INVENTOR:

Leonardo Sala

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 62/185035, titled "Inertial MEMS Sensor Thermal Management Systems and Methods," filed on June 26, 2015 and listing inventor Leonardo Sala, which application is incorporated herein by reference in its entirety.

BACKGROUND

A. Technical Field

[0002] The present invention relates to MEMS sensors and, more particularly, to systems, devices, and methods of temperature-regulating inertial MEMS sensors to increase sensor accuracy over a wide range of ambient temperatures.

B. Background of the Invention

[0003] Existing attempts to provide accurate and stable sensor readings for inertial sensors, such as gyroscopes or accelerometers, that are exposed to environments with fluctuating temperatures, typically utilize designs that place the inertial sensor in a completely closed thermal chamber that covers the sensor shielding it from external temperature variations and maintaining a relatively constant temperature. Oftentimes, the sensor temperature is regulated within the chamber by constantly measuring the chamber temperature with a heating element and regulating the chamber temperature so as to indirectly control the temperature of the sensor. However, such approaches have significant drawbacks, as they drastically increase the overall size and the cost of producing the sensor due to the required additional part count, including the thermal chamber itself, a heater, a temperature loop controller, and oftentimes an ADC. The limitations of such approaches also include the overall achievable temperature accuracy and the drastically increased power consumption of the power-hungry heater.

[0004] What is needed are tools for system designers to overcome the above-described limitations. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Reference will be made to embodiments of the invention, examples of which may be illustrated in the accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in the context of these embodiments, it should be understood that this is not intended to limit the scope of the invention to these particular embodiments.

[0006] FIGURE ("FIG.") 1 illustrates a thermal management system for MEMS sensors comprising a heater element, according to various environment of the invention.

[0007] FIG. 2 is a top view of an exemplary system to manage the temperature of a legacy IC, according to various environment of the invention.

[0008] FIG. 3 is a cross-sectional view of the thermal management system illustrating an exemplary location of a heater element, according to various embodiments of the invention.

[0009] FIG. 4A and 4B are sectional views of exemplary structures of thermal management systems that comprise enclosures, according to various embodiments of the invention.

[0010] FIG. 5 is a block diagram illustrating an exemplary thermal management system, according to various embodiments of the invention.

[0011] FIG. 6A illustrates a thermal management system using an exemplary control circuit, according to various embodiments of the invention.

[0012] FIG. 6B is an electrical circuit model of the thermal management system in FIG.

6A.

[0013] FIG. 7 is a flowchart of an illustrative process for providing thermal management to inertial MEMS sensors, in accordance with various embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] In the following description, for the purpose of explanation, specific details are set forth in order to provide an understanding of the invention. It will be apparent, however, to one skilled in the art that the invention can be practiced without these details. One skilled in the art will recognize that embodiments of the present invention, described below, may be performed in a variety of ways and using a variety of means. Those skilled in the art will also recognize that additional modifications, applications, and embodiments are within the scope thereof, as are additional fields in which the invention may provide utility. Accordingly, the embodiments described below are illustrative of specific embodiments of the invention and are meant to avoid obscuring the invention.

[0015] Reference in the specification to "one embodiment" or "an embodiment" means that a particular feature, structure, characteristic, or function described in connection with the embodiment is included in at least one embodiment of the invention. The appearance of the phrase "in one embodiment," "in an embodiment," or the like in various places in the specification are not necessarily referring to the same embodiment.

[0016] Furthermore, connections between components or between method steps in the figures are not restricted to connections that are affected directly. Instead, connections illustrated in the figures between components or method steps may be modified or otherwise changed through the addition thereto of intermediary components or method steps, without departing from the teachings of the present invention.

