Patent Publication Number: US-2022212920-A1

Title: Liquid-resistant air inlet passive device and methods of making same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This present application is a division of U.S. patent application Ser. No. 16/731,327, filed on Dec. 31, 2019, “Waterproof Microphone And Associated Packing Techniques,” which is a continuation of U.S. patent application Ser. No. 15/354,562, filed on Nov. 17, 2016, “Waterproof Microphone And Associated Packing Techniques,” currently pending, which claims the benefit of U.S. Provisional Application No. 62/257,092, “Pressure Sensor Packaging Design for Water Proof Applications” filed on Nov.18, 2015, and U.S. Provisional Application No. 62/397,186, “A Directional Microphone and Packaging Technique for Creating the Directional Microphone” filed on Sep. 20, 2016, the entire contents of all of the above are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     The mobile communication industry is evolving at a fast phase. One part of the evolution is the introduction of wristwatch mobile phones. Because wristwatch mobile phones are worn on wrists, certain components of the wristwatch mobile phones are more likely to be exposed to liquid damage than that of normal mobile phones which are usually kept in a protective environment (e.g. pockets). Even for normal mobile phones, there are components that have to be exposed to the outside environment. An example of the above mentioned components is the acoustic pressure sensor in a microphone for sensing sound wave pressure. These components have to be protected from being exposed to external damaging factors, such as rains or water, in order to operate properly. 
     Further, surrounding system noises can severely degrade voice quality of a microphone, and therefore a directional microphone offers better voice quality due to its ability to only pick up sound signals in a certain direction. However, a majority of mobile devices use omnidirectional microphones due to unavailability of cost effective directional microphones. In addition, electronic noise-canceling techniques used in omni-directional or directional microphones often fail to keep up with changes in noise and are thus unable to effectively cancel background noise. 
     SUMMARY 
     Aspects of the disclosure provide a waterproof packaging technique which can be used for fabricating waterproof microphones in mobile devices. The waterproof packaging technique employs a liquid-resistant air inlet passive device (LRAPD) which can include a liquid-repellant channel and can be attached to an opening in a housing enclosing an acoustic pressure sensor. In one example, the inner surface of the LRAPD is coated with a self-assembled monolayer (SAM) to realize the waterproof function. 
     A device based on the waterproof packaging technique can include a microelectromechanical system (MEMS) device, a housing enclosing the MEMS device, and a liquid-resistant air inlet passive device (LRAPD) on the housing. The LRAPD can include at least one channel connecting an exterior of the housing with a chamber formed between the housing and the MEMS device. An inside surface of the channel can be coated with a liquid-repellant coating. In some examples, the liquid-repellant coating can be a self-assembled monolayer (SAM) coating. The LRAPD can be attached to an inner side of the housing with an inlet of the channel connected to an opening in the housing, or attached to an outer side of the housing with an outlet of the channel connected to an opening in the housing. Alternatively, the LRAPD can be disposed in an opening of the housing. 
     In one example, the LRAPD includes multiple channels connecting an exterior of the housing with the chamber, and surfaces of the multiple channels are coated with a liquid-repellant coating. In some examples, the housing can include a cover over a substrate supporting the MEMS device, and the LRAPD can be disposed on the substrate. 
     The MEMS device can be a pressure sensor, such as a piezoresistive pressure sensor, a capacitive pressure sensor, and the like in various examples. In one example, the MEMS device is an acoustic pressure sensor with a sensing surface facing the chamber for sensing an acoustic wave, and the LRAPD is formed in a direction of the sensing surface to allow the acoustic wave to reach the sensing surface without dampening the acoustic wave. In another example, a surface of a diaphragm opposite the sensing surface faces the chamber, and the LRAPD is configured to provide an air pressure in the chamber that is equal to atmospheric pressure. Accordingly, in one example, the LRAPD includes a zigzag channel. 
     In a further example, the acoustic pressure sensor includes a cavity between the sensing surface and a housing with an opening in the housing connecting the cavity with the exterior of the housing, and the LRAPD covers the opening. 
     In one example, the LRAPD includes a zigzag channel. In another example, the LRAPD includes a cavity proximate the channel to collect liquid. In a further example, the MEMS device is an acoustic pressure sensor with a sensing surface facing the chamber, and the channel of the LRAPD includes a portion sloping from one end to the other end with respect to the sensing surface to allow an acoustic wave to reach the sensing surface. In one example, the MEMS device is an acoustic pressure sensor with a sensing surface facing the chamber, and the channel of the LRAPD includes a longest portion running parallel from a first end to a second end with respect to the sensing surface to allow an acoustic wave to reach the sensing surface. 
