MEMS CAPACITIVE PRESSURE SENSOR AND MANUFACTURING METHOD

According to an example aspect of the present invention, there is provided a MEMS capacitive pressure sensor (1), comprising a first electrode (17), a deformable second electrode (18) being electrically insulated from the first electrode (17) by means of a chamber (4) between the first electrode (17) and the second electrode (18), and wherein at least one of the first electrode (17) and the second electrode (18) includes at least one pedestal (5) protruding into the chamber (4). According to another example aspect of the present invention, there is also provided a method for manufacturing a MEMS capacitive pressure sensor (1).

FIELD

The present invention relates to a pressure sensor. In particular, the present invention relates to a micro-electro-mechanical (MEMS) capacitive pressure sensor. Further, the present invention relates to a method for manufacturing a MEMS capacitive pressure sensor.

BACKGROUND

MEMS capacitive pressure sensors are known, by means of which pressure can be sensed. MEMS technology facilitates the manufacture of compact pressure sensors. A MEMS capacitive pressure sensor requires two electrodes that move relative to each other under an applied pressure. This configuration is often accomplished by having a fixed electrode formed on a substrate while a moveable electrode is provided in a deformable membrane which is exposed to pressure that is to be sensed.

For example, document US 2015/0008543 A1 discloses a MEMS capacitive pressure sensor. The MEMS capacitive pressure sensor includes a substrate. The MEMS capacitive pressure sensor also includes a first electrode layer on the substrate. The first electrode layer is electrically connected with semiconductor devices in the substrate through electrical interconnection structures. Additionally, the MEMS capacitive pressure sensor includes a second electrode layer on the substrate. A chamber is formed between the first electrode layer and the second electrode layer. The chamber electrically insulates the first electrode layer and the second electrode layer. The first electrode layer, the second electrode layer, and the chamber form a capacitive structure. When a pressure is applied on the second electrode layer, the second electrode layer is deformed. Since the distance between the first electrode and the second electrode changes, the capacitance of the capacitive structure changes. This capacitance is then measured to determine the pressure applied to the deformable second electrode layer. Because the pressure on the second electrode layer is corresponding to the capacitance of the capacitive structure, the pressure on the second electrode layer can be converted into an output signal of the capacitive structure.

The geometry of the structures of such known MEMS capacitive pressure sensors is designed according to an expected pressure range to be measured. The sensibility of the capacitive structure may have a certain limitation. Decreasing the diameter of the second electrode layer and increasing the thickness or mechanical stress of the deformable second electrode layer will deteriorate the sensibility of the pressure sensor. On the other side, high pressure may lead to overloading of the MEMS capacitive pressure sensor. Increasing the diameter of the second electrode layer and decreasing the thickness of the second electrode layer will change the maximum measurable pressure. The sensor is overloaded when the deformable second electrode layer touches the fixed first electrode on the substrate due to bending.

Since the measurable pressure range set by geometry and material properties of the MEMS sensor structure is limited, different MEMS capacitive pressure sensors are typically used in different applications such as measurement of atmospheric pressure and measurement of hydrostatic pressure.

In view of the foregoing, it would be beneficial to provide a single MEMS capacitive pressure sensor which is applicable in an increased operational range.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a MEMS capacitive pressure sensor comprising a first electrode, a deformable second electrode (conductive membrane) being electrically insulated from the first electrode by means of a chamber between the first electrode and the second electrode, and wherein at least one of the first electrode and the second electrode includes at least one pedestal protruding into the chamber.

Various embodiments of the first aspect may comprise at least one feature from the following bulleted list:the sensor is configured to mechanically connect the first electrode and the second electrode at a defined applied pressure by means of the pedestalthe pedestal is made of insulating material or includes an insulating layer which is configured to electrically insulate the first electrode and the second electrodeat least one of the first electrode and the second electrode includes an insulating layer configured to electrically insulate the first electrode and the second electrodethe pedestal is formed annularly or as a ringat least one of an inner diameter of the pedestal, an outer diameter of the pedestal, a diameter of the chamber, a height of the pedestal, a height of the chamber, and a thickness of a deformable membrane is depending on a predetermined measurable pressure rangethe sensor includes two or more pedestals each having a different heightthe height of the pedestals protruding into the chamber increases in a direction radially outwardsthe pressure in the chamber is substantially lower than the atmospheric pressurethe second electrode comprises at least one amorphous polysilicon layerthe first electrode is fixedly attached to a substrate made of insulating materialthe first electrode and the second electrode are electrically connected to a semiconductor device in the substrateat least one of the first electrode and the second electrode comprises a silicon wafer

