Abstract:
A system and method for controlling temperature of a MEMS sensor are disclosed. In a first aspect, the system comprises a MEMS cap encapsulating the MEMS sensor and a CMOS die vertically arranged to the MEMS cap. The system includes a heater integrated into the MEMS cap. The integrated heater is activated to control the temperature of the MEMS sensor. In a second aspect, the method comprises encapsulating the MEMS sensor with a MEMS cap and coupling a CMOS die to the MEMS cap. The method includes integrating a heater into the MEMS cap. The integrated heater is activated to control the temperature of the MEMS sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 61/502,643, filed on Jun. 29, 2011, entitled “INTEGRATED HEATER ON MEMS CAP FOR WAFER SCALE PACKAGED MEMS SENSORS,” which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to Microelectromechanical Systems (MEMS) sensors, and more particularly, to a MEMS sensor that requires rapid heating for temperature characterization, calibration, and compensation. 
       BACKGROUND 
       [0003]    MEMS sensors (e.g. accelerometers, gyros, compasses, pressure sensors, oscillators, etc.) require temperature compensation to reduce output signal changes that result from temperature variations. Conventional methods of temperature compensation are accomplished during production by measuring the output signal of each sensor at known temperatures, determining the temperature dependence of the output signals, and removing the effect of temperature variations by appropriate on chip or off line signal processing. In either case, the temperature dependence data is stored in a non-volatile memory inside the MEMS sensor. 
         [0004]    The drawbacks of these conventional methods include the inefficient and time consuming process of establishing the response of the output signal to temperature variations by applying a known temperature to the MEMS sensor and measuring the resultant output signals. For high volume production, external heating of the MEMS sensor die is prohibitive due to lengthy heating time and complex test setup. 
         [0005]    Therefore, there is a strong need for a cost-effective solution that overcomes the above issues by enabling fast and real-time temperature compensation that is achieved through an integrated heater. The present invention addresses such a need. 
       SUMMARY OF THE INVENTION 
       [0006]    A system and method for controlling temperature of a MEMS sensor are disclosed. In a first aspect, the system comprises a MEMS cap encapsulating the MEMS sensor and a CMOS die coupled to the MEMS cap. The system includes a heater integrated into the MEMS cap. The integrated heater is activated to control the temperature of the MEMS sensor. 
         [0007]    In a second aspect, the method comprises encapsulating the MEMS sensor with a MEMS cap and coupling a CMOS die to the MEMS cap. The method includes integrating a heater into the MEMS cap. The integrated heater is activated to control the temperature of the MEMS sensor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    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. 
           [0009]      FIG. 1  illustrates a system for controlling temperature of a MEMS sensor in accordance with a first embodiment. 
           [0010]      FIG. 2  illustrates a system for controlling temperature of a MEMS sensor in accordance with a second embodiment. 
           [0011]      FIG. 3  illustrates a system for controlling temperature of a MEMS sensor in accordance with a third embodiment. 
           [0012]      FIG. 4  illustrates a system for controlling temperature of a MEMS sensor in accordance with a fourth embodiment. 
           [0013]      FIG. 5  illustrates a system for controlling temperature of a MEMS sensor in accordance with a fifth embodiment. 
           [0014]      FIG. 6  illustrates a diagram showing a temperature rise in a CMOS die in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The present invention relates to Microelectromechanical Systems (MEMS) sensors, and more particularly, to a MEMS sensor that requires rapid heating for temperature characterization, calibration, and compensation. 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. 
         [0016]    A system and method in accordance with the present invention allows for quickly changing the temperature of a fabricated apparatus comprising a MEMS cap, a MEMS sensor, and/or a CMOS die to avoid inefficient external heating. By integrating a heater into the MEMS cap that encapsulates the MEMS sensor and controlling a current through the integrated heater by an external source or by the CMOS die, Joule heating causes the temperature of the heater to increase thereby increasing the temperature of the various components of the apparatus comprising the MEMS cap, the MEMS sensor, and/or the CMOS die. One of ordinary skill in the art readily recognizes that the apparatus can be fabricated using a variety of methodologies including but not limited to wafer scale packaging using the Nasiri Fabrication (NF) platform. 
         [0017]    Due to the high thermal conductivity of the silicon of the MEMS cap, MEMS sensor, and CMOS die and their small masses, the time constant or the time to heat up the masses is very small thereby enabling rapid heating of the whole apparatus. This rapid heating enables more efficient testing and calibration processes of the whole MEMS apparatus to be conducted. 
         [0018]    To describe the features of the present invention in more detail, refer now to the following description in conjunction with the accompanying Figures. 