     Aspects of the disclosure provide another packaging technique for making a directional microphone. The packaging technique employs mechanical structures to cancel undesired background noise to realize directional picking up functions instead of requiring an extra sensor in electronic noise-cancelling techniques. Accordingly, the packaging technique enables a directional microphone with reduced a footprint and cost. 
     A directional microphone device based on the packaging technique can include an acoustic sensor and a housing enclosing the acoustic sensor. The acoustic sensor can include a sensing diaphragm for sensing sound pressure, a cavity below the sensing diaphragm, and a first substrate. The directional microphone device can further includes a channel with an inlet open at an edge of the first substrate and an outlet connected with the cavity. The housing can include a cover attached to a second substrate supporting the first substrate. The cover can include a first opening over the sensing diaphragm and a second opening at a side of the cover. The second opening can be disposed adjacent to the inlet of the channel. 
     In some examples, a first distance of a first path from the second opening to the sensing diaphragm via the channel is configured to be equal to a second distance of a second path from the second opening to the sensing diaphragm via a chamber between the cover and the acoustic sensor. 
     In some examples, the directional microphone device includes multiple channels each having an inlet open at the edge of the first substrate and an outlet connected with the cavity. The multiple channels can extend from the cavity to the edge of the first substrate, and can be evenly distributed from each other. In some examples, the cover includes multiple second openings at sides of the cover. The multiple second openings can be evenly distributed along the edge of the cover. In addition, in some examples, the multiple second openings are positioned adjacent to respective inlets of the multiple channels. 
     The acoustic sensor can be fabricated with MEMS technology. The acoustic sensor can be a capacitive pressure sensor, or a piezoresistive pressure sensor, and the like. In some examples, the microphone device can include an anechoic chip disposed over the cover and configured to absorb sound waves reaching the anechoic chip. 
     In one example, the first substrate is bonded to the second substrate. In another example, the channel is formed between the first substrate and the second substrate. In a further example, the first substrate and the second substrate are a same substrate made from a silicon wafer. 
     In one example, the second substrate further includes a barrier wall disposed outside the housing at an edge of the second substrate and adjacent to the second opening outside the housing. In one example, the barrier wall is configured to block sound waves inside the housing from leaving the housing, and to block sound waves outside the housing from entering the housing. 
     In one example, the acoustic sensor includes sidewalls attached to the sensing diaphragm and the first substrate to form the cavity. In another example, the first substrate includes an opening below the cavity. In a further example, the sensing diaphragm is attached to the first substrate, and the cavity is positioned within the first substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of this disclosure that are proposed as examples will be described in detail with reference to the following figures, wherein like numerals reference like elements, and wherein: 
         FIG. 1  shows a waterproof device according to some examples of the disclosure; 
         FIG. 2A  shows a sectional view of a liquid-resistant air inlet passive device (LRAPD) having a horizontal middle portion according to some examples; 
         FIG. 2B  shows a sectional view of another LRAPD having a sloping middle portion according to some examples; 
         FIG. 3A  shows a top view of a first example of a LRAPD; 
         FIG. 3B  shows atop view of a second example of a LRAPD; 
         FIG. 4A  shows a section view of a third example of a LRAPD; 
         FIG. 4B  shows a section view of a fourth example of a LRAPD; 
         FIG. 5  shows a top view and a side view of a fifth example of a LRAPD; 
         FIG. 6  shows an example of a fabrication process sequence for fabricating a LRAPD; 
         FIG. 7  shows a pressure sensor device using the waterproof packaging technique described herein according to some examples; 
         FIG. 8  shows structural variations of LRAPDs according to some examples; 
         FIG. 9  shows another pressure sensor device using the waterproof packaging technique described above according to some examples; 
         FIG. 10  shows a further pressure sensor device using the waterproof packaging technique described herein according to some examples; 
         FIG. 11  shows structures of an example LRAPD; 
