means of a fastening structure 252, 254 in the center of the respective membrane structure 210, 220, the fastening of the electrodes 210, 220 to the membrane structures 210, 220 can also be carried out by means of several fastening structures, which, depending on the design, can also be arranged in an inner region outside the exact center of the membrane structures 210, 220.

[0039] As can also be seen from Figure 1, mechanical stop structures 290 can also be located in the cavern region 240, which limit the maximum deflections of the membrane structures 210, 220 starting at a predeterminable / definable pressure value. In the present example, the stop structures 290 are constructed in two parts, with the two complementary parts each being attached to the membrane structures 210, 220 by means of their own anchoring structure 291, 292.

[0040] Depending on the application, at least one reference capacitance 260 can also be accommodated in the cavern region 240, which is preferably located in an outer region of the cavern region 240. Such a reference capacitance 260 is preferably produced in the same layer plane as the useful capacitance 250 and comprises a first stationary electrode

[0041] 261 and a second stationary electrode 262 spaced therefrom at a predetermined distance, wherein the first stationary electrode 261 is arranged between the second stationary electrode 262 and the first polysilicon layer 210 forming the first membrane structure 210 and the second stationary electrode

[0042] 262 is arranged between the first stationary electrode 261 and the fourth polysilicon layer forming the second membrane structure 220. The two stationary electrodes 261, 262 can be electrically conductively and / or electrically insulatingly connected to one another and / or to the other layers of the sensing region 200 via separation and anchoring structures 287, 289, wherein the separation and anchoring structures 287, 289 can also act or serve as lateral etch stop structures.

[0043] As can also be seen from Figure 1, the pressure sensor 100 further comprises at least one etching access 150 arranged outside the sensor element 200, which is connected to the cavity region 240 via at least one etching channel 153 extending within the suspension structure 140. The at least one etching access 150 is sealed or closed in a media-tight manner from the environment by a closure 151, so that a gas volume with a defined pressure and a defined composition is located in the cavity region 240. In principle, the etching channel can also be provided between the second and third and / or between the third and fourth polysilicon layers or levels.

[0044] During operation of the pressure sensor, the two membrane structures 210, 220 warp inward or outward depending on the pressure prevailing in the environment 300. As a result, the distance between the two movable electrodes 251, 253 of the useful capacitor 150 decreases or increases, which is measured as a corresponding linear change in the capacitance of the useful capacitor 250. However, due to the arrangement of the stationary electrodes 261, 262 being decoupled from the bending regions of the membrane structures 210, 220, no relevant change in the capacitance of the reference capacitor 260 occurs.

[0045] Figure 2 shows a top view of an exemplary pressure sensor 100 with a sensing region 200 suspended from the semiconductor substrate 110 in a stress-decoupled manner via a suspension structure 140. In the present example, the sensing region 200 is circular and connected to an anchoring region of the surrounding layer system via only one suspension structure. In principle, however, the sensing region 200 can also have a different shape, e.g., square. Furthermore, more than one suspension structure 140 can be used to suspend the sensing region 200. Furthermore, the at least one suspension structure 140 can have any shape and any geometric dimensions.

[0046] With reference to the following Figures 3 to 9, in addition to a method for producing a corresponding pressure sensor, possible alternative embodiments of the pressure sensor are explained by way of example. Figure 3, which illustrates the basic structure of the underlying idea, shows a process stage in which a layer system 120 is already arranged on the semiconductor substrate 110 (Si substrate). To produce the layer system 120, a first silicon dioxide layer (SiO2 layer) 121 is first produced on a semiconductor substrate 110 and then structured. This first SiO2 layer 121 is later removed, at least locally, in order to produce a stress-decoupled and self-supporting sensing region. Subsequently, a first dielectric layer 131, such as a first SiRiN layer (silicon rich nitride), which is etch-resistant to a later SiO2 sacrificial layer etching process, is deposited and structured on the first SiO2 layer 121.In this process, previously created SiO2-free regions in the first SiO2 layer 121 are at least partially filled with the first SiRiN layer 131. These filled regions can later serve, for example, as electrically insulating etch stop boundaries in a sacrificial layer etching process. After structuring the first SiRiN layer 131, the first SiO2 layer 121 and / or the first SiRiN layer 131 are optionally further structured. This creates contact hole structures that can later be used to electrically contact the silicon substrate 110.

