PATENT DOCUMENT

Publication Number: US-10345330-B2
Application Number: US-201615210838-A
Country: US
Kind Code: B2

Title: Mechanical low pass filter for motion sensors

Abstract:
Mechanical low pass filters for motion sensors and methods for making the same are disclosed. In an implementation, a motion sensor package comprises: a substrate; one or more viscous dampers formed on the substrate; one or more mechanically compliant metal springs formed on the substrate; and a sensor stack attached to the one or more metal springs, the sensor stack overlying the one or more viscous dampers and forming a gap between the sensor stack and the one or more viscous dampers and channels between the one or more viscous dampers and metal springs, wherein the one or more metal springs and the one or more viscous dampers provide a mechanical suspension system having a resonant frequency that is higher than a sensing bandwidth of a motion sensor in the sensor stack and lower than a resonant frequency of the motion sensor.

Claims:
What is claimed is: 
     
       1. A motion sensor package comprising:
 a substrate; 
 a plurality of viscous dampers formed on a first side surface of the substrate; 
 one or more mechanically compliant springs, each having a first portion formed on the first side surface of the substrate; and 
 a sensor stack overlying the plurality of viscous dampers and the one or more mechanically compliant springs, 
 wherein each of the one or more mechanically compliant springs has a second portion protruding from a side surface of at least one of the plurality of viscous dampers and attached to a first side surface of the sensor stack and a third portion embedded within the at least one of the plurality of viscous dampers, the first side surface of the substrate faces the first side surface of the sensor stack, 
 wherein a gap is provided between the first side surface of the sensor stack and a top surface of the plurality of viscous dampers, and channels are provided between the plurality of viscous dampers and the mechanically compliant springs, and 
 wherein the one or more mechanically compliant springs and the plurality of viscous dampers provide a mechanical suspension system having a resonant frequency that is higher than a sensing bandwidth of a motion sensor in the sensor stack and lower than a resonant frequency of the motion sensor. 
 
     
     
       2. The motion sensor package of  claim 1 , wherein the gap and the channels are filled with at least one of air, gas, or liquid. 
     
     
       3. The motion sensor package of  claim 1 , wherein a height of the gap, a surface area and a roughness between the plurality of viscous dampers and the sensor stack, and a width and a length of the channels determine a damping coefficient of the mechanical suspension system. 
     
     
       4. The motion sensor package of  claim 1 , wherein the one or more mechanically compliant springs determine a stiffness of the mechanical suspension system. 
     
     
       5. The motion sensor package of  claim 1 , wherein the motion sensor package is hermetically sealed, and the gap and the channels formed by the plurality of viscous dampers are filled with an air, gas, or liquid at a pressure determined at least in part by a sealing pressure. 
     
     
       6. The motion sensor package of  claim 1 , wherein the gap formed between the sensor stack and the plurality of viscous dampers extends in a direction of motion to be dampened. 
     
     
       7. The motion sensor package of  claim 1 , wherein the channels are at least partially surrounding the one or more mechanically compliant springs. 
     
     
       8. The motion sensor package of  claim 1 , wherein the one or more mechanically compliant springs electrically couple the substrate to the sensor stack. 
     
     
       9. The motion sensor package of  claim 1 , wherein the sensor stack comprises:
 an integrated circuit die attached to the one or more mechanically compliant springs; and 
 the motion sensor attached to the integrated circuit die. 
 
     
     
       10. The motion sensor package of  claim 9 , wherein the one or more mechanically compliant springs are attached to the integrated circuit die with at least one of solder, conductive epoxy, or silicone. 
     
     
       11. The motion sensor package of  claim 9 , wherein the motion sensor is a micro-electro-mechanical system (MEMS). 
     
     
       12. The motion sensor package of  claim 1 , wherein the third portion is positioned between the first portion and the second portion. 
     
