MEMS capacitive pressure sensor
View Patent ↗A pressure sensor measures pressure by measuring the deflection of a MEMS membrane using a capacitive read-out method. There are two ways to implement the invention. One involves the use of an integrated Pirani sensor and the other involves the use of an integrated resonator, to function as a reference pressure sensor, for measuring an internal cavity pressure.
1. A pressure sensor, comprising:
a first pressure sensor element having an output and comprising a capacitive Micro Electro Mechanical Systems (MEMS) pressure sensor having a deformable top electrode, and a fixed bottom electrode spaced by a first cavity portion, wherein the top electrode deforms in response to a pressure difference between an external pressure and an internal cavity pressure; and
a second pressure sensor element having an output and housed within a second cavity portion,
wherein:
the second cavity portion has a rigid top cover,
the first and second cavity portions are connected to define a combined sealed cavity,
the second pressure sensor element is configured and arranged with the first pressure sensor element output, such that the second pressure sensor element output provides calibration of the first pressure sensor output,
the second pressure sensor element measures the internal cavity pressure,
a detection routine uses a capacitance input of the first pressure sensor element with data on the internal cavity pressure provided by the second pressure sensor element,
the first and second cavity portions are side-by-side over a Complementary Metal-Oxide Semiconductor (CMOS) integrated circuit,
the top electrode comprises tungsten and a silicon nitride capping layer is applied on the top electrode,
the bottom electrode is made of a metal and an isolation layer is provided over the bottom electrode such that the isolation layer is between the bottom electrode and the first cavity.
2. A sensor as claimed in claim 1 , wherein:
the first and second cavity portions are side by side over a Silicon on Insulator (SOI) substrate arrangement comprising a wafer, an insulator layer over the wafer and a semiconductor layer over the insulator, and
the cavity portions are connected by a channel formed in the insulator layer.
3. A sensor as claimed in claim 2 , wherein each cavity portion connects to, and extends, the channel through a respective via formed in the semiconductor layer.
4. A sensor as claimed in claim 2 , wherein the first cavity portion is over the semiconductor layer, and the second cavity portion is formed in the semiconductor layer.
5. A sensor as claimed in claim 1 , wherein a top metal layer of the CMOS integrated circuit is used for forming electrical connections between at least one external pressure sensor contact and at least one internal pressure sensor electrode.
6. A sensor as claimed in claim 1 , wherein the second pressure sensor element comprises a Pirani gauge.
7. A sensor as claimed in claim 1 , wherein the second pressure sensor element comprises a MEMS resonator, having resonance frequency used to determine the pressure.
8. A sensor as claimed in claim 7 , wherein a quality factor of the resonance frequency is used to determine the pressure.
9. A method of operating a pressure sensor, comprising:
measuring an external pressure using a first pressure sensor element having a capacitive MEMS pressure sensor having a deformable top electrode, and a fixed bottom electrode spaced by a first cavity portion, wherein the top electrode deforms in response to a pressure difference between the external pressure and an internal cavity pressure;
measuring the internal cavity pressure using a second pressure sensor element housed within a second cavity portion, wherein the first and second cavity portions are connected to define a combined sealed cavity; and
combining the external and internal pressure measurements, such that the second pressure sensor element output provides calibration of the first pressure sensor output,
wherein:
the second cavity portion has a rigid top cover,
a detection routine uses a capacitance input of the first pressure sensor element with data on the internal cavity pressure provided by the second pressure sensor element,
the first and second cavity portions are side-by-side over a Complementary Metal-Oxide Semiconductor (CMOS) integrated circuit,
the top electrode comprises tungsten and a silicon nitride capping layer is applied on the top electrode,
the bottom electrode is made of a metal and an isolation layer is provided over the bottom electrode such that the isolation layer is between the bottom electrode and the first cavity.
10. A method as claimed in claim 9 , comprising combining the internal and external pressure measurements with stored calibration parameters in the form of Capacitance-Pressure (C-P) data points or Capacitance-Voltage data points.
11. A method as claimed in claim 9 , wherein:
the second pressure sensor element comprises a Pirani gauge, or
the second pressure sensor element comprises a MEMS resonator, and the method further comprises determining a resonance frequency of the MEMS resonator and using this to estimate the internal pressure, or
the second pressure sensor element comprises a MEMS resonator, and the method comprises determining a quality factor of the resonance and using this to estimate the internal pressure.