[0047] After the optional second structuring of the first SiO2 layer 121 and / or the first SiRiN layer 131, the first polysilicon layer 122 is deposited and structured. A first membrane structure 210 is later formed using this first polysilicon layer 122. Furthermore, further structures can be realized using the first polysilicon layer 122, such as at least one electrical substrate contact or at least one conductor track structure. A second SiO2 layer 123 is then produced and structured on this first polysilicon layer 122. This SiO2 layer 123 serves, among other things, as a sacrificial layer in a cavity region 240 and is later removed again in the cavity region. Subsequently, the deposition and structuring of a second polysilicon layer 124 takes place. In this process, the SiO2-free regions in the second SiO2 layer 123 are at least partially filled with material from the second polysilicon layer 124.The regions thus filled in the second SiO2 layer 123 can serve to create electrically conductive connections between the first and second polysilicon layers / levels 122, 124, lateral etch limiting structures, partial regions of later stop structures, a partial region of an anchoring structure of the first and second membrane structures 210, 220, a partial region of a sidewall 230 surrounding the cavity region 240, a partial region of an outer etch stop structure 286, and / or as anchoring structures of a later freely movable first electrode structure 251 and / or a first stationary electrode 261 of an integrated reference capacitor structure 260. The structuring of the second polysilicon layer 124 serves to produce at least one later movable first electrode structure 251.At the same time, a first conductor track structure (not shown here) and / or a first stationary electrode 261 of the reference capacitor structure 260 can also be created. Alternatively, before the deposition of the second polysilicon layer 124, at least one further structuring of the second SiO2 layer 123 and the deposition and structuring of a further SiRiN layer 134, which is at least partially covered by the second polysilicon layer 124 or embedded therein, can be carried out (see Figure 4).In this way, at least anchors of stop structures 292 in the cavern region 240 and / or lateral etch limiting structures provided between the first and second polysilicon layers / levels 122, 124 and / or anchoring structures of the later movable first electrode structure 210 and / or the stationary first electrode 261 of the reference capacitor structure 260 and / or a partial region of an anchoring structure of the first and second membrane structures 210, 220 and / or a partial region of a side wall 230 surrounding the cavern region 240 and / or a partial region of an outer etch stop structure 286 can be designed to be electrically insulating.

[0048] A third SiO2 layer 125 is then deposited and structured on the structured second polysilicon layer 124. This SiO2 layer 125 essentially defines the distance between the two later movable electrode structures 251, 253 of the useful capacitor 250 as well as the distance between the stationary electrodes 261, 262 of one or more optional reference capacitors 260. As already described, the stationary electrodes 261, 262 can be located at least partially in the cavern region and / or alternatively can also be provided outside the cavern region 240.Alternatively, the third SiO2 layer can be designed to have different thicknesses locally / regionally by means of an etching process and / or by depositing and structuring at least two SiO2 layers in order to produce a different distance between the movable electrode structures 251, 253 of the useful capacitor 250 and between the stationary electrodes 261, 262 of one or more optional reference capacitors 260.

[0049] The structuring of the third SiO2 layer 125 can be carried out in such a way that the third SiO2 layer 125 is not completely removed in areas of the cavity region 240 in which nubs or vertical stop structures 290 are to be provided. The remaining thickness of the third SiO2 layer 125 can be used to adjust the maximum deflection of the membrane structures 210, 220 as a result of pressure application, i.e., how far the membrane structures 210, 220 can later move toward each other during pressure measurement.

[0050] After the third SiO2 layer 125 has been structured, a second SiRiN layer 132 is deposited. In the third SiO2 layer 125, the SiO2-free regions or the depressions previously created therein are at least partially eliminated in order to be able to create electrically insulating etch limiting structures and / or a partial region of vertical stop structures 290 and / or anchoring structures of the stationary first and second electrodes 261, 262 of the reference capacitor structure 260 and / or a partial region of an anchoring structure of the first and second membrane structures 210, 220 and / or a partial region of a side wall 230 surrounding the cavern region 240 and / or a partial region of an outer etch stop structure 286.

