The present invention relates to Microelectromechanical Systems (MEMS) devices, and more particularly, to MEMS devices that sense pressure.
MEMS devices comprise a moveable microstructure that moves in response to forces such as inertial, electrostatic, magnetic or differential pressure. There is a strong need for a cost-effective solution that improves the detection of forces such as pressure. The present invention addresses such a need.
A system and method for providing a MEMS device with integrated electronics are disclosed. In a first aspect, the MEMS device comprises an integrated circuit substrate and a MEMS subassembly coupled to the integrated circuit substrate. The integrated circuit substrate includes at least one circuit coupled to at least one fixed electrode. The MEMS subassembly includes at least one standoff formed by a lithographic process, a flexible plate with a top surface and a bottom surface, and a MEMS electrode coupled to the flexible plate and electrically coupled to the at least one standoff. A force acting on the flexible plate causes a change in a gap between the MEMS electrode and the at least one fixed electrode.
In a second aspect, the method comprises providing an integrated circuit substrate and coupling a MEMS subassembly to the integrated circuit substrate. The integrated circuit substrate includes at least one circuit coupled to at least one fixed electrode. The MEMS subassembly includes at least one standoff formed by a lithographic process, a flexible plate with a top surface and a bottom surface, and a MEMS electrode coupled to the flexible plate and electrically coupled to the at least one standoff. A force acting on the flexible plate causes a change in a gap between the MEMS electrode and the at least one fixed electrode.
The accompanying figures illustrate several embodiments of the invention and, together with the description, serve to explain the principles of the invention. One of ordinary skill in the art will recognize that the particular embodiments illustrated in the figures are merely exemplary, and are not intended to limit the scope of the present invention.
The present invention relates to Microelectromechanical Systems (MEMS) devices, and more particularly, to MEMS devices that sense pressure. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the described embodiments and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and features described herein.
A system and method in accordance with the present invention provides force sensitive and force exerting MEMS devices with integrated electronics. By bonding an integrated circuit substrate that includes at least one fixed electrode to a MEMS subassembly that includes a lithographically formed standoff and a flexible plate with coupled MEMS electrode, a sealed cavity is formed with a reference pressure. Accordingly, a force acting on the flexible plate causes a deflection of the flexible plate and in turn, causes a change in a gap size formed by the sealed cavity between the MEMS electrode and the at least one fixed electrode.
The flexible plate of the MEMS devices deforms and deflects due to a variety of external forces acting on the portion of the flexible plate that is disposed externally and subject to the ambient surrounding environment. These external forces include but are not limited to pressure differences between the reference pressure and a pressure of the ambient surrounding environment, shear forces acting on the flexible plate, and other forces acting on the flexible plate via micro-flow and acceleration exertions.
Additionally, a system and method in accordance with the present invention describes a class of MEMS devices, sensors, and actuators including but not limited to pressure sensors, self-testing pressure sensors, accelerometers, force sensors, shear sensors, fluidic sensors, and micro-speakers that are hermetically sealed and bonded to integrated circuits, that use capacitive sensing and electrostatic actuation, and that have a flexible plate between the hermetically sealed cavity and the surrounding environment to allow the device to interact with the surrounding environment.
Features that enhance performance of the MEMS devices include but are not limited to electrode configurations for detecting and rejecting variations of gap between the MEMS electrode and the fixed electrode of the integrated circuit substrate, pressure sensor offset temperature dependence canceling techniques, pressure sensor self-testing and self-calibrating techniques, and pressure sensor particle filters that eliminate undesirable environmental factors.
To describe the features of the present invention in more detail, refer now to the following description in conjunction with the accompanying figures.
In
The gap between the MEMS electrode 104 and the fixed electrode 118 is determined by the at least one standoff 110 height. The combination of the MEMS electrode 104 and the fixed electrode 118 forms a capacitor. Deformation of the flexible plate 126 due to a force 102, including but not limited to pressure changes, causes changes in the gap between the MEMS electrode 104 and the fixed electrode 118. These changes in the gap in turn cause capacitance changes within the capacitor that are measured by a capacitive measurement process. In one embodiment, the capacitive measurement process includes connecting the capacitor to the integrated circuit substrate 114 with the embedded electronic circuit 116 then measuring a capacitance to indicate the amount of plate deformation resulting from the force 102.