         [0019]      FIG. 1  illustrates a system  100  for controlling temperature of a MEMS sensor in accordance with a first embodiment. The system  100  includes a CMOS die  102 , a MEMS cap  104  coupled to the CMOS die  102 , and a heater  106  integrated into the MEMS cap  104 . The MEMS cap  104  encapsulates the MEMS sensor. In one embodiment, the heater  106  is integrated into a top surface of the MEMS cap  104 . By activating and/or deactivating the heater  106 , the temperature of any of the MEMS cap  104 , encapsulated MEMS sensor, and the CMOS die  102  can be increased and/or decreased respectively. 
         [0020]    In one embodiment, the heater  106  is activated by applying a current through two terminals (not shown in  FIG. 1 ) of the heater  106 . Once the heater  106  is activated, the temperature of the heater  106  is increased due to Joule heating which in turn increases the temperature of the MEMS cap  104  and the CMOS die  102  coupled to the MEMS cap  104 . The high thermal conductivity of the silicon components of the system  100  and the small masses involved in the system  100  results in a time constant that is very small including but not limited to the order of milliseconds. Accordingly, the system  100  is rapidly heated by the heater  106 . 
         [0021]    One of ordinary skill in the art readily recognizes that the heater  106  can be integrated into the MEMS cap  104  during production of the system  100  or post-production by an end-user and that would be within the spirit and scope of the present invention. In one embodiment, the MEMS cap  104  is made of a variety of materials including but not limited to silicon. 
         [0022]    In one embodiment, the heater  106  is a resistive heater with terminals connected to either the CMOS die  102  via the MEMS cap  104  or directly to package pads. In this embodiment, the two terminal heater is integrated with the MEMS caps to allow current to pass across the two terminals to increase the temperature.  FIG. 2  illustrates a system  200  for controlling temperature of a MEMS sensor in accordance with a second embodiment. The system  200  includes a CMOS die  102 , a MEMS cap  104  coupled to the CMOS die  102 , a heater  106  integrated into the MEMS cap  104 , an outer package  208  housing the CMOS die  102 , the MEMS cap  104 , and the heater  106 , and at least one package pad  210  connected to the assembly of the MEMS cap  104 /CMOS die  102  structure via at least one wire bond  212 . 
         [0023]    By running a current through the heater  106 , the heater  106  is activated to control the temperature of the system  200 . The activation of the heater  106  is controlled by the CMOS die  102 . In one embodiment, the CMOS die  102  includes a temperature sensor, an electronic circuit for measuring the temperature sensor&#39;s output signals, an electronic circuit for measuring the MEMS sensor&#39;s output signals, and an electronic circuit for energizing and/or activating the heater  106 . In one embodiment, the on chip temperature sensor measures the temperature of the system  200 . In another embodiment, an external thermocouple that is in contact with the top of the outer package  208  measures the temperature of the system  200 . 
         [0024]    One of ordinary skill in the art readily recognizes that the CMOS die  102  can include a variety of different types of electronic circuit components and that would be within the spirit and scope of the present invention. In another embodiment, the activation of the heater  106  is controlled by an external source. There are different ways to integrate the resistive heater into the MEMS cap including but not limited to micromachining and screen printing methodologies. In one embodiment, the resistive heater is an aluminum film deposited into a top surface of the MEMS cap. In this embodiment, the resistive heater is any material that conducts or semi-conducts current including but not limited to polysilicon, various metals, various metallic silicides, and other silicon based films, and resistive patches. In another embodiment, the resistive heater is a high resistance material including but not limited to a polymer with metal dust. 
         [0025]      FIG. 3  illustrates a system  300  for controlling temperature of a MEMS sensor in accordance with a third embodiment. The system  300  includes a CMOS die  102 , a silicon MEMS cap  304  coupled to the CMOS die  102 , an aluminum film heater  306  integrated into the silicon MEMS cap  304 , a first wire bond  308  coupled to a first terminal of the aluminum film heater  306 , and a second wire bond  310  coupled to a second terminal of the aluminum film heater  306 . 
         [0026]    In one embodiment, the aluminum film heater  306  is deposited into a top surface of the silicon MEMS cap  304 . In this embodiment, the aluminum film heater  306  is split into two discontinuous pieces by a cut  320  of varying degree including but not limited to a shallow cut and a deep cut. The cut provides a discontinuity in the aluminum film. The current path is then subject to pass through a portion of the silicon MEMS cap  304  in the region of the area of discontinuity. The resistance between the first and second heater terminals is determined by the doping level of the silicon MEMS cap  304 . In one embodiment, the aluminum film heater  306  is deposited after implantation of the silicon MEMS cap  304  to get good contact between the deposited aluminum film heater  306  and the silicon MEMS cap  304 . 