[0051] This is followed by structuring the second SiRiN layer 132, followed by further optional structuring of the second SiRiN layer 132 and / or the third SiO2 layer 125 to produce contact hole structures. After this process step, a third polysilicon layer 126 is deposited and structured. This third polysilicon layer 126 serves to produce a second movable electrode structure 253 of the useful capacitor 250 and a second electrode 262 of the optional reference capacitor structure 260. Furthermore, by depositing material from the third polysilicon layer 126 into corresponding contact hole structures in the third SiO2 layer 125, electrically conductive connections between the second and third polysilicon layers / levels 124, 126 and / or at least one conductor track structure can be realized.

[0052] Alternatively, after the structuring of the second SiRiN layer 132 and / or the third SiO2 layer 125, the structures exposed in the process can be at least partially filled with material of the third polysilicon layer / level 124, 126 by the second SiRiN layer 132 and / or the third SiO2 layer 125 in order to be able to form electrical connections, lateral etch limiting structures, a partial region of later stop structures, a partial region of an anchoring structure of the first and second membrane structures 210, 220, a partial region of a side wall 230 surrounding the cavern region 240 and / or a partial region of an outer etch stop structure 286 between the second and third polysilicon layers / levels 124, 126 in an electrically conductive manner.

[0053] In order to avoid an etching attack on the third SiO2 layer 125 during the structuring of the second SiRiN layer 132, a further polysilicon layer can be deposited on the third SiO2 layer 125 before the deposition of the second SiRiN layer 132 and structured together with the third SiO2 layer 125. During the subsequent structuring of the second SiRiN layer 132, the etching process stops at / in the further polysilicon layer, and incipient etching of the third SiO2 layer 125 can be avoided. This is particularly advantageous because the third SiO2 layer 125 is used to define the electrode spacings and its thickness therefore determines the capacitance of the useful capacitor 250 and thus indirectly also the measuring sensitivity and measuring range of the pressure sensor 100. Subsequently, a deposition of a third polysilicon layer 126 and a structuring of the third polysilicon layer 126 together with the layer used as an etch stop orserving additional polysilicon layer.

[0054] After structuring the third polysilicon layer 126 together with the further polysilicon layer, a fourth SiO2 layer 127 is deposited and structured, followed by the deposition of a third SiRiN layer 133. Previously created SiO2-free regions in the fourth SiO2 layer 127 are at least partially filled with a third SiRiN layer 133. These filled regions can later serve, for example, as electrically insulating etch stop boundaries in a sacrificial layer etching process. After structuring the third SiRiN layer 133, the fourth SiO2 layer 127 and / or the third SiRiN layer 133 are optionally further structured.In this case, contact hole structures, partial regions of later stop structures and / or anchoring structures are produced, which can later serve for electrical contact between the third and fourth polysilicon layer / level 126, 128, for mechanically suspending a movable electrode on the second membrane structure, as lateral etch stop structures, as a partial region of an anchoring structure of the first and second membrane structure 210, 220, as a partial region of a side wall 230 surrounding the cavern region 240 and / or as a partial region of an outer etch stop structure 286.

[0055] After the production of these structures, a fourth polysilicon layer 128 is deposited and structured. In this process, at least partially SiO2-free regions in the fourth SiO2 layer 127 are filled, and the aforementioned structures consisting of material from the fourth polysilicon layer 128 are created. The fourth polysilicon layer 128 also serves to realize the second membrane structure 220 and to produce at least one electrical conductor structure (not shown here). During the structuring of the fourth polysilicon layer 128, isolation trenches can be created, such as, among others, the isolation trench 129 shown in Figure 3 and serving to electrically insulate the second membrane structure 220. Further layer deposition can then take place, which are not shown in detail here in Figure 3. They can, for example,for the production of bond pad structures, electrical conductor tracks and insulation structures for their electrical insulation and / or for the production of diffusion barriers, which are used, for example, as moisture barriers.

