DE102019135606B3 - Semiconductor-based differential pressure sensor and method for its manufacture - Google Patents

Abstract

The invention relates to a differential pressure sensor comprising a first (01) and a second substrate plate (02), each of which has a carrier layer (03), a component layer (04) and a separating layer (05) located between them. The thickness of the component layer (04) of the first substrate plate (01) differs significantly from the thickness of the component layer (04) of the second substrate plate (02). A circuit structure with at least one strain-sensitive sensor element (07) is formed in one of the component layers (04) and, when this component layer is deformed, delivers a sensor signal that correlates with the deformation. The carrier layer (03) of the first and second substrate plates (01, 02) is removed in a membrane area, so that a first pressure space (11) in the first substrate plate (01) and a second pressure space (12) in the second substrate plate (02) is trained. The first and second substrate plates (01, 02) are connected to one another at their respective component layers (04), so that a membrane (13) consisting of the component layers (04) is formed in the membrane area. The invention also relates to a method for producing such a device Differential pressure sensor.

Description

The present invention relates to a miniaturized differential pressure sensor based on so-called wafers and resistant to corrosive media. The invention also relates to a method for producing such a sensor, in particular using processes typical in semiconductor technology.

In practice, different sensors are used to determine the differential pressure between two media spaces. In typical silicon wafer-based piezoresistive differential pressure sensors, the pressures p ₁ and p ₂ to be compared are applied to the front and rear of a membrane etched into silicon. The output signal of a measuring bridge arranged on the membrane is proportional to the pressure difference. Such sensors are well suited for reliable differential pressure measurement in dry air and other non-corrosive, non-aggressive and electrically insulating media. The sensitive components on the membrane, in particular the measuring bridge and the metallization including the bonding wires for electrical contacting, do not allow the use of such sensors under harsh environmental conditions.

From the DE 100 62 637 B4 a micromechanical differential pressure sensor for measuring a pressure difference in two separate rooms or media is known. This differential pressure sensor comprises a substrate with at least one cavity which is arranged by surface micromechanical methods on a main surface of the substrate and is delimited on one side by a movable membrane. A first pressure can be applied to the movable membrane starting from the main surface of the substrate. Furthermore, at least one opening is provided in the main surface of the substrate to which a second pressure can be applied. This publication also describes that an evaluation circuit can be arranged on the substrate. The publication presents designs of differential pressure sensors that can be used in principle and their manufacture using micromechanical processes.

From the EP 1 783 782 A1 a pressure sensor is known which has a cell with an expandable surface. Resistors are applied to this surface in the form of a thin film, this thin film consisting of electrically conductive particles within a dielectric medium and having a thickness of about three to ten times the average size of the particles. In this case, too, there results a change in the electrical resistance of the thin layer when pressure is applied to the stretchable surface.

The DE 10 2017 122 264 A1 describes a pressure sensor chip and a method for its manufacture. The pressure sensor chip consists of a sensor plate and a reinforcement plate connected to it, which are formed from a first and a second wafer. A pressure chamber is formed in the sensor plate and a through opening opening into the pressure chamber is formed in the reinforcement plate. A membrane with a predetermined membrane thickness is formed on a membrane side of the sensor plate above the pressure chamber, the pressure chamber depth corresponding to the difference between the thickness of the first wafer and the membrane thickness.

The US 2015/0122038 A1 Figure 12 shows a pressure sensor comprising a first substrate with processing circuitry and a cap mounted on the first substrate. The cap comprises a container, a holder and one or more suspension elements for hanging the container on the holder. The container has a cavity and a deformable membrane. Furthermore, the pressure sensor comprises detection means and a second substrate.

The DE 10 2016 203 232 A1 shows a 3D stacked piezoresistive pressure sensor comprising a MEMS pressure measuring element with a first and a second side. A flexible membrane and Wheatstone bridge circuit are on the first page. The pressure sensor further comprises a first integrated circuit having a substrate with a first and a second side. An electronic circuit is formed in a predetermined portion on the first side. There is a recess on the second side. The first integrated circuit is attached on its second side to the first side of the MEMS pressure measuring element. The recess of the first integrated circuit and the first side of the MEMS pressure measuring element define an evacuated cavity. A first plurality of conductive contacts is formed in the substrate with the first integrated circuit. The first plurality of contacts extends through the substrate and electrically connects the Wheatstone bridge circuit to the circuit formed on the first side of the first integrated circuit.