         FIG. 12  shows another pressure sensor device using the waterproof packaging technique described above according to some examples; 
         FIG. 13A  shows an example of a sound pressure sensor based on capacitive sensing in a first operating state; 
         FIG. 13B  shows an example of a sound pressure sensor based on capacitive sensing in a second operating state; 
         FIG. 14  shows an example of an omnidirectional microphone device; 
         FIG. 15A  shows a capacitive pressure sensor with sound pressure exerted on one side of a sensing surface; 
         FIG. 15B  shows how sound pressures of two synchronized sound waves are cancelled at a capacitive pressure sensor; 
         FIG. 16  shows an example capacitive pressure sensor for illustrating a principle for making a directional microphone; 
         FIG. 17  shows a sectional view of an example unidirectional microphone device according to some examples; 
         FIG. 18  shows a sectional view of an example substrate structure; 
         FIG. 19  shows a perspective view of an example unidirectional microphone device; 
         FIG. 20A  shows an example of a unidirectional microphone device in a sectional view; 
         FIG. 20B  shows an example of a unidirectional microphone device in a perspective sectional view; 
         FIG. 20C  shows an example of a unidirectional microphone device in a perspective view; and 
         FIG. 21  shows an example directional microphone device with LRAPDs according to some examples. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows a waterproof device  100  according to some examples of the disclosure. The device  100  includes a microelectromechanical system (MEMS) device  130 , a cover  120 , and a substrate  140 . The cover  120  and the substrate  140  form a housing  160  enclosing the MEMS device  130 . A chamber  161  is thus formed between the cover  120  and the MEMS device  130 . The housing  160  can prevent outside particles, for example, water droplets, dust, and the like, from entering the chamber  161  to damage the MEMS device  130  or degrade performance of the MEMS device  130 . 
     The housing  160  includes an opening  121  that provides a passage connecting the chamber  161  with the environment outside the housing  160 . For example, the MF.MS device  130  can be an acoustic pressure sensor for sensing a sound waves coming from outside the housing  160 . The opening  121  can provide a path for the sound waves to reach the MEMS device  130 . 
     According to an aspect of the disclosure, in dry air applications, such as air pressure sensors used in mobile phones, walkie-talkies, or wristwatch mobile phones, a sensing surface of the MEMS device  130  needs to be exposed to ambient environment to receive a sound wave pressure, or/and an opposite surface of the sensing surface needs to be vented to the atmosphere in order to operate properly. At the same time, the MEMS device  130  needs to be protected from damage caused by liquid, such as water, in the ambient environment. To serve this purpose, a waterproof packing technique is provided in the disclosure. Specifically, a liquid-resistant air inlet passive device (LRAPD)  110  is employed for packaging the device  100 . As shown, the LRAPD  110  is attached to the cover  120  below the opening  121 . The LRAPD  110  includes a liquid-resistant channel  111  which connects the chamber  161  with the opening  121  providing a path between the outside environment and the interior of the housing  160 . The liquid-resistant channel  111  exposes or vents the MEMS device  130  to the air while repulsing any liquid from entering into the chamber  161 . 
     In some pressure sensing applications, a waterproof gel coating can be used to cover an outer surface of a pressure sensor, such as the MEMS device  130  in  FIG. 1 , to protect the pressure sensor. However, for ultra-low pressure sensing applications, such as microphones for sound pressure sensing, thickness of a sensing diaphragm of a pressure sensor is in a range of a few micrometers, and any gel coating may deteriorate performance of such type of pressure sensor. In such scenario, a LRAPD  110  can be more advantageous than a gel coating on the pressure sensor. 
     In various examples, the MEMS device  130  enclosed by the housing  160  can be any suitable MEMS devices which utilizes exposure to external environment of the housing  160 . Generally, an MEMS device contains parts or components of a size ranging from 1 micrometer to 1 millimeter that perform engineering functions by electromechanical means. A MEMS device can include micro sensors or actuators made of micromechanical structures, and auxiliary integrated circuits for device function controls and signal transductions. In some examples, the MEMS device  130  is configured to be a pressure sensor, such as a capacitive pressure sensor, a piezoresistive pressure sensor, and the like. Specifically, in some examples, the MEMS device  130  is configured to be acoustic pressure sensor for sensing a sound wave pressure. 