         [0027]    One of ordinary skill in the art readily recognizes that between the first and second heater terminals, a variety of resistances and voltage differences can be utilized to determine the amount of power generated for heating of the aluminum film heater  306  and that would be within the spirit and scope of the present invention. Additionally, one of ordinary skill in the art readily recognizes that a variety of outer package masses and heating times can be utilized and that would be within the spirit and scope of the present invention. 
         [0028]    In one embodiment, a resistance of 100 ohms and a 10 volt (V) voltage difference between the first and second heater terminals is assumed. Combining the equations Power (P)=Current (I)×Voltage (V) and I=V/Resistance (R), results in P=V 2 /R or 1 Watt of power being generated for the heating of the aluminum film heater  306 . Additionally, in this embodiment, if the outer package housing the CMOS die  102 , the silicon MEMS cap  304 , and the aluminum film heater  306 , is assumed to be mostly plastic with a specific heat of 1.2 Joules/Grams×Degree Celsius (J/g*C) and a total mass of 30 milligrams (mg), 5 seconds of heating the aluminum film heater  306  will result in a 140 degree C. temperature increase being achieved. 
         [0029]    To alter the resistance between the first and second heater terminals and thus the duration required to heat the MEMS cap and/or CMOS die, additional cuts can be made into the heater that is integrated into the silicon MEMS cap.  FIG. 4  illustrates a system  400  for controlling temperature of a MEMS sensor in accordance with a fourth embodiment. The system  400  resembles the system  300  of  FIG. 3  and includes a CMOS die  102 , a silicon MEMS cap  404  coupled to the CMOS die  102 , a heater  406  integrated into the silicon MEMS cap  404 , a first wire bond  408  coupled to a first terminal of the heater  406 , and a second wire bond  410  coupled to a second terminal of the heater  406 . 
         [0030]    In one embodiment, the heater  406  integrated into the silicon MEMS cap  404  includes two cuts. One of ordinary skill in the art readily recognizes that a plurality of cuts can be utilized and that would be within the spirit and scope of the present invention. Additionally, the heater  406  resistance is maintained at a higher level than the other resistances along the current path so that enough power is dissipated across the heater  406  and external contacts and cables do not get hotter than the heater  406 . The power dissipated by the heater  406  is I 2 ×R. If the heater  406  resistance is too small (e.g. &lt;1 ohm), then heating circuitry could supply a current for Joule heating rather than supplying a voltage. 
         [0031]      FIG. 5  illustrates a system  500  for controlling temperature of a MEMS sensor in accordance with a fifth embodiment. The system  500  includes a CMOS die  102 , a MEMS cap  504  coupled to the CMOS die  102 , a heater  506  integrated into the MEMS cap  504 , a first wire bond  508  coupled to a first terminal of the heater  506 , and a second wire bond  510  coupled to a second terminal of the heater  506 . In one embodiment, the heater  506  comprises aluminum layers lithographically patterned on a top surface of the MEMS cap  504 . 
         [0032]    By lithographically patterning the aluminum layers, a resistive heater with desired resistance is attained. In one embodiment, the top surface area of the MEMS cap  504  is 2 millimeters (mm) by 1 mm and 10 segments of 1 mm long aluminum layers are lithographically patterned on the top surface of the MEMS cap  504 . In this embodiment, assuming the resistivity of aluminum is 2.81 ohm×meter and the aluminum layers have 4 micron widths and 1 micro thicknesses, the resistance of the heater  506  is 70 ohms. 
         [0033]      FIG. 6  illustrates a diagram  600  showing a temperature rise in a CMOS die in accordance with an embodiment. The temperature rise is measured by a temperature sensor within CMOS die circuitry or coupled externally to the CMOS die. In the diagram  600 , the resistance of the heater integrated into the MEMS cap is adjusted to be 200 ohms and the vertical scale is 1 Celsius (C)=280 least significant bits (LSB). When an external power supply of 10 V is connected to the heater, the temperature of the CMOS die increases rapidly and within 5 seconds, an approximately 36 degree Celsius temperature rise is achieved. 
         [0034]    As above described, the system and method allow for rapidly controlling temperature compensation of wafer scale packaged MEMS sensors to more efficiently and more accurately measure and calibrate component signal outputs of the whole MEMS apparatus. By integrating a heater into the MEMS cap of the wafer scale packaged MEMS sensor, temperature compensation in volume production of MEMS sensors can achieve temperature rises in approximately one second without the usage of complicated equipment. In comparison, conventional methods typically require approximately 20-30 seconds for the temperature rises and use complicated equipment such as well isolated ovens and/or contact heaters. 
         [0035]    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.