In one embodiment, the flexible plate 126 is formed on a device layer such as a single crystal silicon device layer and is made from doped Silicon (Si) with a thickness range including but not limited to 1 micrometer (um) to 100 um. In this embodiment, the doped Si enables the flexible plate 126 to also serve as the MEMS electrode 104. The flexible plate 126 has at least one standoff 110 lithographically formed on its bottom surface 108. In this embodiment, the fixed electrode 118 is formed from a top metal layer of the CMOS integrated circuit substrate 114. One of ordinary skill in the art readily recognizes that the flexible plate 126 can be made to be responsive to various forces and that would be within the spirit and scope of the present invention.
The at least one standoff 110 is bonded to the integrated circuit substrate 114 by the bond 112 that is conductive to create an electrical connection between the integrated circuit substrate 114 and the MEMS electrode 104. One of ordinary skill in the art readily recognizes that the bond 112 can be a variety of different conductive bonds including but not limited to an aluminum-germanium eutectic bond and that would be within the spirit and scope of the present invention.
The MEMS device 400 of
In
In an embodiment, sealed cavity 512 may be formed by the flexible plate 504, standoffs 520 and 522 and CMOS substrate 502 and sealed at a certain pressure. A second sealed cavity enclosing second MEMS device 506 may be separately sealed at a different pressure than the sealed cavity 512. In an embodiment, the sealed cavity 512 and the second sealed cavity may be sealed at the same pressure by opening a portion of the standoffs. In an embodiment, sealed cavity 512 is bounded by the standoffs 520 and 522. In another embodiment, a portion of standoff 520 is opened (not shown in
The MEMS device 500 of
The electronic circuit, embedded within the CMOS integrated circuit substrate 502 and shown in the bottom part of
The electronic circuit, embedded within the integrated circuit substrate 602 and shown in the bottom part of
In one embodiment, particle filter 810 is used as a stationary electrode. In
In one embodiment, channels of the particle filter 810 are partially filled with a soft protective gel or oil that are kept in place by an adhesive or surface tension forces. The soft protective gel or oil acts as an impermeable barrier against particles and moisture while still transmitting pressure difference variations without any significant attenuation. One of ordinary skill in the art readily recognizes that the channels of the particle filter 810 may be partially filled at varying levels and by a variety of materials and that would be within the spirit and scope of the present invention.
The flexible plate 926 is deformed due to an ambient environment pressure (Pamb) being greater than a reference pressure (Pref) or a Pamb>Pref condition. These two pressures are separated by the flexible plate 926. In one embodiment, the flexible plate 926 is thin. One of ordinary skill in the art readily recognizes that the thinness of the flexible plate 926 can be of varying degrees and that would be within the spirit and scope of the present invention.
The sealed cavity 920 disposed on one side of the flexible plate 926 is sealed during factory manufacturing at a reference pressure of Pref including but not limited to 0.1 to 100 millibar (mbar) or 10.1 Pascal (Pa) to 10.1 kPa. The other side of the flexible plate 926 is exposed to an ambient environment pressure of Pamb. In one embodiment, Pamb is atmospheric pressure which at sea level is approximately 1 atm or 101 ·kPa. One of ordinary skill in the art readily recognizes that Pamb changes as a result of meteorological conditions and as a function of elevation and these changes would be within the spirit and scope of the present invention.
The flexible plate 926 deforms due to a pressure difference Pamb−Pref and a maximal deflection point of the flexible plate 926 is described by the following equation, where keff is an effective stiffness of the flexible plate 926:
In one embodiment, the effective stiffness of a square membrane with fixed edges, thickness of h, and side length of b is described by the following equation, where E is the Young's modulus:
The flexible plate 926 deflection also changes due to a temperature variation of the structural material stiffness of the flexible plate 926 and due to a temperature variation of the reference pressure. In one embodiment, there is a vacuum in the sealed cavity 920 and so the reference pressure Pref is 0. In this embodiment, the deflection of the flexible plate 926 is influenced only by a temperature variation of the structural material stiffness of the flexible plate 926. One of ordinary skill in the art readily recognizes that most materials become softer with temperature rises and so the deflection of the flexible plate 926 will increase with temperature and that would be within the spirit and scope of the present invention.