     In one example, the substrate  140  is a part of the MEMS device  130 . For example, the substrate  140  is a portion of a silicon wafer which is used for fabricating one or more parts of the MEMS device  130 , and the cover  120  is bonded and sealed to the substrate  140  to form the chamber  161 . In another example, the MEMS device  130  is fabricated on a silicon die that is attached to the substrate  140  that is a part of a packaging housing, and the cover  120  is then sealed to the packaging substrate  140  to form the chamber  161 . In the above two examples, the cover or the package housing can be made of any suitable materials, such as plastics, metals, ceramics, glass, and the like. 
     In various examples, the MEMS device  130  can include additional microelectronics  150  (e.g., electronic circuits or components) cooperating with the MEMS device  130  to fulfill various functions. 
       FIG. 2A  shows a sectional view of a LRAPD  200 A according to some examples. The LRAPD  200 A includes a channel  201 . The channel  201  includes a first portion  210 , a second portion  220  (a middle portion) and a third portion  230  sequentially connected to form the channel  201 . In one example, the middle portion is perpendicular to the first portion  210  and the third portion  230  as shown in  FIG. 2A . The channel  201  includes an inlet  211  and an outlet  231 . In one example, the channel is formed in a piece of silicon material  250 . The LRAPD  200 A further includes a liquid-repellant coating  240  covering at least part or all of an inner surface of the channel  201 . In addition, a size  202  of the channel  201  (e.g., diameter of the channel) can vary depending on requirements of various applications. In some examples, size of a channel in a LRAPD can be in a range of 2-50 micrometers, while in other examples, size of a channel in a LRAPD can be in a range of 50-500 micrometers. 
     According to an aspect of the disclosure, the liquid-repellant coating  240  can cause the hydrophobic effect when a water droplet is disposed at the surface of the liquid-repellant coating  240  such that the droplet water tends to form a spherical shape. As a result, when the size of the water droplet is larger than the size  202  of the channel  201 , the water droplet will be repelled from entering the channel  201 . For example, when the LRAPD  200 A is installed proximate an opening of a housing enclosing an acoustic pressure sensor for a mobile device, water droplets caused by rain or other ambient factors will be repelled from entering the housing when the mobile device is exposed to the ambient environment. In some instances, it is possible that water droplets with a smaller size may enter through the inlet  211 , however, these can be prevented from passing further through the channel  201  due to the liquid-repellant coaling  240  and the structure of the channel  201 . 
     In some examples, the liquid-repellant coating  240  is made of a self-assembled monolayer (SAM) coating, for example, generated from a wet chemistry based process. In one example, the SAM coated surface of the channel  201  is extremely hydrophobic, and water contact angles thus formed can be greater than 90°. In addition, the thickness of a SAM coating can be molecularly thin and in a range of 2-3 nanometers. 
       FIG. 2B  shows a sectional view of another LRAPD  200 B according to some examples. The LRAPD  200 B includes a channel  262  that has an inlet  261  and an outlet  263  at two ends of the channel  262 . The channel  262  includes a first portion  264 , a second portion  265  (a middle portion) and a third portion  266  sequentially connected to form the channel  262 . The LRAPD  200 B can also include a liquid-repellant coating covering at least part or all of an inner surface of the channel  262 . The channel  262  can be formed in a piece of silicon material  250 . However, different from  FIG. 2A  example, the second portion  265  of the channel  262  slopes from one end to the other end. 
     In one example, the LRAPD  200 B is attached to a cover and is parallel to a sensing surface of an MEMS device. Accordingly, the middle portion  265  is sloped with respect to the sensing surface of the MEMS device  130 . In one example, the LRAPD includes a cavity  270  below the bottom portion of the channel  262  for collecting water passing through the inlet  261 . In this way, the water inside the channel  262  will not block sound waves from passing through the channel  262 . The stored water may later disappear gradually through evaporation. In one example, absorbent materials are filled in the cavity for holding liquid. The absorbent materials can include Polyvinyl Alcohol (PVA) sponges, polyester sponges, and the like. In other examples, the LRAPD may not include the cavity and in the case that a water droplet having a size smaller than that of the inlet  261  enters the channel  262 , the water droplet can be retained at a bottom portion at the lower end of the second portion  265 . As a result, the water droplet is prevent from further going through the channel  262  after entering through the inlet  261  due to the sloped channel. 