According to a second aspect of the present invention, there is provided a method for manufacturing a MEMS capacitive pressure sensor, the method comprising forming a first electrode, forming a deformable second electrode, which is electrically insulated from the first electrode by means of a chamber between the first electrode and the second electrode, and forming at least one pedestal protruding into the chamber from at least one of the first electrode and the second electrode.

Considerable advantages are obtained by means of certain embodiments of the present invention. Certain embodiments of the present invention provide a single MEMS capacitive pressure sensor which is applicable in an increased operational range. Pressure measurement can be, for example, performed in different applications such as measurement of atmospheric pressure and hydrostatic pressure. Two different pressure sensors for measuring atmospheric pressure and hydrostatic pressure can be e.g. replaced by a single pressure sensor, thus reducing the footprint and production costs of the component.

Certain embodiments of the present invention further provide a method for manufacturing a MEMS capacitive pressure sensor. The method is capable of being performed simply and cost effectively. The MEMS capacitive pressure sensors can be manufactured in industrial scale.

EMBODIMENTS

Certain embodiments of the present invention relate to a MEMS capacitive pressure sensor which is applicable in an increased operational pressure range. The sensor comprises a pedestal protruding from at least one of a first electrode (bottom electrode) and a deformable second electrode (top electrode) into a chamber of the sensor. The pedestal will mechanically connect both electrodes at a specific pressure, thus stiffening the structure of the sensor. Measurement can be continued after mechanically connecting the electrodes via the pedestal. The sensor may be, for example, used in measurement of atmospheric pressure before mechanically connecting the electrodes by means of the pedestal. Measurement of hydrostatic pressure may take place after mechanically connecting the electrodes by means of the pedestal, for instance. The sensor provides an increased operational pressure range. Further, certain embodiments of the present invention relate to a method for manufacturing a MEMS capacitive pressure sensor.

InFIG. 1a schematic view of a MEMS capacitive pressure sensor1is illustrated, wherein a deformable electrode18includes a pedestal5in accordance with at least some embodiments of the present invention. The sensor1also includes a first electrode17which is fixedly attached to a substrate19. The substrate19is a standard silicon wafer. The substrate19may further include semiconductor devices (not shown). Further, the sensor1includes a deformable second electrode18which is supported by spacers20. The spacers20are made of insulating material and configured to electrically insulate the first electrode17and the second electrode18. A chamber4is formed between the first electrode17and the second electrode18. The chamber4electrically insulates the first electrode17and the second electrode18. Additionally, the second electrode18includes a pedestal5protruding from the second electrode18into the chamber4. The pedestal5is formed as a single ring.

The first electrode17, the second electrode18, and the chamber4form a capacitive structure. When a pressure P is applied on the second electrode18, the second electrode18is deformed. Since the distance between the first electrode17and the second electrode18changes, the capacitance of the capacitive structure changes. This capacitance is then measured to determine the pressure P applied to the deformable second electrode18. According to certain embodiments, the first electrode17includes an insulating layer21on the opposite side of the pedestal5. The insulating layer21is configured to electrically insulate the first electrode17and the second electrode18.

InFIG. 2a schematic view of a MEMS capacitive pressure sensor1is illustrated, wherein a fixed electrode17includes a pedestal5in accordance with at least some embodiments of the present invention is illustrated. The sensor1includes a first electrode17which is fixedly attached to a substrate19. The substrate19a standard silicon wafer. Further, the sensor1includes a deformable second electrode18which is supported by spacers20. The spacers20are made of insulating material and configured to electrically insulate the first electrode17and the second electrode18. A chamber4is formed between the first electrode17and the second electrode18. The chamber4electrically insulates the first electrode17and the second electrode18. Additionally, the first electrode17includes a pedestal5protruding from the first electrode17into the chamber4. The pedestal5is formed as a single ring.