From the DE 10 2013 207 683 A1 a method for producing a pneumatically actuatable microfluidic analysis cartridge is known. The method comprises a step of providing, a step of closing, a step of aligning and a step of connecting. In the provision step, a fluidic part of the analysis cartridge and a pneumatic part of the analysis cartridge are provided. The fluidic part is for this trained to carry out fluidic basic operations of a biochemical analysis process. The pneumatic part is designed to control the basic operations using air pressure. In the closing step, a joining side of the fluidic part is closed with a first fluid-tight elastic membrane. Additionally or alternatively, in the closing step, one side of the pneumatic part is closed with a second membrane. In the alignment step, the joining side of the fluidic part and the joining side of the pneumatic part are aligned with one another. In the connecting step, the fluidic part and the pneumatic part are connected to one another in order to obtain the analysis cartridge.

From the DE 10 2006 022 379 A1 a micromechanical pressure transducer is known which comprises a chip with a deflectable membrane and a chip with an evaluation circuit. The evaluation circuit can process information about the deflection of the membrane. At least one of the two chips contains a cavity that is open on the substrate side. The two chips are electrically and mechanically connected to one another in such a way that the at least one cavity for the deflectable membrane forms a back volume and the information about the deflection of the membrane is applied to the evaluation circuit.

Previously known differential pressure sensors, which can be produced with known semiconductor technologies on the basis of wafers, are either not resistant to corrosive media or require special design measures, for example a structure with a separating membrane and a so-called oil reservoir. The latter increases the costs for the built-up measuring cell many times over, which means that it is practically impossible to use it in typical mass applications. Depending on the application, the volume or the transmission length of the oil-filled chamber can also be very large, which can lead to measurement errors due to undefined volume expansion of the oil reservoirs, uneven volume expansion of the front and rear oil reservoirs or undefined temperature gradients. Other solutions only show insufficient long-term stability, e.g. B. due to mechanical and thermomechanical stresses.

One object of the present invention, based on the prior art, is therefore to provide an improved differential pressure sensor which can be manufactured inexpensively using conventional semiconductor technologies, is resistant to corrosive media and has high long-term stability.

The stated object is achieved by a differential pressure sensor according to the attached claim 1 or by a method for producing such a sensor according to claim 7.

The differential pressure sensor according to the invention comprises a first and a second substrate plate, which are preferably formed from an SOI wafer (SOI = silicon on insulator). The substrate plates each consist of at least one carrier layer, a component layer and a separating layer lying between them. It is important that the thickness of the component layer of the first substrate plate differs significantly from the thickness of the component layer of the second substrate plate. A circuit structure with preferably a plurality of strain-sensitive sensor elements is formed in at least one of the component layers in such a way that the sensor elements deliver a sensor signal that correlates with the deformation when this component layer is deformed. The circuit structure preferably comprises one or more piezoresistive resistors as sensor elements. In a membrane area, the carrier layers of both the first and the second substrate plate are removed, so that a first pressure space is formed in the first substrate plate and a second pressure space is formed in the second substrate plate. The first and second substrate plates are connected to one another at their respective component layers, so that a membrane consisting of the separating layers and the component layers of the two substrate plates is formed in the membrane area. In the operating state, this membrane is acted upon on both sides with the pressure that prevails in the adjacent pressure chamber, so that the membrane is deflected depending on a pressure difference, which leads to mechanical stresses in the component layers and thus generates the desired sensor signal.

The inventive construction of the differential pressure sensor achieves resistance to aggressive media, since there is a spatial separation between the first pressure chamber with the medium present therein under a pressure p ₁ , the second pressure chamber with the medium present therein under a pressure p ₂ and the circuit structure in the component layer is ensured. The respective separating layers ensure the appropriate separation.