     
       13. An apparatus comprising:
 a motion sensor including:
 a substrate; 
 a plurality of viscous dampers formed on a first side surface of the substrate; 
 one or more mechanically compliant springs, each having a first portion formed on the first side surface of the substrate; and 
 a sensor stack overlying the plurality of viscous dampers and the one or more mechanically compliant springs, 
 wherein each of the one or more mechanically compliant springs has a second portion protruding from a side surface of at least one of the plurality of viscous dampers and attached to a first side surface of the sensor stack and a third portion embedded within the at least one of the plurality of viscous dampers, the first side surface of the substrate faces the first side surface of the sensor stack, 
 wherein a gap is provided between the first side surface of the sensor stack and a top surface of the plurality of viscous dampers, and channels are provided between the plurality of viscous dampers and the mechanically compliant springs, and 
 wherein the one or more mechanically compliant springs and the plurality of viscous dampers provide a mechanical suspension system having a resonant frequency that is higher than a sensing bandwidth of a motion sensor in the sensor stack and lower than a resonant frequency of the motion sensor; 
 a processor coupled to the motion sensor; 
 memory coupled to the processor and configured to store instructions, which when executed by the processor, cause the processor to perform operations comprising:
 obtaining a motion signal from the motion sensor; and 
 determining one or more of position, velocity, speed or orientation of the apparatus based at least in part on the motion signal. 
 
 
 
     
     
       14. The apparatus of  claim 13 , wherein the gap and the channels are filled with at least one of air, gas, or liquid. 
     
     
       15. The apparatus of  claim 13 , wherein a height of the gap, a surface area and a roughness between the plurality of viscous dampers and the sensor stack, and a width and a length of the channels determine a damping coefficient of the mechanical suspension system. 
     
     
       16. The apparatus of  claim 13 , wherein the one or more mechanically compliant springs determine a stiffness of the mechanical suspension system. 
     
     
       17. The apparatus of  claim 13 , wherein the motion sensor package is hermetically sealed, and the gap and the channels formed by the plurality of viscous dampers are filled with an air, gas, or liquid at a pressure determined at least in part by a sealing pressure. 
     