[0017] FIG. 1 illustrates a thermal management system for MEMS sensors comprising a heater element, according to various environment of the invention. System 100 comprises substrate 102, ASIC 104, heating element 106, and MEMS sensor 108. MEMS sensor 108 may be any MEMS sensor, such as an inertial MEMS sensor (e.g., a six degrees-of-freedom gyroscope or accelerometer). In embodiments, MEMS sensor 108 measures temperature directly, such as by using a Boltzmann constant related electronic effect. However, this is not intended as a limitation as MEMS sensor 108 may equally measure temperature indirectly, for example, by taking advantage of the relationship between temperature and figure-of-merit variations detected in a MEMS resonator. ASIC 104 is located on substrate 102 and represents any integrated circuit, e.g., a microcontroller. Heating element 106 is disposed on ASIC 104. It is noted that heating element 106 may be placed on substrate 102, embedded into ASIC 104, or embedded into MEMS 108. In addition, MEMS 108 may be located between substrate 102 and ASIC 104.

[0018] In operation, heating element 106 is controlled by a controller (not shown in FIG. 1) to provide heat to ASIC 104 in a manner such as to cause ASIC 104 to maintain a uniform temperature across MEMS sensor 108. Although heating element 106 is depicted in FIG. 1 as having a rectangular shape along the perimeter of ASIC 104, heating element 106 may be designed to have any desired shape to reduce temperature gradients and maximize temperature uniformity across MEMS sensor 108. For example, heating element 106 may be designed to have a spiral shape, have a fin structure at various locations, or be constructed as a combination of multiple individual sections that each may be controlled independently. Temperature uniformity across MEMS sensor 108 may be achieved, for example, by effectively zeroing out the temperature coefficients of the MEMS sensor 108.

[0019] One skilled in the art will appreciate that thermal management system 100 may be employed not only to control the temperature uniformity of MEMS sensors, but any other type of sensor or integrated circuit. FIG. 2 is a top view of an exemplary system to manage the temperature of a legacy IC, according to various environment of the invention. System 200 comprises substrate 204, IC 210, temperature controller 208, heating element 206, and temperature sensor 212. In embodiments, heating element 206 is located on top of substrate 202 and is controlled by temperature controller 208 to uniformly heat substrate 202 to a desired target temperature. IC 210 is attached to substrate 202 in good thermal contact so as to enable heat exchange between substrate 202 and IC 210. Temperature sensor 212 is placed in good thermal contact with IC 210 to provide accurate temperature readings to temperature controller 208 that controls the temperature of substrate 202 and, thus, the temperature of IC 210 via heating element 206.

[0020] While temperature sensor 212 is shown as being placed central to heating element 216, a person of skill in the art will appreciate that temperature sensor 212 may equally be placed off-center and at any other location, such as at the underside of substrate 202, for example, to take advantage of only minor temperature gradients that may exist between temperature sensor 212 and any other device in thermal contact with substrate 202.

[0021] FIG. 3 is a cross-sectional view of the thermal management system illustrating an exemplary location of a heater element, according to various embodiments of the invention. System 300 comprises heating element 306 and component 304. Component 304 may be any part of an electronic circuit, such as a substrate, an integrated IC, or a MEMS sensor that is to be kept at a constant temperature during operation. In embodiments, heating element 306 is placed on top of to-be-heated component 304 and exposes at least its upper surface.

[0022] In order to facilitate thermal exchange between heating element 302 and component 304 (e.g., an IC), in embodiments, heating element 302 is made of aluminum or copper and is implemented as a resistive element that operates based on the principle of I ² R heating. Heating element 302 may be implemented, for example, as an integrated low resistance resistor. It is understood that any suitable material or metal alloy may be used as resistor material. In embodiments, heating element 306 is designed to have a thermal expansion coefficient that is close to that of component 304 and/or a material that maybe used to affix heating element 306 to component 304, such as a thermal epoxy.

[0023] Heating element 302 may be powered by a heat generation system that utilized an electrical power source, such as an on-board battery, a rail voltage, etc. As will be discussed further below, heating element 302 may be driven by a heater driver controlled by a dedicated temperature control system.