The first electrode17, the second electrode18, and the chamber4form a capacitive structure. When a pressure P is applied on the second electrode18, the second electrode18is deformed. Since the distance between the first electrode17and the second electrode18changes, the capacitance of the capacitive structure changes. This capacitance is then measured to determine the pressure P applied to the deformable second electrode18. According to certain embodiments, the second electrode18includes an insulating layer21on the opposite side of the pedestal5. The insulating layer21is configured to electrically insulate the first electrode17and the second electrode18.

InFIG. 3a schematic view of a MEMS capacitive pressure sensor1in accordance with at least some embodiments of the present invention is illustrated, wherein a pedestal5of a first electrode17or a second electrode18is mechanically in contact with the respective other electrode17,18. The sensor1is configured to mechanically connect the first electrode17and the second electrode18at a defined applied pressure by means of the pedestal5. Mechanical connection of the first electrode17and the second electrode18will stiffen the deformable second electrode18in order to avoid overloading of the sensor1.

When the deformable second electrode18of the sensor1is deformed to a certain point at a defined pressure, the first electrode17and the second electrode18will be mechanically connected via the pedestal5. Subsequently, the internal part of the deformable second electrode18within the pedestal ring5and the external part of the deformable second electrode outside the pedestal ring5can be considered as different membranes. These membranes are much stiffer in comparison with the full membrane before mechanically connecting the electrodes17,18. Thus, the different membranes can be used for measurement of higher pressure. The pedestal5is made from insulating material or includes an insulating layer configured to electrically insulate the first electrode17and the second electrode18. According to certain embodiments, at least one of the first electrode17and the second electrode18includes an insulating layer on the opposite side of the pedestal5. The insulating layer is configured to electrically insulate the first electrode17and the second electrode18during mechanical connection.

Pressure measurement can continue after mechanically connecting the first electrode17and the second electrode18. The second electrode18can further deflect within and outside of the pedestal ring5of the first electrode17. Changes of the capacitance can be measured after mechanically connecting the electrodes17,18, thus increasing the operational pressure range of the sensor1.

The sensor1shown allows measurement of low pressures, e.g. atmospheric pressure, when the full membrane is used. Additionally, the sensor allows measurement of high pressure, e.g. hydrostatic pressure, when the second electrode18is mechanically connected to the first electrode17and the stiffened parts of the membrane are used at the same time. Parameters of the sensor1such as an inner diameter dinnerof the pedestal5, an outer diameter douterof the pedestal5, a diameter dchamberof the chamber4, a height hpedestalof the pedestal, a height hchamberof the chamber4, and a thickness tmembraneof a deformable membrane affect the measurable pressure range.

InFIG. 4a schematic cross sectional view of a MEMS capacitive pressure sensor1in accordance with at least some embodiments of the present invention is illustrated. A pedestal5is formed as a ring having an inner diameter dinner, an outer diameter douter, and a height hpedestal. According to certain embodiments, the sensor1may comprise two or more pedestals5. In this case, each pedestal5has a different inner diameter dinner, outer diameter douter, and height hpedestal. The height hpedestalof each pedestal5protruding into the chamber4then increases in a direction radially outwards from a central axis of the chamber4. With increasing pressure the outermost pedestal ring will mechanically connect the first electrode17and the second electrode18first. Subsequent mechanical connections may be made under increasing pressure by pedestals arranged in a direction radially inwards from the outermost pedestal.

A first manufacturing method of a MEMS capacitive pressure sensor in accordance with at least some embodiments of the present invention is illustrated inFIGS. 5 to 18.

InFIG. 5a schematic view of a first manufacturing step of a MEMS capacitive pressure sensor according to an embodiment of the present invention is illustrated. A first substrate is used to start the manufacturing. The first substrate is typically a first silicon wafer2.