     
       18. The apparatus of  claim 13 , wherein the third portion is positioned between the first portion and the second portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of and claims priority to U.S. patent application Ser. No. 14/866,378, filed Sep. 25, 2015, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to integrated circuit (IC) packaging. 
     BACKGROUND 
     Motion sensors in consumer electronics are subjected to interference due to their integration into compact and highly complex systems, such as smart phones, electronic tablets and wearable devices. In particular, mechanical vibrations from speakers, vibrators or other system components can impact the desired motion signal by inducing noise and error in the motion sensor output. Moreover, the resonance behaviors of the motion sensor can amplify the mechanical vibrations further increasing the noise and error in motion sensor output. A conventional approach to address this issue is to implement an electrical low-pass filter at the output of the sensor to attenuate the signal that is out of a defined bandwidth (BW) of the motion sensor. The electrical low-pass filter, however, may not be sufficient and consumes power which limits its utility in low power applications. 
     SUMMARY 
     Mechanical low pass filters for motion sensors and methods for making the same are disclosed. 
     In an implementation, a motion sensor package comprises: a substrate; one or more viscous dampers formed on the substrate; one or more mechanically compliant metal springs formed on the substrate; and a sensor stack attached to the one or more metal springs, the sensor stack overlying the one or more viscous dampers and forming a gap between the sensor stack and the one or more viscous dampers and channels between the one or more viscous dampers and metal springs, wherein the one or more metal springs and the one or more viscous dampers provide a mechanical suspension system having a resonant frequency that is higher than a sensing bandwidth of a motion sensor in the sensor stack and lower than a resonant frequency of the motion sensor. 
     In an implementation, a method of fabricating a mechanical suspension system with viscous dampers comprises: (a) depositing a sacrificial material with a defined thickness on a surface of a substrate; (b) patterning the sacrificial material; (c) depositing a seed layer onto the surface and the sacrificial layer; (d) depositing a first photoresist layer onto the seed layer; (e) patterning the first photoresist layer to define a spring pattern; (f) forming a first metal layer of a first defined thickness onto the seed layer to form a metal spring; (g) removing the first photoresist layer; (h) depositing a second photoresist layer on the seed layer and the metal spring; (i) patterning the second photoresist layer to define a viscous damper pattern; (j) forming a second metal layer of second defined thickness onto the seed layer to form viscous dampers; (k) grinding or milling a resulting structure fabricated by the preceding steps (a)-(j) to create a flat surface on the resulting structure; (l) removing the second photoresist layer and the seed layer from the resulting structure; and (m) removing the sacrificial layer from the resulting structure to release the metal spring. 
     In an implementation, an apparatus comprises: a motion sensor including: a substrate; one or more viscous dampers formed on the substrate; one or more mechanically compliant metal springs formed on the substrate; a sensor stack attached to the one or more metal springs, the sensor stack overlying the one or more viscous dampers and forming a gap between the sensor stack and the one or more viscous dampers and channels between the one or more viscous dampers and metal springs, wherein the one or more metal springs and the one or more viscous dampers provide a mechanical suspension system having a resonant frequency that is higher than a sensing bandwidth of a motion sensor in the sensor stack and lower than a resonant frequency of the motion sensor; a processor coupled to the motion sensor; memory coupled to the processor and configured to store instructions, which when executed by the processor, causes the processor to perform operations comprising: obtaining a motion signal from the motion sensor; and determining one or more of position, velocity, speed or orientation of the apparatus based at least in part on the motion signal. 
     Particular implementations disclosed herein provide one or more of the following advantages. A mechanical low pass filter utilizing a mechanical suspension system is built into a sensor package. The mechanical suspension system isolates the motion sensor from out-of-band vibration and package strain and improves sensor stability. In an embodiment, viscous dampers (e.g., air dampers) are used to provide small gaps and/or channels filled with air, gas or liquid that can be used in place of, or together with, mechanically compliant dampers to achieve a mechanical suspension system that has a desired resonant frequency range (e.g., between 500 Hz to 1 kHz) and quality Q (e.g., less than 1). 
     The details of the disclosed implementations are set forth in the accompanying drawings and the description below. Other features, objects and advantages are apparent from the description, drawings and claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a plot showing frequency responses of a motion sensor, a mechanical filter and a combined sensor and filter. 
         FIGS. 2A and 2B  are cross-section views of example configurations of a mechanical low pass filter with mechanically compliant dampers assembled into a package. 
         FIG. 3  is a cross-section view of an example alternate mechanical low pass filter with mechanically compliant dampers assembled into a package. 
         FIGS. 4A-4H  are an example process flow for fabricating the mechanical suspension system with mechanically compliant dampers shown in  FIGS. 2 and 3 . 
         FIGS. 5A-5H  are example metal spring patterns that provide the desired mechanical filter frequency response shown in  FIG. 1 . 
         FIG. 6  is a plot showing example frequency responses of a simulated mechanical suspension system that includes only metal springs and a simulated mechanical suspension system that includes metal springs and mechanically compliant dampers. 
         FIG. 7  is example apparatus that includes a motion sensor as described in reference to  FIGS. 1-6 . 
         FIG. 8  is a cross-section view of an example configuration of a mechanical low pass filter with viscous dampers assembled into a package. 
         FIG. 9  is a quarter model of example configuration of springs and viscous dampers. 
         FIGS. 10A-10D  are examples of metal spring and viscous damper designs that meet the design goal on mechanical resonant frequency and Q. 
         FIGS. 11A and 11B  are example simulated frequency responses of the mechanical suspension system. 
         FIGS. 12A-12I  are cross-section views of a process flow to fabricate the mechanical suspension system with viscous dampers. 
     