[0024] FIG. 4A and 4B are sectional views of exemplary structures of thermal management systems that comprise enclosures, according to various embodiments of the invention. System 400 comprises enclosure 120 that partially or completely covers a structure that comprises substrate 102, ASIC 104, and MEMS sensor 106. Enclosure 120 may be designed to have any shape that forms a partial or complete cavity 430 that may aid in reflecting heat or preventing heat from flowing away from components underneath enclosure 120. In embodiments, enclosure 120 is designed as an open cavity that allows air to circulate around one or more components, such as MEMS sensor 106, in order to reduce potential thermal hot spots and, thus, increase temperature uniformity across MEMS sensor 106 via convection and/or conduction. In FIG. 4B, enclosure 120 is shown as comprising a flat top section 470, for example a plastic cover, that may serve as a lid or temperature shield.

[0025] In embodiments, enclosure 120 is supported exclusively by substrate 102; however, this is not intended as a limitation, because enclosure 120 may be placed on -and supported by- any other structure. For example, enclosure 120 may rest on ASIC 104 in scenarios where a heater element (not shown) is located on ASIC 104. In embodiments, enclosure 120 may be designed to capture radiated heat and conduct it back to substrate 102 to reduce power consumption of the heater element, thereby, increasing energy efficiency.

[0026] FIG. 5 is a block diagram illustrating an exemplary thermal management system, according to various embodiments of the invention. System 500 comprises ASIC 502, temperature sensor 506, serial interface 512, front-end 514, temperature control loop 520, and heat generation system 522.

[0027] Serial interface 512 represents any communication interface known in the art. ASIC 502, which may be any integrated circuit, is coupled to MEMS sensor 504 via heat generation system 522. Heat generation system 522 comprises a heat source that, in operation, maintains a MEMS sensor (not shown) at a precise temperature. In embodiments, heat generation system 522 is controlled by temperature control loop 520 based on temperature data that temperature control loop 520 receives from temperature sensor 506. In embodiments, the temperature of the MEMS sensor is maintained above the ambient temperature to ensure proper operation of temperature control loop 520 at all times.

[0028] In embodiments, serial interface 512 is used to communicate between analog front-end circuit 514 and temperature control loop 520 to regulate the power output by heat generation system 522. Analog front-end circuit 514 may be coupled to a digital signal processing unit that provides programming features to system 500. It is noted that although temperature control loop 520 and heat generation system 522 are depicted as add-on components to ASIC 502 in FIG. 5, any component shown in system 500 may be integrated with ASIC 502. It is further noted that although temperature may be defined as a digital word through a digital interface, the temperature controller can be equally implemented in the analog domain, or as a mixed signal system.

[0029] FIG. 6A illustrates a thermal management system using an exemplary control circuit, according to various embodiments of the invention. System 600 comprises power source 602, heating element 604, switching element 606, driver 608, a dithering circuit 610, and MEMS sensor 630. Power source 602 may be any source of electrical power, e.g., a voltage regulator capable of providing energy to heating element 604. Switching element 606 is a switch to control the power delivered to heating element 604. In embodiments, switching element 606 is a power MOSFET may be driven by MOSFET driver 608, for example, via pulse-width modulation (PWM).

[0030] In embodiments, control circuit 610 comprises dithering circuit 610 and comparator 612, both are coupled between MOSFET driver 608 and sensor 630. Summing element 616 in dithering circuit 614 receives temperature signal 624 from MEMS sensor 630. Sensor 630, in this example, is an accelerometer that generates an accelerometer signal 632.