12. A method of manufacturing a pressure sensor, comprising:
forming a first pressure sensor element comprising a capacitive MEMS pressure sensor having a deformable top electrode, and a fixed bottom electrode spaced by a first cavity portion;
forming a second pressure sensor element housed within a second cavity portion, and forming a rigid top cover over the second cavity portion; and
connecting the first and second cavity portions to define a combined sealed cavity,
wherein:
the combined sealed cavity is formed via a sacrificial etch, by deposition of a sacrificial layer and subsequent removal of the sacrificial layer,
the first and second cavity portions are side-by-side over a Complementary Metal-Oxide Semiconductor (CMOS) integrated circuit,
the top electrode comprises tungsten and a silicon nitride capping layer is applied on the top electrode,
the bottom electrode is made of a metal and an isolation layer is provided over the bottom electrode, such that the isolation layer is between the bottom electrode and the first cavity,
the sacrificial layer is silicon oxide, and
a second pressure sensor element output provides calibration of a first pressure sensor output.
13. A method as claimed in claim 12 , comprising providing a SOI substrate arrangement comprising a wafer, an insulator layer over the wafer and a semiconductor layer over the insulator, wherein:
the first cavity portion is formed over the semiconductor layer,
the second cavity portion is formed in the semiconductor layer, and
the first and second cavity portions are connected by a channel formed in the insulator layer to define the combined sealed cavity.
14. A method as claimed in claim 12 , comprising providing a CMOS circuit comprising a wafer with a multilevel interconnect arrangement, wherein:
the first cavity portion is over the multilevel interconnect arrangement, and
the interconnect arrangement is used to form electrical connections to the pressure sensor elements.
15. A method as claimed in claim 12 , wherein forming the second pressure sensor element comprises forming a Pirani gauge or a MEMS resonator having a resonance frequency dependent on pressure.
16. A pressure sensor, comprising:
a first pressure sensor element having an output and comprising a capacitive Micro Electro Mechanical Systems (MEMS) pressure sensor having a deformable top electrode, and a fixed bottom electrode spaced by a first cavity portion, wherein the top electrode deforms in response to a pressure difference between an external pressure and an internal cavity pressure; and
a second pressure sensor element having an output and housed within a second cavity portion,
wherein:
the second cavity portion has a rigid top cover, wherein the first and second cavity portions are connected to define a combined sealed cavity,
the second pressure sensor element is configured and arranged with the first pressure sensor element output, such that the second pressure sensor element output provides calibration of the first pressure sensor output,
the second pressure sensor element measures the internal cavity pressure,
a detection routine uses a capacitance input of the first pressure sensor element with data on the internal cavity pressure provided by the second pressure sensor element,
the first and second cavity portions are side-by-side over a Complementary Metal-Oxide Semiconductor (CMOS) integrated circuit,
the top electrode comprises tungsten and is positioned on a silicon nitride membrane,
a silicon nitride passivation layer is applied on the top electrode such that the top electrode is between the silicon nitride membrane and the silicon nitride passivation layer, and
the bottom electrode is made of a metal and an isolation layer is provided over the bottom electrode such that the isolation layer is between the bottom electrode and the first cavity.
17. A pressure sensor, comprising:
a first pressure sensor element having an output and comprising a capacitive Micro Electro Mechanical Systems (MEMS) pressure sensor having a deformable top electrode, and a fixed bottom electrode spaced by a first cavity portion, wherein the top electrode deforms in response to a pressure difference between an external pressure and an internal cavity pressure; and
a second pressure sensor element having an output and housed within a second cavity portion,
wherein:
the second cavity portion has a rigid top cover,
the first and second cavity portions are connected to define a combined sealed cavity,
the second pressure sensor element is configured and arranged with the first pressure sensor element output, such that the second pressure sensor element output provides calibration of the first pressure sensor output,
the second pressure sensor element measures the internal cavity pressure,
a detection routine uses a capacitance input of the first pressure sensor element with data on the internal cavity pressure provided by the second pressure sensor element,
the first and second cavity portions are side-by-side over a Complementary Metal-Oxide Semiconductor (CMOS) integrated circuit,
the top electrode comprises tungsten and a silicon nitride capping layer is applied to the top electrode,
the bottom electrode is made of a metal and an isolation layer is provided over the bottom electrode such that the isolation layer is between the bottom electrode and the first cavity,
the second pressure sensor element comprises a Pirani gauge, and
the Pirani gauge is realized as a surface micromachined silicon Pirani device that is arranged as a wire with gaps between the wire and side heat sinks.