    
    
     The same reference symbol used in various drawings indicates like elements. 
     DETAILED DESCRIPTION 
     The disclosed implementations provide a mechanical low-pass filter for motion sensors to attenuate out-of-band vibrations (i.e., outside the bandwidth of the motion sensor). In some implementations, the mechanical low-pass filter is built into the package of the motion sensor by creating a mechanically compliant suspension system to attenuate vibration. The mechanically compliant suspension system is designed to have a resonant frequency that is higher than the sensing bandwidth of the motion sensor (f sensor ) but lower than the resonant frequency of the motion sensor (represented by response curve  102 ). In the examples that follow, the motion sensor is a micro-electrical-mechanical system (MEMS). Some example MEMS are a MEMS accelerometer for sensing acceleration and a MEMS gyro for sensing rotation rate. 
       FIG. 1  is a plot showing frequency responses  101 ,  102 ,  103  of a mechanical low pass filter, motion sensor and a combined sensor and mechanical filter, respectively. The mechanical suspension system disclosed herein and that is used to create the low-pass filter will provide a −40 dB/dec attenuation after input frequencies (vibration frequencies) pass the resonant frequency of the filter (f filter ). The combined sensor plus filter frequency response (represented by frequency response curve  103 ) will have an attenuated resonant peak at the resonant frequency of the motion sensor (f MEMS ). The combined sensor and filter frequency response (curve  103 ) has a steeper roll-off (−80 dB/dec) after f MEMS . Moreover, the compliance of the mechanical suspension system will absorb most of the strain caused by the assembly process of the motion sensor package and improve the motion sensor stability. 
       FIGS. 2A and 2B  are cross-section views of example configurations of a mechanical low pass filter assembled into a package.  FIG. 2A  shows an example two-die MEMS device, where MEMS die  208  and an application specific integrated circuit (ASIC) die  206  are assembled into package  200 . MEMS die  208  and ASIC die  206  are connected using bond wire  210 .  FIG. 2B  shows an example single-die MEMS device, where MEMS  218  and integrated circuit  217  are monolithically integrated into a single die which is then assembled into package  211 . A bond wire is not required to connect MEMS  218  to ASIC  217 . Package  200  and  211  provide the functions of mechanical suspension and electrical connection in metal springs  203  and  214  and it is better suited for devices with only a few electrical connections to the solder pads of package  200  and  211 . 
     Referring to  FIG. 2A , in some implementations package  200  includes substrate  201  (e.g., a ceramic substrate), mechanically compliant dampers  202 , metal springs  203 , solder bumps  204 , integrated circuit die  206 , shock absorbing die-attachment film (DAF)  207 , MEMS die  208  and package cover  209 . The combination of integrated circuit die  206 , DAF  207  and MEMS die  208  are also referred to herein as a sensor stack. The overall stiffness and quality factor Q of the mechanical suspension system is determined by the designs of metal springs  203  and mechanically compliant dampers  202 . The damping coefficient is determined by the material properties, design and location of dampers  202  on substrate  201 . Metal springs  203  can be shaped in a variety of ways as shown in  FIGS. 5A-5H  to achieve the desired low-pass filter characteristics shown in  FIG. 1 . Bond wire  210  electrically connects MEMS die  208  to integrated circuit die  206 . 
     In this example implementation, metal springs  203  serve as both the mechanical suspension and electrical connection to package  200 . In some implementations, through silicon vias (TSVs)  205  can be formed in integrated circuit die  206  to electrically connect integrated circuit die  206  to metal springs  203  through solder bumps  204 . Metal springs  203  can be electrically coupled to package pads (not shown) to allow signals from integrated circuit die  206  to be output on one or more pins (not shown) of package  200 . 
     Referring to  FIG. 2B , in some implementations package  211  includes substrate  212  (e.g., a ceramic substrate), mechanically compliant dampers  213 , metal springs  214 , solder bumps  215 , integrated circuit  217 , MEMS  218  and package cover  219 . MEMS  218  and integrated circuit  217  are monolithically integrated into a single die which is then assembled into package  211 . The single die is also referred to herein as a sensor stack. The overall stiffness and quality factor of the mechanical suspension system is determined by the designs of metal springs  214  and mechanically compliant dampers  213 . The damping coefficient is determined by the material properties, design and location of dampers  213  on substrate  212 . Metal springs  214  can be shaped in a variety of ways as shown in  FIGS. 5A-5H  to achieve the desired low-pass filter characteristics shown in  FIG. 1 . 