The invention is based, inter alia, on the knowledge that the proposed use of two substrate plates made from SOI wafers enables the circuit structure, for example in the form of a measuring bridge, to be completely embedded in bulk silicon, which means that the spatial separation of the medium in the Pressure rooms is guaranteed. An additional passivation as well as complete electrical insulation is buried through the Separation layers (buried oxide, BOX) of the SOI wafer with a thickness of typically 0.5 ... 1.0 µm are guaranteed. At the same time, these separating layers serve as an etch stop during the formation of the membrane by etching the carrier layers and thus ensure a membrane thickness that is very homogeneous over the entire silicon wafer. The membrane thickness defines important sensor parameters such as sensitivity, linearity of the characteristic curve and overload resistance. Another advantage of the proposed sensor geometry is the mechanically symmetrical clamping of the membrane from the front and rear. This avoids asymmetries in the differential pressure characteristic with front and rear loading.

According to a preferred embodiment, the thickness of the component layer of the first substrate plate differs by a factor of <= 0.8, preferably <= 0.5, from the thickness of the component layer of the second substrate plate. With the same thickness of the silicon component layers of the two SOI wafers, the strain-sensitive sensor elements, in particular piezoresistors, would be in the tension-free, neutral fiber of the membrane, so that a deformation of the membrane would not generate a sensor signal correlating to the strain. Therefore, the thickness of the component layers must be significantly different.

In a modified embodiment, circuit structures are formed in each of the component layers of the two substrate plates. These circuit structures can generate redundant signals to reduce the probability of failure or also fulfill different measuring functions, for example by realizing a temperature measurement in addition to the strain sensor system.

The circuit structure preferably comprises a plurality of piezoresistive resistors which are connected to one another as a measuring bridge.

A modified embodiment is characterized in that, in the area of the connection between the two component layers, a passivation layer is also formed, which electrically separates the component layers from one another. This passivation layer is produced, for example, by an oxidation step before the component layers are connected to one another. The component layers are preferably connected by bonding.

An advantageous embodiment has, on the side of the carrier layer facing away from the component layer, at least one of the substrate plates a cover plate which closes the pressure space. In the cover plate there is an access channel opening into the pressure chamber in order to supply the pressurized medium in the operating state.

The method according to the invention for producing such a differential pressure sensor initially comprises steps of providing a first and a second SOI wafer. These SOI wafers each have at least one carrier layer, one component layer and a separating layer between them. The SOI wafers are selected such that the thickness of the component layer of the first SOI wafer differs significantly from the thickness of the component layer of the second SOI wafer. In the next step, a circuit structure with at least one strain-sensitive sensor element is produced in at least one of the component layers. In a further step, the two SOI wafers are connected to one another at their respective component layers, preferably by bonding. After this connection has been established, the carrier layers of the two SOI wafers are partially removed in membrane areas, each beginning from the side of the carrier layer facing away from the component layer. In this way, a plurality of membranes is formed in each case in opposite sections of the wafer, which membranes are formed by sections of the opposite component layers that are connected to one another. After these steps and, if necessary, further optional steps, the differential pressure sensors produced in this way can be separated.

The removal of the carrier layers in the membrane area and further structuring steps are preferably carried out exclusively by conventional, anisotropic etching processes using potassium hydroxide solution (KOH) for deep structuring of silicon.

It is obvious to a person skilled in the art that a suitable number of contact pads must be produced in order to make contact with the circuit structure produced. Lowered contact pads are preferably produced outside the pressure spaces produced, which are used for electrical contacting optionally by means of wire bonding or spring contacts.

The chip has a media feed on the front and back as well as sufficiently large sealing surfaces that allow simple sealing by means of elastomer seals. The membrane is located in the middle of the sensor and is mechanically largely decoupled from the housing. It is subjected to _{pressures p 1} and p ₂ from one side each.

According to a preferred embodiment, boron-doped bridge resistors are electrically connected to a metallization via highly doped lead regions.

According to a modified embodiment, channeling effects can be used during the implantation in order to increase the high levels required for highly doped connections or supply lines To achieve penetration depths. By varying the angle of incidence and simultaneous rotation of the wafer during the implantation process and by dispensing with a scattering oxide or using a very thin scattering oxide, a high level of doping that is homogeneous in depth can be achieved. This is followed by moderate tempering, which is only used for electrical activation and not for further diffusion of the dopants.