     It is noted that a LRAPD can have various configurations depending on packaging requirements of various applications. For example, the channel can be in any form and shape. The size and length of the channel can vary depending on the application. For devices likely to be subject to wetter conditions, the channel can be made longer to reduce liquid absorption and transfer into the chamber of the housing. For devices less likely to be subject to wet conditions, the channel can be shorter to reduce manufacturing costs. The number of channels of a LRAPD can be larger than one. The channel can be either strait or zigzag, or any other forms. In addition, a LRAPD can be mounted outside or inside a housing enclosing a MEMS device, or in the plane of an opening in the housing thus being merged with the housing. When mounted outside the housing, the outlet of the LRAPD can be aligned with an opening of the housing, while when mounted inside the housing the inlet of the LRAPD can be aligned with the opening of the housing. Further, in some examples, an opening in a housing can be positioned at any suitable locations, for example, top, bottom, or side of the housing, for disposing a LRAPD proximate the opening. Furthermore, a LRAPD can be made of any suitable materials in addition to silicon materials, such as plastics, metals, ceramics, glass, polysilicon, silicon dioxide, and the like. 
       FIGS. 3A / 3 B/ 4 A/ 4 B/ 5  show some examples of LRAPDs with different designs.  FIG. 3A  shows a top view of a first version of a LRAPD  300 A. The LRAPD  300 A includes an inlet  301  opening upward, an outlet  303  facing downward, and two separate channels  302  arranged in parallel connecting with the inlet  301  and the outlet  303  at opposite ends.  FIG. 3B  shows a top view of a second version of a LRAPD  300 B. Similarly, the LRAPD  300 B includes an inlet  311  opening upward, an outlet  313  facing downward and a channel  312  connecting with the inlet  311  and the outlet  313 . However, the channel  312  has a zigzag form, and is longer and narrower than either one of the channels  302 . Accordingly, the LRAPDs  300 A and  300 B can be used for different purposes in acoustic pressure sensors. For example, the LRAPD  300 A can be used for exposing sound pressure to a sensing surface of a pressure sensor such that the sound wave pressure can reach the sensing surface without being hindered. In contrast, the LRAPD  300 B can be used for venting a chamber behind the sensing surface to the atmosphere such that the chamber has an air pressure equal to the atmospheric pressure. 
       FIG. 4A  shows a section view of a third version of a LRAPD  400 A. The LRAPD  400 A includes a channel  402  that has a zigzag form in vertical direction. Due to the vertical zigzag structure, when a water droplet enters the channel  402  from an inlet  401 , the water droplet can be retained at a bottom portion  410  of the channel  402 , thus reducing possibility for the water droplet to reach an outlet  403  of the channel  402 . The retained water droplet can evaporate later.  FIG. 4B  shows a section view of a fourth version of a LRAPD  400 B which has a form similar to that of the LRAPD  400 A. The LRAPD  400 B includes a channel  422  having an inlet  421  and an outlet  423 . The channel  422  may have a bottom portion  430  as shown in  FIG. 4B . However, different from LRAPD  400 A, the LRAPD  400 B may additionally include a cavity  440  at below the bottom portion  430 . The cavity  440  can function as a liquid reservoir for receiving water droplets entering the inlet  421  and storing the received water temporarily. In this way, more water droplets can be retained at the cavity, and sound waves can still pass through the channel  422  without being blocked by the retained water. Water stored in the cavity  440  can later evaporate into the atmosphere and/or the cavity  440  can store a material such as a sponge to absorb liquid. 
       FIG. 5  shows a top view and a side view of a fifth version of a LRAPD  500  on the left and right side of  FIG. 5 , respectively. The LRAPD  500  takes a form of a square chip with multiple channels  501  going through the chip from one side to another side. In various examples, the length, size, number and position arrangement of the multiple channels  501  may vary. This allows sound to travel through the LRAPD  500  while preventing or hindering liquid from passing through to the interior chamber. The third version LRAPD  500  provides multiple channels  501  for sound waves going through without being dampened. An application example of the third version LRAPD  500  is shown in  FIG. 12 . 
       FIG. 6  shows an example of a fabrication process sequence  600  for fabricating a LRAPD. Standard semiconductor processing technology can be used to fabricate a LRAPD in some examples. The fabrication process sequence  600  can include multiple steps, and diagrams (a) to (e) in  FIG. 6  represent part of the multiple steps. 