[0056] Alternatively, after the deposition of a fourth SiO2 layer 127 and the deposition of a third SiRiN layer 133, the structuring of the fourth SiO2 layer 127 and the third SiRiN layer 133 can be carried out such that the aforementioned structures consist of the material of the fourth polysilicon layer 128. In this case, the elements / structures formed between the third and fourth polysilicon layers / levels 126, 128 are preferably electrically conductive.

[0057] As shown in Figure 5, by providing at least one etching access 150 outside the anchoring region of the membranes 210, 220 and by providing at least one etching channel 153 between the etching access 150 and the cavity region 240, the SiO2 material present in the cavity region 240 can be removed in a sacrificial layer etching process. In this process, the SiO2 material of the second, third, and fourth SiO2 layers 123, 125, 127 is removed between the first and second membrane structures 210, 220, between the first and second movable electrodes 251, 253 of the useful capacitor structure 250, between corresponding stop structures, and optionally also between the first and second stationary electrodes 261, 262 of a reference capacitor structure 260.

[0058] As further shown in Figure 6, this at least one etching access 150 can subsequently be closed again by means of at least one layer deposition and / or by locally melting at least a portion of the layer system 120, e.g., using a laser. In this case, a defined pressure can be set and / or a defined gas or gas mixture can be enclosed in the cavity region 240.

[0059] After closing the etching access 150, the layer system 120 can be at least partially removed around the first and second membrane structures 210, 220 and their anchoring regions using known structuring methods, down to the first SiO2 layer 121. After the first SiO2 layer 121 is subsequently removed, a self-supporting sensing region 200 with two movable membranes 210, 220 can be realized between the first membrane structure 210, its anchoring regions, the at least one suspension structure 140, and the Si substrate 110. Such a self-supporting sensor structure 200 is shown in Figure 7. The suspension of the self-supporting region 200 realized by the at least one suspension structure 140 can be designed as desired with regard to shape and / or geometric dimensions.The electrical conductor tracks (not shown here) for connecting the electrodes of the useful capacitance 250 and the one or more reference capacitances 260 run through at least one of the suspension structures 140. Electrically conductive layers of the layer stack forming the respective suspension structure 140 can serve as conductor tracks. As can be seen from Figure 7, the at least one etching channel 153, which connects the cavern region 240 to the at least one etching access and via which the SiO2 sacrificial oxide layers are removed from the cavern region, also runs through at least one suspension structure 140. In principle, it is also possible to remove the sacrificial oxide layers in the cavern region 240 and beneath the first membrane structure 210 in a common etching process. In this case, it is possible to first etch the layer system 120 (e.g.around the first and second membrane structures 210, 220 and their anchoring regions) at least partially down to the first SiO2 layer 121 and subsequently creating the at least one etching access 150 for removing the sacrificial oxide layers 123, 125, 127 in the cavity region 240. In this variant, the sacrificial oxides from the cavity region 240 and the first SiO2 layer 121 between the first membrane structure 210, its anchoring regions, the at least one suspension structure 140, and the Si substrate 110 can be removed simultaneously. The at least one etching access 150 can then be closed.

[0060] The provision of an etching access 150 outside the anchoring region of the membranes and the provision of at least one etching channel 153 which connects the etching access 150 to the cavern region 240 between the membrane structures 210, 220 can be dispensed with if at least one etching access 150 is provided in the anchoring region and / or in the second membrane structure 220, through which the sacrificial oxide layers in the cavern region 240 can be removed and which is then closed again in a media-tight manner.

[0061] As further shown by way of example in Figure 7, it is also possible to provide the anchors 252, 254 of the movable electrodes 251, 253 of the useful capacitance 250 and the anchors 291, 292 of the mechanical stop structures 290 on the respective membrane structure 210, 220 not from polysilicon, but from SiRiN. In this case, the electrical connection of the movable electrodes 251, 253 can be made via flexible conductor structures (not shown here) in the second and third polysilicon layers / levels 124, 126.