     In this example implementation, metal springs  214  serve as both the mechanical suspension and electrical connection to package  211 . In some implementations, through silicon vias (TSVs)  216  can be formed in integrated circuit die  217  to electrically connect integrated circuit die  217  to metal springs  214  through solder bumps  215 . Metal springs  214  can be electrically coupled to package pads (not shown) to allow signals from integrated circuit die  217  to be output on one or more pins (not shown) of package  211 . 
       FIG. 3  is a cross-section view of a second example implementation of a mechanical low pass filter assembled into a package. Package  300  is better suited for devices with many electrical connections to the solder pads of package  300 . In some implementations, package  300  includes substrate  301  (e.g., a ceramic substrate), mechanically compliant dampers  302 , metal springs  303 , integrated circuit die  304 , shock absorbing DAF  305 , MEMS  306  and package cover  309 . Like package  200 , the overall stiffness and quality factor Q of the mechanical suspension system shown in  FIG. 3  is determined by the designs of both metal springs  303  and dampers  302 . The damping coefficient is determined by the design and location of dampers  302  on substrate  301 . Metal springs  303  can be shaped in a variety of ways as shown in  FIGS. 5A-5H  to achieve the desired low-pass filter characteristics shown in  FIG. 1 . In some implementations, metal springs  303  are attached to integrated circuit die  304  by solder, conductive epoxy or silicone. 
     Unlike package  200 , metal springs  303  are only used for mechanical suspension and not for electrical connections with package pads  310 . In some implementations, wire bonds  312  electrically couple MEMS  306  and integrated circuit die  304  to package pads  310 . 
       FIGS. 4A-4H  are an example process flow for fabricating the mechanical suspension system shown in  FIGS. 2 and 3 . Referring to  FIG. 4A , the process flow begins with a silicon wafer or general package substrate  400 . Damping material  401  is dispensed on the top surface of wafer or substrate  400  with a defined thickness, as shown in  FIG. 4B . Damping material  401  can be cured at an appropriate temperature. Damping material  401  is then patterned using, for example, a CO 2  laser to form dampers  402 , as shown in  FIG. 4C . 
     First photoresist layer  403  is deposited on the top surface of damping material  401  with a thickness that is greater than a thickness of damping material  401 , as shown in  FIG. 4D . First photoresist layer  403  is then patterned by photolithography technology to define opening areas. First photoresist layer  403  is then developed to etch away unwanted areas. 
     Seed layer  404  is deposited by physical vapor deposition (PVD) onto the top surface first photoresist layer  403 , as shown in  FIG. 4E . Second photoresist layer  405  is deposited on seed layer  404 , as shown in  FIG. 4F . The same lithographic technique used with first photoresist layer  403  is used with second photoresist layer  405  to define one or more metal spring patterns. 
     Metal layer  406  of defined thickness is electrode-plated onto seed layer  404  to form the metal spring. Second photoresist layer  405  is removed by chemical etching and seed layer  404  is removed by sputtering or chemical etching, as shown in  FIG. 4G . Lastly, first photoresist layer  403  is removed to release the metal spring, as shown in  FIG. 4H . 
       FIGS. 5A-5H  are example metal spring patterns that help provide the desired mechanical filter frequency response shown in  FIG. 1 . A variety of metal spring patterns can be used to obtain the filter characteristics shown in  FIG. 1 . The example patterns shown in  FIGS. 5A-5H  were simulated using motion sensor silicon dimensions as the device to be vibration isolated to achieve the desired filter characteristics shown in  FIG. 1 , where the mechanical suspension system has a resonant frequency higher than the motion sensor bandwidth (e.g., 500 Hz) but lower than the resonant frequency (e.g., 1 KHz) of the motion sensor. Some examples of metal spring patterns that provide the desired filter characteristics include radial straight beam ( FIG. 5A ), radial L-shaped beam ( FIG. 5B ), radial asymmetric L-shaped beam ( FIG. 5C ), radial S-shaped beam ( FIG. 5D ), radial dual beam ( FIG. 5E ), radial curved dual beam ( FIG. 5F ), radial folded beam ( FIG. 5G ) and peripheral dual beam ( FIG. 5H ). Other metal spring patterns may also be used provided they can provide the desired filter characteristics. 