In another embodiment, the flexible plate 926 is made from a very soft material, moves essentially as a piston, and the sealed cavity 920 is sealed while containing gas at a reference pressure Pref. In this embodiment, when temperature rises, the pressure exerted by the gas on the flexible plate 926 rises as well which pushes the flexible plate 926 away from the integrated circuit substrate 914. This results in a reduction in the deflection of the flexible plate 926.
In another embodiment, the sealed cavity 920 is sealed while containing gas at a particular pressure that results in the deflection of the flexible plate 926 being insensitive to temperature variation due to a canceling of two effects. The first effect of a temperature variation of the deflection of the flexible plate 926 is described by the following equation:
According to the ideal gas law, the second effect of a pressure in the sealed cavity 920 is proportional to an absolute temperature and is described by the following equation:
At a specific reference pressure, these two effects cancel each other out resulting in a deflection of the flexible plate 926 that is temperature independent. This specific reference pressure is described by the following equation:
In one embodiment that uses a silicon material, the typical variation of Young's modulus is −40 ppm/K. In this embodiment, at a temperature of T=300 Kelvin (K), the reference pressure Pref that provides a cancellation of these two effects is approximately described by the following equation:
P
ref=0.0118*Pamb (6).
As a result, a MEMS device utilizing equation (6) to cancel the two aforementioned effects does not require independent temperature measurement by an on-board temperature sensor. One of ordinary skill in the art readily recognizes that a variety of temperatures and varying materials will result in changes to these aforementioned equations and that would be within the spirit and scope of the present invention.
The flexible plate 1126 comprises a top surface 1106 and a bottom surface 1108. The MEMS subassembly is bonded to the integrated circuit substrate 1114 via a bond 1112 which forms a sealed cavity 1120. A handle substrate 1122 is bonded to the flexible plate 1126 via an oxide layer 1124. An additional mass 1128 is also coupled to the top surface 1106 of the flexible plate 1126 to enable sensing of additional forces. In one embodiment, these additional forces include but are not limited to accelerations parallel and normal to the flexible plate 1126 and shear forces. In another embodiment, the additional mass 1128 is coupled to a joystick to enable tracking of joystick motion.
In one embodiment, three fixed electrodes are disposed under the flexible plate 1126: a first fixed electrode disposed under a middle portion of the flexible plate 1126, a second fixed electrode disposed to the left of the first fixed electrode, and a third fixed electrode disposed symmetrically to the right of the first fixed electrode. In this embodiment, shear forces cause capacitance of the second fixed electrode and the third fixed electrode to change oppositely, i.e. when the capacitance of the second fixed electrode increases the capacitance of the third fixed electrode decreases. In this embodiment, normal forces cause all capacitance to change (i.e. increase or decrease) similarly. In another embodiment, four fixed electrodes are disposed under the flexible plate 1126 on the CMOS substrate to measure motion normal to the substrate and motion in a first direction parallel to the substrate and motion in a second direction parallel to the substrate and orthogonal to the first direction.
Electrostatic actuation of the aforementioned MEMS devices is enabled by forming at least one more electrode on the integrated circuit substrate. This self-testing feature can also be used for self-calibration of the device at the factory, which lowers the cost of testing by eliminating the need for external pressure chambers. Electrostatic actuation of the flexible plate can be used for self-testing and self-calibration of the MEMS device and can be used to create a variety of devices including but not limited to micro-speakers, micro-mirrors, and MEMS devices that modulate light.
Referring back to
The resulting function and emulated pressure is used for the self-testing of the pressure sensor MEMS device and is described by the following equation, where Pst is a self-testing pressure, FS, is a self-testing force, Amemb is an area of the flexible plate, Ast is an area of the self-testing electrode ST, ∈ is dielectric permittivity (e.g. 8.85e-12F/m for vacuum); Vst is self-testing potential, g is gap between the bottom of the reference sealed cavity/chamber and flexible plate, and k is a coefficient taking into account that the flexible plate bends as opposed to moving in a piston-like motion:
The aforementioned MEMS devices are utilized for a variety of applications including but not limited to pressure sensor devices, shear force sensor devices, and force exerting devices. As force exerting devices, the MEMS devices operate as micro-speakers, micro-mirrors, and micro-light modulating devices.