[0062] As Figure 8 illustrates, it is an alternative to the previously described

[0063] In some embodiments, it is also possible not to remove the third SiO2 layer 125 between the stationary electrodes 261, 262 of the at least one reference capacitance 260. This allows the capacitance of the reference capacitance structure 260 to be increased or, with the same capacitance, the reference capacitance structure 260 to be reduced in area, which in turn allows the movable electrode structures 251, 253 of the useful capacitance 250 to be made larger in area.

[0064] Figure 9 illustrates a further embodiment of the manufacturing process. As can be seen here, special structures 162 can also be introduced into the surface 115 of the Si substrate 110 beneath the layer system 120, at least in the region of the sensor structure 200. These structures 162, which are not completely filled with a first SiO2 layer 121, can be introduced. These structures 162, which are formed, for example, in the form of trenches or channels, preferably serve to distribute an etching medium for removing the first SiO2 layer 121 over the surface as quickly as possible. In this way, the full-surface removal of the first SiO2 layer 121 beneath the first membrane structure 210 can be achieved in a significantly shorter time. In Figure 9, such structures 162 are shown as examples in the form of regular trench structures.

[0065] Alternatively, channel-like SiO2-free structures for a rapid areal distribution of an etching medium for removing the first SiO2 layer 121 within the first SiO2 layer 121 and for removing sacrificial oxide layers 123, 125, 127 in the cavern region 240 can optionally be provided, for example, within the second SiO2 layer 123.

[0066] In the embodiments described so far, an external etching access 150 for removing the SiO2 sacrificial layers in the cavern region 240 is described, which is located outside the stress-decoupled sensing region 200, which further extends at least through the fourth polysilicon layer 128 and which leads into the cavern region 240 via an etching channel 153 running in the suspension structure 140 serving for stress decoupling. In principle, however, it is also possible to provide at least one etching access 150 on the later stress-decoupled sensing region 200 through at least the fourth polysilicon layer 128, via which the SiO2 sacrificial layers 123, 125, 127 in the cavern region 240 can be removed and which can be closed by means of at least one layer deposition and / or a laser reseal process after the removal of the SiO2 sacrificial layers.Figure 10 shows, by way of example, a pressure sensor 100 with a correspondingly designed sensing region 200. As can be seen here, the etching accesses 150 are arranged, if possible, at the edge of the sensing region 200 in a region between the inner and outer anchoring structures 289, 288, in order not to change or influence the properties of the second membrane structure 220 anchored to the inner anchoring structures 289. The number, size, and distribution of the etching accesses 150 can vary depending on the application.

[0067] As can be seen in Figure 11, between the internal etching accesses 150 and the center of the membrane structures 210, 220, between the individual polysilicon layers 124, 126, 128, at least partially / regionally, additional anchoring structures 289 for the first and second membrane structures 210, 220, which are made of polysilicon and / or SiRiN, can be provided. The anchoring structures 289 can be aligned centrally with respect to one another or at least partially deviate from a central alignment.The advantage of these optional anchoring structures 289 is that the closure region of the at least one etching access 150 can be placed between the membrane anchoring of the second membrane structure 220 and the lateral etch stop structures 288 made of polysilicon and / or SiRiN provided at least on the circumference of the stress-decoupled sensing region, and thereby negative influences of the closure region on the membrane properties of the second membrane 220 can be avoided.

[0068] Figure 12 shows a top view of a pressure sensor 100 designed according to the embodiment shown in Figures 10 and 11. The pressure sensor 100 shown here as an example has a circular sensing region 200, which is suspended in a stress-decoupled manner via a suspension structure 140. As shown here as an example, a total of four etching accesses 150 are provided in an edge region of the sensing region 200, distributed along the circumference of the sensing region 200 and arranged between outer and inner anchoring structures 288, 289.

[0069] In principle, the circumferential contour of the stress-decoupled sensor structure / sensing region 200 can be arbitrary and does not have to correspond to the shape / contour of the first and / or second membrane surface 210, 220 defined by the further anchoring structures 289.