       FIG. 6  is a plot showing example frequency responses of a simulated mechanical suspension system that includes only metal springs and a simulated mechanical suspension system that includes metal springs and mechanically compliant dampers. In these example simulations, the mechanical suspension systems include metal springs with radial L-shaped beam patterns, as shown in  FIG. 5B . As shown by the plot, the mechanical suspension system with only the metal spring (no dampers) has a resonant frequency at about 786 Hz. When the damper is incorporated, the resonant frequency can increase to about 1131 Hz, but the vibration amplitude is greatly attenuated at resonant frequency resulting in a quality factor of about 11.6. 
       FIG. 7  is an example apparatus that includes one or more motion sensors, as described in reference to  FIGS. 1-6 . In some implementations, motion sensor packages  200 ,  300 ,  800  can be implemented in an apparatus, such as smart phone, tablet computer, wearable computer and the like. The apparatus can have a system architecture  700  that includes processor(s)  701 , memory interface  702 , peripherals interface  703 , one or more motion sensors  704   a - 704   n , wireless communication subsystem  705 , audio subsystem  715 , Input/Output (I/O) interface  707 , memory  708 , display device  713  and input devices  714 . 
     Motion sensors  704   a - 704   n  (e.g., MEMS accelerometer, MEMS gyroscope) may be coupled to peripherals interface  703  to facilitate multiple motion sensing functionalities of the apparatus. Location processor  706  can include a global navigation satellite system (GNSS) receiver. Wireless communications subsystem  705  may include radio frequency (RF) receivers and transmitters (or RF transceivers) and/or optical (e.g., infrared) receivers and transmitters. Wireless communication subsystem  705  can operate over a variety of networks, such as global system for mobile communications (GSM) network, GPRS network, enhanced data GSM environment (EDGE) network, IEEE 802.xx network (e.g., Wi-Fi, Wi-Max, ZigBee™), 3G, 4G, 4G LTE, code division multiple access (CDMA) network, near field communication (NFC) network, Wi-Fi Direct network and Bluetooth™ network. 
     I/O interface  707  may include circuitry and/or firmware for supporting wired mediums and implement various communication protocols and include ports for UART, Serial, USB, Ethernet, RS-232 and the like. 
     Memory interface  702  is coupled to memory  708 . Memory  708  may include high-speed random access memory or non-volatile memory, such as one or more magnetic disk storage devices, one or more optical storage devices, or flash memory (e.g., NAND, NOR). Memory  708  may store operating system  709 , such as Darwin, RTXC, LINUX, UNIX, OS X, iOS, WINDOWS, or an embedded operating system such as VxWorks. Operating system  709  may include instructions for handling basic system services and for performing hardware dependent tasks. In some implementations, operating system  709  may include a kernel (e.g., UNIX/LINUX kernel). 
     Memory  708  may also store communication instructions  710  to facilitate communicating with one or more additional devices in a network topology and one or more computers or servers over wired and wireless mediums. Communication instructions  710  can include instructions for implementing all or part of a wireless communications software stack. 
     Memory  708  may include sensor processing instructions  711  to facilitate motion sensor-related processing and functions on motion signals received from motion sensors  704   a - 704   n.    
     Other instructions  712  can include instructions for a variety of applications that use the motion signals provided by motion sensors  704   a - 704   n . For example, other instructions can include application instructions that take the motion signals from motion sensors  704   a - 704   n  and compute the current location, speed and orientation of the apparatus in a reference coordinate frame (e.g., geodetic, local level). The application instructions can display a map on display device  713  with a marker indicating the location of the apparatus along with other information such as turn-by-turn directions for a route. Audio subsystem  715  can provide speech output for the application that provides, for example, audible turn-by-turn directions. 
     Other applications can make other uses of motion signals from motion sensors  704   a - 704   n  and will benefit from motion signals that are less noisy and have less errors due to the mechanical filter designs disclosed herein. For example, an electronic pedometer application can benefit from improved motion signals provided by the mechanical filter designs disclosed herein. 