InFIG. 6a schematic view of a second manufacturing step of a MEMS capacitive pressure sensor according to an embodiment of the present invention is illustrated. A masking layer comprising a first oxide layer6and a nitride layer7is made on a surface of the first silicon wafer2. The first oxide layer6is arranged between the first silicon wafer2and the nitride layer7. The thickness of the first oxide layer6may be 500 [nm] and the thickness of the nitride layer7may be 300 [nm], for instance. Then patterning of the masking layer takes place. The masking layer is required to prepare the first silicon wafer2for a local oxidation process (LOCOS process) at a later stage.

InFIG. 7a schematic view of a third manufacturing step of a MEMS capacitive pressure sensor according to an embodiment of the present invention is illustrated. Local oxidation (LOCOS) of the first silicon wafer2takes place in the areas where the surface of the first silicon wafer2is not coated by the masking layer. The local oxidation may be, for example, performed at a temperature of about 1000 [° C.]. A silicon oxide layer8is formed in the areas selected by means of the patterned masking layer.

InFIG. 8a schematic view of a fourth manufacturing step of a MEMS capacitive pressure sensor according to an embodiment of the present invention is illustrated. The masking layer in the centre part is removed. In other words, the oxide layer6and the nitride layer7are only removed between the areas where a silicon oxide layer8has been formed.

InFIG. 9a schematic view of a fifth manufacturing step of a MEMS capacitive pressure sensor according to an embodiment of the present invention is illustrated. A second local oxidation is performed in order to form silicon oxide between the previously formed silicon oxide areas. The local oxidation may be, for example, performed at a temperature of about 1000 [° C.]. The thickness of the previously formed silicon oxide layer8is greater than the thickness of the subsequently formed silicon oxide.

InFIG. 10a schematic view of a sixth manufacturing step of a MEMS capacitive pressure sensor according to an embodiment of the present invention is illustrated. The masking layer on the surface of the first silicon wafer2, i.e. the first oxide layer6and the nitride layer7, is removed.

InFIG. 11a schematic view of a seventh manufacturing step of a MEMS capacitive pressure sensor according to an embodiment of the present invention is illustrated. The silicon oxide layer8is wet etched. By means of removing the silicon oxide a cavity9is formed in the first silicon wafer2. Additionally, a pedestal5protruding from the first silicon wafer2into the cavity9is formed as a ring.

InFIG. 12a schematic view of an eighth manufacturing step of a MEMS capacitive pressure sensor according to an embodiment of the present invention is illustrated. The manufacturing is continued by providing a second silicon wafer3.

InFIG. 13a schematic view of a ninth manufacturing step of a MEMS capacitive pressure sensor according to an embodiment of the present invention is illustrated. A second oxide layer10is thermally deposited on the surface of the second silicon wafer3. Subsequently, the second oxide layer10is patterned. An insulating layer21formed as a ring is provided to the second silicon wafer3. The insulating layer21may also be, for example, an oxide layer.

InFIG. 14a schematic view of a tenth manufacturing step of a MEMS capacitive pressure sensor according to an embodiment of the present invention is illustrated. Aligned fusion bonding of the first silicon wafer2and the second silicon wafer3takes place, thus forming a chamber4between the wafers2,3. Bonding is performed under complete or partial vacuum conditions. Therefore, a vacuum is created in the chamber4, i.e. the pressure in the chamber4is substantially lower than the atmospheric pressure. The pedestal5protrudes from the first substrate2into the chamber4.

InFIG. 15a schematic view of a eleventh manufacturing step of a MEMS capacitive pressure sensor according to an embodiment of the present invention is illustrated. Grinding and polishing of the surface of the first silicon wafer2facing away from the second silicon wafer3is performed. The thickness tmembraneof the deformable membrane, i.e. the portion of the first silicon wafer2covering the chamber4, between the chamber4and the surface of the first silicon wafer2facing away from the second silicon wafer3is depending on the expected pressure range. Other parameters affecting the pressure range are e.g. the diameter dchamberof the chamber4, the inner diameter dinnerof the pedestal5, the outer diameter douterof the pedestal5, the height hpedestalof the pedestal5, and the height hchamberof the chamber4.