Additionally, disposing part of the flexible plate of these MEMS devices to the environment enables these MEMS devices to sense and actuate more than just pressure changes and variations. The MEMS devices described by
In another embodiment, the top surface of the flexible plate(s) of these aforementioned MEMS devices is coated with a chemical compound that is capable of absorbing selective gas or fluid species from the environment. Based upon deformations of the flexible plate resulting from these absorptions, the chemical compound coating enables the MEMS device for micro-balanced chemical sensing.
In one embodiment, the capacitive gap determined by the standoffs of the aforementioned MEMS devices is subject to manufacturing tolerances and variability that ordinarily lead to capacitance variations and therefore variability in sensor outputs. To combat this variability, in one embodiment, the aforementioned MEMS devices utilize common gap values for both capacitors C1(P) and C3(P) by the implemented differential sensing.
One of the merits of any force sensing MEMS device is the ability to resolve very small variations in forces which result in very small ymax variation sensing capabilities on the level of picometers (˜10−12 m) and in very small C1(P) sensing on the level of a atto-Farads (˜10−18 F). These small electrical signals are difficult to measure and are easily overwhelmed by various environmental factors. To combat these various environmental factors, in one embodiment, the aforementioned MEMS device utilize sensing nodes of the capacitive bridge formed by the first and second fixed electrodes which are electrically shielded from the environment by the conductive flexible plate and by the third and fourth fixed electrodes. The third and fourth fixed electrodes are the driven nodes of the capacitive bridge and carrying higher potentials than the first and second fixed electrodes. This results in the third and fourth fixed electrodes being less susceptible to these various environmental factors.
As above described, the system and method allow for more efficient and more accurate force sensing and force exerting MEMS devices that are capable of self-testing and self-calibrating. By coupling an integrated circuit substrate with a fixed electrode to a MEMS subassembly that includes a MEMS electrode coupled to a flexible plate, a hermetically sealed cavity/chamber is formed between the fixed electrode and the MEMS electrode that enables the sensing of various forces by a force responsive capacitive pressure sensor device. Thus, on one side of the flexible plate of this force responsive capacitive pressure sensor device is the hermetically sealed cavity/chamber and the other side is exposed to the surrounding ambient environment.
The at least one standoff formed on the MEMS subassembly determines the gap size of the hermetically sealed cavity/chamber and forms an electrical connection to the integrated circuit substrate. This capacitive pressure sensor can be integrated with other MEMS devices including but not limited to force-responsive devices on the same MEMS subassembly. This capacitive pressure sensor can also be integrated with other CMOS-based sensors including but not limited to temperature, light, and proximity sensors.
Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.
This application claims the benefit of U.S. Provisional Patent Application No. 61/502,616, filed on Jun. 29, 2011, entitled “HERMETICALLY SEALED MEMS DEVICE WITH A PORTION EXPOSED TO THE ENVIRONMENT AND WITH VERTICALLY INTEGRATED ELECTRONICS,” which is incorporated herein by reference in its entirety. This application is related to U.S. Provisional Patent Application No. 61/502,603 filed Jun. 29, 2011, docket # IVS-154PR (5027PR), entitled “DEVICES AND PROCESSES FOR CMOS-MEMS INTEGRATED SENSORS WITH PORTION EXPOSED TO ENVIRONMENT,” and U.S. patent application Ser. No. ______, docket #IVS-154 (5027P), entitled “PROCESS FOR A SEALED MEMS DEVICE WITH A PORTION EXPOSED TO THE ENVIRONMENT,” filed concurrently herewith and assigned to the assignee of the present invention, all of which are incorporated herein in their entireties.
Number | Date | Country | |
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61502616 | Jun 2011 | US | |
61502603 | Jun 2011 | US |