       FIG. 8  is a cross-section view of an example configuration of a mechanical low pass filter with viscous dampers assembled into a package. A viscous damper can use air and/or any gases and/or liquids with suitable viscosities. In some implementations package  800  includes substrate  801  (e.g., a ceramic substrate), viscous dampers  802 , metal springs  803 , solder bumps  804 , integrated circuit die  806 , shock absorbing die-attachment film (DAF)  807 , MEMS die  808  and package cover  809 . The combination of integrated circuit die  806 , DAF  807  and MEMS die  808  are also referred to herein as a sensor stack. The overall stiffness and damping coefficient of the mechanical suspension system are determined by the designs of metal springs  803  and viscous dampers  802 , respectively. Bond wire  810  electrically connects MEMS die  808  to integrated circuit die  806 . 
     In this example implementation, metal springs  803  serve as both the mechanical suspension and electrical connection to package  800 . In some implementations, TSVs  805  can be formed in integrated circuit die  806  to electrically connect integrated circuit die  806  to metal springs  803  through solder bumps  804 . Metal springs  803  can be electrically coupled to package pads (not shown) to allow signals from integrated circuit die  806  to be output on one or more pins (not shown) of package  800 . 
     Package  800  is an alternative design to mechanically compliant dampers that achieves or improves the overall damping for mechanical suspension based on viscous damping effects. In some implementations, viscous dampers  802  can be used in place of, or together with, mechanically compliant dampers  202  shown in  FIG. 2A . In package  800 , metal springs  803  do not have physical contact with viscous dampers  802 . Rather, viscous dampers  802  create small vertical gaps  811  to the bottom side of the sensor stack and channels  812  around metal springs  803 . As a result, the movement of MEMS die  808  driven by external vibrations would experience damping due to squeeze film, slide film and viscous drag. With hermetic sealing of package  800 , the damping coefficient is determined by the gap height, surface area and roughness between one or more viscous dampers and the sensor stack, and the channel width and length. The damping coefficient can be further optimized with the sealing pressure. 
       FIG. 9  shows an example of the spring and viscous damper design in a quarter model. In this example, viscous dampers  902  are attached to the bottom side of sensor stack  900 , creating small vertical gaps  904  to the substrate  903  and channels  901  around metal springs  905 . In some implementations, as shown in  FIG. 8 , viscous dampers  802  can be attached to the side of substrate  801 , creating vertical gaps  811  to the bottom side of sensor stack  806 - 808 . 
       FIGS. 10A-10D  are examples of viscous damper configurations that meet the design goal for mechanical resonant frequency and quality factor Q. Full design examples of metal spring and viscous dampers are shown in  FIGS. 10A-10D  which take into consideration state-of-art motion sensor silicon dimensions as the device to be vibration isolated. The design goal is to achieve a mechanical suspension system with an overall resonant frequency to be in the range of about 500 Hz to about 1 kHz and low quality factor Q (e.g., less than 1.0) for a better low-pass filtering effect. The design and location of the viscous dampers determine the damping coefficient as well as the quality factor Q and, together with the design and number of metal springs, define the overall stiffness and resonant frequency of the mechanical suspension system. 
     Referring to  FIGS. 10A-10D , different viscous damper configurations are presented based on the geometry of metal springs to create channels  1004   a - 1004   d  (between spring and viscous dampers or between springs) and gaps (between viscous dampers and the sensor stack). Channels  1004   a - 1004   d  will increase the travel length of the air, gas or liquid during sensor motion and therefore increases the viscous drag. Viscous dampers  1005 ,  1007 ,  1009  and  1011  create small gaps in the direction of sensor motion that needs to be damped out. Small gaps enhance the air damping through squeeze-film and slide-film damping effects. 
       FIG. 11A  is an example simulated frequency response of a metal spring suspension using viscous dampers, showing a resonant frequency at about 700 Hz and a quality factor Q of about 1.3. The simulation shows that a metal spring suspension system using viscous damping (e.g., air damping) can achieve the design goal for resonant frequency and quality factor Q. 