• 1 eine vereinfachte Schnittdarstellung einer ersten Ausführungsform eines erfindungsgemäßen Differenzdrucksensors;
• 2 eine vereinfachte perspektivische Ansicht des Differenzdrucksensors gemäß 1;
• 3 eine vereinfachte Schnittdarstellung einer zweiten Ausführungsform des Differenzdrucksensors.

Further advantages and details of the invention emerge from the following description of preferred embodiments with reference to the drawing. Show it:

• 1 a simplified sectional view of a first embodiment of a differential pressure sensor according to the invention;
• 2 a simplified perspective view of the differential pressure sensor according to FIG 1 ;
• 3 a simplified sectional view of a second embodiment of the differential pressure sensor.

1 shows in a greatly simplified, not to scale sectional view a first embodiment of a differential pressure sensor according to the invention. The differential pressure sensor initially comprises a first substrate plate 01 and a second substrate plate 02 each produced from an SOI wafer. Both substrate plates have a carrier layer 03 , a component layer 04 and between them a separating layer 05 . The component layer 04 the first substrate plate 01 is significantly thinner than the component layer in this embodiment 04 the second substrate plate 02 . For example, the thinner component layer is only about half as thick as the thicker component layer.

To generate the embodiment according to 1 all electrically relevant circuit structures were implemented in the SOI wafer with the thinner component layer. In particular, are in the component layer 04 the first substrate plate 01 Piezoresistors as strain-sensitive sensor elements 07 implemented. In this case, a passivation layer was used to protect the circuit structure 08 generated.

In a subsequent manufacturing step, the two SOI wafers were bonded face-to-face with the component layers. The passivation layer 08 is thus located in the composite area between the substrate plates 01 , 02 .

A deep silicon structuring then takes place in the composite produced. This creates the backing layers 03 both substrate plates removed in a membrane area, creating the first substrate plate 01 a first pressure room 11 and in the second substrate plate 02 a second pressure room 12th is formed. The printing room 11 , 12th extends in depth to the respective separating layer 05 . A deformable membrane remains in the membrane area 13th , which through the interconnected component layers 04 is formed. It can be seen that the circuit structure implemented in the component layer is protected from any media that flow into the first or second pressure chamber.

In a modified embodiment, additional passivation on both sides can optionally take place. This can preferably be a silicon nitride passivation (Si2N4), which ensures a very good chemical passivation, among other things through resistance to moisture and other ionic species. This additional coating is applied after the substrate plates have been connected and the two SOI wafers have been etched, i.e. before any cover plates are attached.

In order to make contact with the circuit structure, metallization takes place over into the component layer 04 implanted vias 14th . For this purpose, in the embodiment shown, an additional lead implantation with high energy and adapted post-heating was carried out to achieve high penetration depths.

In the embodiment according to 1 were in a further optional step on that of the membrane 13th facing away from the carrier layers 04 each cover plate 15th appropriate. The top panels 15th are produced from further wafers, which are structured and then bonded against the SOI wafer assembly. The structuring of the cover plates in particular creates access channels 16 created which the media supply to the pressure rooms 11 , 12th with the respective pressures p ₁ and p ₂ . The top panels 15th also cause additional mechanical stiffening and mechanical decoupling of the membrane 13th from the sensor housing. This sub-step can be omitted if the available sealing surface is covered by the carrier layers 04 is provided is sufficient.

2 FIG. 11 shows a perspective, partially sectioned view of the differential pressure sensor in the embodiment according to FIG 1 .

3 shows in a greatly simplified, not to scale sectional view a second embodiment of the differential pressure sensor, the structure of which is basically that of FIG 1 corresponds to the embodiment shown. In addition, this embodiment includes the implanted vias 14th another through-hole plating level for contacting the circuit structure 20th with inverse doping for electrical connection of the substrate. In addition, there is no passivation layer at the interface between the two component layers 04 , whereby the entire bulk silicon surrounding the circuit structure is at the same electrical potential. This causes electrical shielding of the sensor elements on all sides 07 against electrical fields from the environment. The substrate potential is defined by connecting the substrate contact. In the in 3 The embodiment shown, the substrate contact is connected to the positive potential of the bridge feed via the chip metallization. In this embodiment, the silicon direct bonding for joining the two SOI wafers is preferably carried out with hydrophobic surfaces and in a high vacuum or ultra-high vacuum.