[0070] Figure 13 shows a further possible embodiment in which, by providing additional lateral etch stop structures 231 made of polysilicon and / or SiRiN between the individual polysilicon layers 124, 126, 128 within the sensing region 200, defined edge regions 232 can be created in which SiO2 sacrificial layers are not removed. These SiO2-filled edge regions 232 can be used, for example, to reinforce or make more stable the peripheral edge 230 of the sensor structure 200 formed by the outer etch stop structures.

[0071] The lateral etch stop structures made of SiRiN can not only be provided between polysilicon layers, but can also completely penetrate polysilicon layers, at least partially / in some areas.

[0072] Figure 14 shows a simplified flow diagram of the manufacturing method 400. In a first step 410, a semiconductor substrate 110 is first provided. In step 420, a layer system 120 is created on the semiconductor substrate 110 by depositing and structuring suitable materials, such as silicon dioxide, polysilicon, and SiRiN (silicon rich nitride), etc. By structuring the layers, a double membrane structure 210, 220 with a cavity region 240 is created in a functional region 111 of the layer system 120. In a further step 430, an SiO2 material present in the cavity region 240 is removed via a previously created etching access 150. In step 440, the etching access 150 is closed by depositing material or by means of a laser reseal process.Finally, in step 450, a self-supporting sensor structure 200 is created by etching a SiO2 material present beneath the double membrane structure 210, 220.

[0073] The structure of the pressure sensor has not been fully described in the foregoing description and the associated drawings. Rather, the manufacturing method may comprise further steps which will be apparent to a person skilled in the art. For example, after the closure of at least one etching access for removing the sacrificial oxide layers in the cavern region, further process steps may be carried out in order to provide or implement, for example, bond pad structures, electrical conductor tracks and wiring levels, electrical insulation structures and insulation layers, diffusion barriers and diffusion barrier layers, and the like. Furthermore, after each deposition, the deposited layer surface can optionally be planarized, for example by a CMP process (chemical mechanical polishing), to create a flat surface. The deposited polysilicon layers can also optionally be polished, at least locally or partially, to increase the electrical conductivity.be doped in regions using known methods and optionally additional structures such as planar regions and / or at least one conductor track made of polysilicon can be provided within the first silicon dioxide layer 121.

[0074] Since these are well-known procedures, a detailed description has been omitted.

[0075] Although the invention has been illustrated and described in detail by the preferred embodiments, the invention is not limited to the disclosed examples. Rather, other variations may be derived therefrom by those skilled in the art without departing from the scope of the invention.

Claims

Claims 1 . Micromechanical pressure sensor (100) comprising: - a substrate (110) with a layer system (120) arranged thereon, - a sensor structure (200) which is self-supporting and fastened to the layer system (120) by means of at least one suspension structure (140), said sensor structure comprising a first membrane structure (210), a second membrane structure (220) and a cavern region (240) arranged between the two membrane structures (210, 220) and closed off by a side wall (230) running in an edge region (212, 222) of the two membrane structures (210, 220), wherein the sensor structure (200) comprises a useful capacitor (250) arranged in the cavern region (240) between the two membrane structures (210, 220), said capacitor having a first electrode (251) fastened to the first membrane structure (210) and a second electrode (253) arranged between the first electrode (251) and the second membrane structure (220) and fastened to the second membrane structure (220).

2. Micromechanical pressure sensor (100) according to claim 1, wherein the electrodes (251, 253) are each fastened to the respective membrane structure (210, 220) by means of at least one fastening structure (252, 254) in an inner region (211, 221) of the respective associated membrane structure (210, 220).

3. Micromechanical pressure sensor (100) according to claim 1 or 2, wherein in an edge region (242) of the cavern region (240) at least one reference capacitor (260) with two oppositely arranged stationary electrodes (261, 262) is further provided, wherein each of the stationary electrodes is arranged between the respective other stationary electrode and one of the membrane structures.

4. Micromechanical pressure sensor (100) according to claim 3, wherein the reference capacitor (260) is anchored at least partially or in regions in the side wall (230) and is arranged in a self-supporting manner and aligned plane-parallel to the membrane structures (210, 220).

5. Micromechanical pressure sensor (100) according to claim 4, wherein a silicon dioxide layer consisting of the material of the third silicon dioxide layer (125) is arranged between the stationary electrodes (261, 262) of the reference capacitor (260).