       FIG. 11B  shows an example simulated frequency response for another spring design using mechanically compliant dampers, together with a frequency response when no mechanically compliant dampers are used (e.g., metal springs only). The mechanical suspension system with only the metal springs has a resonant frequency at about 786 Hz. When the damper is incorporated, the vibration amplitude is greatly attenuated showing a Q of about 11.6. The resonant frequency, however, increases to about 1131 Hz, which is a design tradeoff when incorporating mechanically compliant dampers. In some implementations, mechanically compliant dampers can be used together with viscous dampers to achieve desired damping. 
       FIGS. 12A-12I  are cross-section views of a process flow to fabricate a mechanical suspension system with viscous dampers, described in reference to  FIGS. 8-11 . 
     Referring to  FIG. 12A , the package begins with a flat surface. In the example shown, the flat surface is provided general package substrate  1200 . In other embodiments, the flat surface can be provided by a silicon wafer. A sacrificial material  1201  is deposited with a defined thickness and pattern. 
     Referring to  FIG. 12B , thin seed layer  1202  is deposited by physical vapor deposition (PVD) onto the surface of substrate  1200  and sacrificial layer  1201 . 
     Referring to  FIG. 12C , first photoresist layer  1203  is deposited onto seed layer  1202 . First photoresist layer  1203  is patterned by photolithography technology to define a metal spring pattern. 
     Referring to  FIG. 12D , a first metal layer of defined thickness is electrode-plated onto seed layer  1202  to form metal springs  1204  and first photoresist layer  1203  is removed by chemical etching, as shown in  FIG. 12E . 
     Referring to  FIG. 12F , second photoresist layer  1206  is deposited onto seed layer  1202  and metal springs  1204 . Second photoresist layer  1206  is patterned by photolithography technology to define a sequential viscous damper pattern. 
     Referring to  FIG. 12G , a second metal layer of a second defined thickness is electrode-plated on to seed layer  1202  to form viscous dampers  1207 . A substantially flat top surface is created on the resulting structure by grinding or milling the top surface of the structure as shown in  FIG. 12H . 
     Referring to  FIG. 12I , second photoresist layer  1206  is removed by chemical etching and seed layer  1202  is removed by sputtering or chemical etching. Lastly, sacrificial layer  1201  is removed to release metal springs  1204 . 
     As described herein, a mechanical low pass filter for motion sensors can provide out-of-band vibration attenuation as well as package strain isolation. The mechanical suspension structure can be made from any material that can be deposited with thin-film deposition technology. The materials include but are not limited to: copper, copper alloy, aluminum, aluminum alloy, iron, silicon, nickel and nickel alloy. The mechanical suspension system utilizes mechanically compliant dampers and/or viscous dampers to further attenuate the resonance behavior of the mechanical filter. The basic structure design for the mechanical filter can be manufactured in a sensor package. 
     While this document contains many specific implementation details, these details should not be construed as limitations on the scope of what may be claimed but rather as descriptions of features that may be specific to particular implementations. Certain features that are described in this specification in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable sub combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can, in some cases, be excised from the combination, and the claimed combination may be directed to a sub combination or variation of a sub combination.

Metadata:
Filing Date: 20160714
Publication Date: 20190709
Grant Date: 20190709
Priority Date: 20150925
Inventors: CHEN, KUAN-LIN
LAI, YUN-JU
Assignee: APPLE INC
CPC Classifications: [{"code": "B81B7/0048", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01P2015/0882", "inventive": false, "first": false, "tree": "[]"}, {"code": "B81B2201/0235", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01P15/0802", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01P1/003", "inventive": true, "first": false, "tree": "[]"}, {"code": "B81B2201/0242", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2224/48091", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01C19/5783", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01P1/003", "inventive": true, "first": false, "tree": "[]"}, {"code": "B81B7/0048", "inventive": true, "first": false, "tree": "[]"}, {"code": "B81B2201/0242", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01C19/5783", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2224/48091", "inventive": false, "first": false, "tree": "[]"}, {"code": "B81B2201/0235", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01P2015/0882", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01P15/0802", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01L2924/00014", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 58408817