In a further modified embodiment, as an alternative to the structure described above, the bridge from the sensor elements can also be in the thicker component layer 04 the second substrate plate 02 be implanted. The additional implantation for producing the plated-through holes could thus be dispensed with. Instead, when the contact holes are opened, not only the cover layer system but also the thin silicon layer would have to be opened.

List of reference symbols

01

first substrate plate

02

second substrate plate

03

Carrier layer

04

Component layer

05

Separating layer

06

-

07

strain-sensitive sensor elements / piezoresistors

08

Passivation layer

09

-

10

-

11

first printing room

12th

second pressure chamber

13th

membrane

14th

Through-hole plating

15th

Cover plate

16

Access channel

17th

-

18th

-

19th

-

20th

further via level

Claims (9)

Comprehensive differential pressure sensor, - A first (01) and a second substrate plate (02), which are sections of a first and second SOI wafer (SOI = silicon on insulator) and each have a carrier layer (03), a component layer (04) and one between the respective carrier layer (03) and the respective component layer (04) have a separating layer (05), the thickness of the component layer (04) of the first substrate plate (01) deviating from the thickness of the component layer (04) of the second substrate plate (02); - A circuit structure with at least one strain-sensitive sensor element (07), which is formed in one of the component layers (04) and, when mechanical stresses occur in this component layer, supplies a sensor signal that correlates therewith; wherein the carrier layer (03) of the first and the second substrate plate (01, 02) is removed in a membrane area, so that a first pressure space (11) in the first substrate plate (01) and a second pressure space (12) in the second substrate plate (02 ), and wherein the first and second substrate plates (01, 02) are connected to one another at their respective component layers (04) so that a membrane (13 ) is formed. Differential pressure sensor according to Claim 1 , characterized in that the thickness of the component layer (04) of the first substrate plate (01) differs by a factor of <= 0.8, preferably <= 0.5, from the thickness of the component layer (04) of the second substrate plate (02). Differential pressure sensor according to Claim 1 or 2 , characterized in that circuit structures are formed in each of the component layers (04) of the two substrate plates (01, 02). Differential pressure sensor according to one of the Claims 1 to 3 , characterized in that the sensor elements (07) are piezoresistive resistors, which are preferably connected to one another as a measuring bridge. Differential pressure sensor according to one of the Claims 1 to 4th , characterized in that in the area of the connection between the two component layers (04) a passivation layer (08) is formed which electrically separates the component layers from one another. Differential pressure sensor according to one of the Claims 1 to 5 , characterized in that on the side of the carrier layer (03) facing away from the component layer (04) at least one of the substrate plates (01, 02) is attached a cover plate (15) which delimits the pressure space (11, 12), wherein in the Cover plate (15) an access channel (16) opening into the pressure chamber is present. Method for producing a differential pressure sensor, comprising the following steps: - Provision of a first and a second SOI wafer (SOI = silicon on insulator), each having a carrier layer (03), a component layer (04) and a separating layer (04) between the respective carrier layer (03) and the respective component layer (04). 05), the thickness of the component layer of the first SOI wafer differing from the thickness of the component layer of the second SOI wafer; - Generating a circuit structure with at least one strain-sensitive sensor element (07) in at least one of the component layers (04); - Connecting the two SOI wafers to their respective component layers (04); - Partial removal of the carrier layers (03) of the two SOI wafers in membrane areas in order to form membranes (13) which are formed by interconnected sections of the opposing component layers (04) of the two SOI wafers. Procedure according to Claim 7 , characterized in that after the partial removal of the carrier layers (03) and before the differential pressure sensors are separated, a cover plate wafer is attached to the remaining sections of the carrier layer (03) at least one of the SOI wafers. Procedure according to Claim 7 or 8th , characterized in that the differential pressure sensor to be produced according to one of the Claims 1 to 6th is configured.

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