6. Micromechanical pressure sensor (100) according to one of the preceding claims, wherein in the layer system (120) at least one etching access (150) to the cavern region (240) is formed outside a deflectable sensing surface (201) of the second membrane structure (220), and wherein the etching access (150) has a closure (151) which is formed by at least one material deposited on the surface of the layer system (120) and / or introduced into the etching access (150) and / or by at least one locally melted and re-solidified material of the layer system (120).

7. Micromechanical pressure sensor (100) according to claim 6, wherein the etching access (150) is arranged outside the sensor structure (200) and is connected to the cavern region (240) by means of an etching channel (153) extending through the suspension structure (140).

8. The micromechanical pressure sensor (100) of claim 6, wherein the at least one etching access (150) is arranged within the at least one suspension structure (140).

9. Micromechanical pressure sensor (100) according to claim 6, wherein the at least one etching access (150) is arranged in an edge region (242) of the sensor structure (200).

10. Micromechanical pressure sensor (100) according to claim 9, wherein the sensor structure (200) has additional anchoring structures (289) for fastening the membrane structures (210, 220), and wherein the etching access (150) is arranged between the circumferential side wall (230) and the additional anchoring structures (289).

11. Micromechanical pressure sensor (100) according to one of the preceding claims, wherein the circumferential side wall (230) is formed by outer anchoring structures (288), inner anchoring structures (289) and silicon dioxide material of the silicon dioxide layers (123, 125, 127) of the layer system (120) remaining between the outer and inner anchoring structures (288, 289).

12. Micromechanical pressure sensor (100) according to one of the preceding claims, wherein in the cavern region (240) at least one mechanical stop structure (290) is further fastened to at least one membrane structure (210, 220), which stops a relative movement between the two membrane structures (210, 220) from a predetermined pressure value.

13. A method for producing a micromechanical pressure sensor (100) configured according to one of claims 1 to 12, wherein a semiconductor substrate (110) is provided, wherein a layer structure (120) is produced on the semiconductor substrate (110) by depositing and structuring layers (121-128), wherein in a functional region (111) of the layer structure (120), a sensor structure (200) is produced which is connected to the remaining layer structure (120) via a suspension structure (140), said sensor structure comprising a first membrane structure (210) structured from a first polysilicon layer (121), a second membrane structure (220) structured from a fourth polysilicon layer (128), and a side wall (230) extending in an edge region (212, 222) of the membrane structures (210, 220). limited cavern area (240),wherein in the cavern region (240) a useful capacitor (250) with a first electrode (251) structured from a second polysilicon layer (124) and attached to the first membrane structure (210) and a third electrode (252) Polysilicon layer (126) structured and attached to the second membrane structure (220), wherein subsequently, with the aid of an etching process, silicon dioxide layers (123, 125, 127) arranged between the polysilicon layers (122, 124, 126, 128) in the cavern region (240) are removed via at least one etching access (150) having an opening (152) in the fourth polysilicon layer (128), wherein furthermore, with the aid of an etching process, a first silicon dioxide layer (121) arranged between the first membrane structure (210) and the surface (115) of the semiconductor substrate (110) is removed.

14. The method according to claim 13, wherein the etching access (150) is closed by depositing at least one material on the surface of the layer system (120) and / or by locally melting at least one material of the upper layers of the layer system (120) in the region of the etching access (150).

15. The method according to claim 13 or 14, wherein the removal of the silicon dioxide layers (123, 125, 127) in the cavern region (240) and the removal of the first silicon dioxide layer (121) below the first membrane structure (210) are carried out in a common etching process.

16. The method according to any one of claims 13 to 15, wherein trench structures (162) are produced in the surface (115) of the semiconductor substrate (110) beneath the sensor structure (200), which trench structures are not completely destroyed during the deposition of the first silicon dioxide layer (121), and wherein the trench structures (162) cause or promote a distribution of the etching medium beneath the first silicon dioxide layer (121) during the etching of the first silicon dioxide layer (121) beneath the first membrane structure (210).

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