InFIG. 16a schematic view of a twelfth manufacturing step of a MEMS capacitive pressure sensor according to an embodiment of the present invention is illustrated. The first silicon wafer2is partially deep etched and the second oxide layer10is partially removed.

InFIG. 17a schematic view of a thirteenth manufacturing step of a MEMS capacitive pressure sensor according to an embodiment of the present invention is illustrated. Conductive material layers are deposited on the first silicon wafer2and the second silicon wafer3, thus forming contact structures11. The contact structures11can include one, two or several layers of one, two or several metals. The contact structures11may be made of aluminum, for instance. The contact structures11are typically applied using a mechanical mask. Of course, any other suitable method can be used. The thickness of the contact structures11may be, for example, about 1 [μm]. Other possible metals include, but are not limited to, molybdenum, gold, and copper, for instance.

InFIG. 18a schematic view of a fourteenth manufacturing step of a MEMS capacitive pressure sensor according to an embodiment of the present invention is illustrated. Wire bonding of the manufactured structure is performed as last manufacturing step of the MEMS capacitive pressure sensor1. A sensor1comprising a pedestal5protruding from the second electrode18into the chamber4is provided as a result. The first silicon wafer2including the pedestal5represents a deformable electrode18comprising a deformable membrane. The second silicon wafer3represents a fixed electrode17.

A further manufacturing method of a MEMS capacitive pressure sensor in accordance with at least some embodiments of the present invention is illustrated inFIGS. 19 to 33.

InFIG. 19a schematic view of a first manufacturing step of a MEMS capacitive pressure sensor according to another embodiment of the present invention is illustrated. A first substrate is used to start the surface micromechanical process. The first substrate is typically a first silicon wafer2.

InFIG. 20a schematic view of a second manufacturing step of a MEMS capacitive pressure sensor according to another embodiment of the present invention is illustrated. A patterned masking layer comprising a first oxide layer6and a nitride layer7is made on a surface of the first silicon wafer2. The first oxide layer6is arranged between the first silicon wafer2and the nitride layer7. The thickness of the first oxide layer6may be in the range between 300 [nm] and700[nm], for example 500 [nm], and the thickness of the nitride layer7may be in the range between 200 [nm] and400[nm], for example 300 [nm]. The masking layer is required to prepare the first silicon wafer2for a double local oxidation process (LOCOS process) at a later stage.

InFIG. 21a schematic view of a third manufacturing step of a MEMS capacitive pressure sensor according to another embodiment of the present invention is illustrated. Double local oxidation (LOCOS) of the first silicon wafer2takes place in the areas where the surface of the first silicon wafer2is not coated by the masking layer. The local oxidation may be performed at a temperature in a range between 800 [° C.] and1200[° C.], for example at a temperature of 1000 [° C.]. A silicon oxide layer8is formed in the areas selected by means of the patterned masking layer.

InFIG. 22a schematic view of a fourth manufacturing step of a MEMS capacitive pressure sensor according to another embodiment of the present invention is illustrated. The masking layer in the centre part is removed. In other words, the oxide layer6and the nitride layer7are only removed between the areas where a silicon oxide layer8has been formed.

InFIG. 23a schematic view of a fifth manufacturing step of a MEMS capacitive pressure sensor according to another embodiment of the present invention is illustrated. A second local oxidation is performed in order to form silicon oxide between the previously formed silicon oxide areas. The local oxidation may be performed at a temperature in a range between 800 [° C.] and 1200 [° C.], for example at a temperature of 1000 [° C.]. The thickness of the previously formed silicon oxide layer8is greater than the thickness of the subsequently formed silicon oxide.

InFIG. 24a schematic view of a sixth manufacturing step of a MEMS capacitive pressure sensor according to another embodiment of the present invention is illustrated. The nitride layer7is removed. The first oxide layer6will remain on the surface of the first silicon wafer2and form a united oxide structure with the silicon oxide8. An insulating layer (not shown) made of electrically insulating material is additionally made on top of the united oxide structure.