[0031] In operation, the components in system 600 are configured in a control loop that permits accelerometer 630 to remain at a precise temperature. In detail, accelerometer 630 outputs temperature signal 624, for example, via a temperature sensor designed to measure the surface temperature of accelerometer 630. Temperature signal 624 is input to control circuit 610 that compares it to reference signal 620. In embodiments, if a deviation of temperature signal 624 from reference signal 620 is detected, control circuit 610 outputs PWM signal 622 to MOSFET driver 608 to control the operation of switching element 606 so as to adjust the amount of power that heating element 604 receives from power source 602. The heat generated by heating element 604 is used to reduce the deviation between the temperature of accelerometer 630 and a target temperature provided by reference signal 620, thereby, maintaining a constant temperature on accelerometer 630. [0032] In embodiments, dithering circuit 610 is used to prevent control circuit 610 from operating in a limit cycle. In detail, due to thermal time constant(s) inherent in system 600 that involve an interplay between thermal resistances and capacities, e.g., between an IC and air cavity located above sensor 630, low frequency temperature fluctuations may be induced. The presence of fluctuations or overtones typically results in heat generated by heating element 604 and, thus, temperature of sensor 630 periodically changing at a more or less constant frequency. The fluctuations are hard to predict due to the unknown and difficult to model thermal phase delays of the various elements in the loop. In order to avoid such fluctuations, which may induce unwanted mechanical stresses in various parts of thermal system 600, such as in MEMS sensor 630, in embodiments, dithering circuit 610 is used and configured to suppress the creation of low frequency fluctuations.

[0033] In embodiments, dithering circuit 610 is adjusted such as to spread the energy associated with such tones over a relatively broad range of frequencies to, ultimately, reduce peak- to-peak variations in temperature and prevent artifacts at the output of MEMS sensor 630.

[0034] One skilled in the art will appreciate that placing heating element 604 relatively close to MEMS sensor 630, as compared to, e.g., adjacent to the package walls, will increase the heat transfer efficiency and, thus, reduce the overall amount of heating power that is required during operation.

[0035] FIG. 6B is an electrical circuit model of the thermal management system in FIG. 6A. For clarity, components similar to those shown in FIG. 6A are labeled in the same manner. For purposes of brevity, a description or their function is not repeated here. In a manner similar to FIG. 6A, control circuit 610 in circuit 650 in FIG. 6B comprises dithering circuit 614 and comparator 612. Control circuit 610 is coupled between heater driver 608 and low-pass filter 670 that represents an electrical equivalent of the thermal resistances and capacitances of the circuit comprising the MEMS sensor.

[0036] FIG. 7 is a flowchart of an illustrative process for providing thermal management to inertial MEMS sensors, in accordance with various embodiments of the invention. Process 700 starts at step 702 by providing a temperature sensor that is in thermal contact with an IC or a MEMS sensor.

[0037] At step 704, the temperature sensor measures a temperature to outputs a temperature signal, for example, in form of a representative voltage signal.

[0038] At step 706, the temperature signal is provided to a temperature controller that compares the temperature signal to a target temperature. In embodiments, the temperature controller uses a comparator that compares the measured temperature signal to a reference or target signal to determine, at step 708, whether the temperature of the temperature sensor deviates from the reference temperature.

[0039] If a deviation is detected, then, at step 710, the heater element generates a heat flow that heats the MEMS sensor.

[0040] At step 712, the controller controls the power to a heater element in order to reduce the difference between the target temperature and the measured temperature. In embodiments, the controller uses a dithering circuit to perform the control function.

[0041] It will be appreciated by those skilled in the art that fewer or additional steps may be incorporated with the steps illustrated herein without departing from the scope of the invention. For example, various power generation and filtering functions may be provided to increase signal accuracy. No particular order is implied by the arrangement of blocks within the flowchart or the description herein.

[0042] It will be further appreciated that the preceding examples and embodiments are exemplary and are for the purposes of clarity and understanding and not limiting to the scope of the present invention. It is intended that all permutations, enhancements, equivalents, combinations, and improvements thereto that are apparent to those skilled in the art, upon a reading of the specification and a study of the drawings, are included within the scope of the present invention. It is therefore intended that the claims include all such modifications, permutations, and equivalents as fall within the true spirit and scope of the present invention.