InFIG. 25a schematic view of a seventh manufacturing step of a MEMS capacitive pressure sensor according to another embodiment of the present invention is illustrated. An LPCVD silicon nitride layer13or other insulator is deposited on the silicon oxide8. The thickness of the LPCVD silicon nitride layer13may be in the range between 300 [nm] and500[nm], for instance.

InFIG. 26a schematic view of an eighth manufacturing step of a MEMS capacitive pressure sensor according to another embodiment of the present invention is illustrated. The LPCVD silicon nitride layer13is patterned in order to provide holes14for sacrificial oxide removal at a later stage. Patterning typically takes place by etching the LPCVD silicon nitride layer13locally.

InFIG. 27a schematic view of a ninth manufacturing step of a MEMS capacitive pressure sensor according to another embodiment of the present invention is illustrated. Porous polysilicon15is deposited in the holes14. The thickness of the porous polysilicon15may be in the range between 50 [nm] and150[nm], for instance.

InFIG. 28a schematic view of a tenth manufacturing step of a MEMS capacitive pressure sensor according to another embodiment of the present invention is illustrated. Sacrificial silicon oxide removal is partially performed by HF-vapor etching, thus forming a cavity9between the LPCVD silicon nitride layer13and the first silicon wafer2. A pedestal5is further formed as a ring. The pedestal5protrudes from the first silicon wafer2into the cavity9. The pressure in the cavity9equals the atmospheric pressure. An insulating layer (not shown) faces the LPCVD silicon nitride layer13in order to electrically insulate the LPCVD silicon nitride layer13and the pedestal5during mechanical connection.

InFIG. 29a schematic view of an eleventh manufacturing step of a MEMS capacitive pressure sensor according to another embodiment of the present invention is illustrated. An amorphous polysilicon layer16is deposited on the LPCVD silicon nitride layer13. The thickness of the polysilicon layer16may be in the range between 300 [nm] and500[nm], for instance. Deposition is performed in a partial vacuum or complete vacuum in order to provide a sealed evacuated chamber4between the first silicon wafer2, the LPCVD silicon nitride layer13, and the polysilicon layer16. The pressure in the chamber4is substantially lower than the atmospheric pressure.

InFIG. 30a schematic view of a twelfth manufacturing step of a MEMS capacitive pressure sensor according to another embodiment of the present invention is illustrated. The LPCVD silicon nitride layer13and the polysilicon layer16are patterned.

InFIG. 31a schematic view of a thirteenth manufacturing step of a MEMS capacitive pressure sensor according to another embodiment of the present invention is illustrated. The oxide arranged on the surface of the silicon wafer2is patterned.

InFIG. 32a schematic view of a fourteenth manufacturing step of a MEMS capacitive pressure sensor according to another embodiment of the present invention is illustrated. Conductive material is deposited in the pattern of the oxide arranged on the surface of the silicon wafer2and on the polysilicon layer16, thus forming contact structures11. The contact structures11can include one, two or several layers of one, two or several metals. The contact structures11may be made of aluminium, for instance. The thickness of the contact structures11may be, for example, about 1 [μm]. Other possible metals include, but are not limited to, molybdenum, gold, and copper, for instance.

InFIG. 33a schematic view of a fifteenth manufacturing step of a MEMS capacitive pressure sensor according to another embodiment of the present invention is illustrated. Wire bonding of the manufactured structure is performed as last manufacturing step of the MEMS capacitive pressure sensor1. A sensor1comprising a pedestal5protruding from the first electrode18into the chamber4is provided as a result. The first silicon wafer2including the pedestal5represents a fixed electrode17. The LPCVD silicon nitride layer13and the polysilicon layer16represent a deformable electrode18comprising a deformable membrane. An insulating layer (not shown) faces the LPCVD silicon nitride layer13in order to electrically insulate the LPCVD silicon nitride layer13and the pedestal5during mechanical connection.

INDUSTRIAL APPLICABILITY

At least some embodiments of the present invention find industrial application in production of wrist watches. Two different pressure sensors for measuring atmospheric pressure and hydrostatic pressure can be replaced by a single pressure sensor, for instance.

Acronyms List

MEMS micro-electro-mechanical systemLOCOS local oxidization of siliconLPCVD low pressure chemical vapor deposition

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