     First, a first silicon wafer  601  is prepared as shown in diagram (a). The silicon wafer  601  can be of any suitable type as known by one of ordinary skill in the art. Second, a channel  602  is patterned and etched in the first wafer  601  as shown in diagram (b). Third, optionally, a silicon oxide layer can be grown on the entire first wafer  601 . Next, as shown in diagram (c), a second wafer  603  can be bonded over the first wafer  601  thus forming a channel  602 . Similarly, the second wafer  603  can be of any suitable types. Any suitable bonding mechanisms can be used for the fabrication. Then, both wafers  601  and  603  can be ground down into a thin substrate such that thickness of the LRAPD is reduced. This step can be optional and the thickness of the final LRAPD can vary depending on interior space of a housing enclosing a MEMS device. Next, as shown in diagram (d), air inlet ports  604  and  605  are etched in both wafers  601  and  603  at the opposite ends of the channel  602  to form a channel  606 . Finally, as shown in diagram (e), a SAM coating is formed on all or part of the inner surface of the channel  606 , the inlet  604  and outlet  605 . 
     It is noted that silicon materials are used in the  FIG. 6  example, however fabrication of a LRAPD is not limited to silicon materials. Any suitable materials, such as plastics, metals, ceramics, and the like, can be used for fabricating a LRAPD with various fabrication processes. 
       FIG. 7  shows a pressure sensor device  700  using the waterproof packaging technique described herein according to some examples. The device  700  can include a housing  710  formed by a cover  711  and a substrate  712 , and a pressure sensor  720  enclosed in the housing  710 . In addition, an opening  713  can be formed on the cover  721 , and a LRAPD  701  is disposed in the opening  713 . The LRAPD  701  can include one or more channels coated with a liquid-repellant coating, such as a SAM coating. 
     In various examples, the pressure sensor  720  can be an absolute, gage or differential measurement device. Generally, a pressure sensor measures a certain pressure in comparison to a reference pressure, and pressure sensors can be categorized into three types of devices: absolute, gage and differential devices. Absolute pressure sensors measure the pressure with respect to a high vacuum reference sealed in a cavity behind a sensing diaphragm. Gage pressure sensors measure the pressure relative to ambient atmospheric pressure. Differential pressure sensors measure a difference between two pressures on opposite sides of a sensing diaphragm. For gage pressure sensor, one side opposite the sensing side of the diaphragm has to be exposed to atmospheric pressure. In addition, the pressure sensor  720  can be sensors based on various physical mechanisms. For example, the pressure sensor  720  can be a capacitive pressure sensor or a piezoresistive sensor in different examples. 
     In  FIG. 7 , a capacitive pressure sensor  720  is used as an example for illustrating the waterproof technique. The capacitive pressure  720  is fabricated with MEMS technology and includes micromechanical structures. As shown, the capacitive pressure sensor  720  includes a diaphragm (moving plate)  721  and a back plate (fixed plate)  722 . Two electrodes attached to the diaphragm  721  and the back plate  722  form a capacitor. The back plate  722  is perforated such that a cavity  723  below the back plate  722  is connected with the gap between the diaphragm  721  and the back plate  722 . In addition, the capacitive pressure sensor  720  is attached to the substrate  712  by some sealing materials  724 . In one example, the capacitive pressure sensor  720  is fabricated on a silicon die separated from a silicon wafer that includes multiple units of pressure sensors generated from a fabrication process. 
     While the air is allowed to go through the LRAPD  701 , water droplets can be prevented from entering the housing  710  due to liquid-resistant function of the LRAPD  701 . 
     In one example, the capacitive pressure sensor  720  is configured to be an acoustic pressure sensor  720  performing gage pressure measurement. The acoustic pressure sensor  720  includes a venting path (not shown) which provides a path allowing air coming from the opening  713  to enter the cavity  723 . At the same time, the air coming from the opening  713  can also reach the sensing surface (upper surface in  FIG. 7 ). As a result, both the sensing surface and back surface (bottom surface in  FIG. 7 ) of the diaphragm  721  are exposed to the same atmospheric pressure, which provides a condition required for gage pressure measurement. In another example, the capacitive pressure sensor  720  is a barometric pressure sensor  720  performing absolute pressure measurement. Accordingly, the chamber  723  may be sealed to form a vacuum. 
     In one example, the capacitive pressure sensor  720  is an acoustic pressure sensor  720  for sensing sound wave pressure, and the device  700  is configured to be a microphone, for example, used in a mobile device. During the operation of the acoustic pressure sensor  720 , sound pressure is allowed to enter the housing  710  through a channel of the LRAPD  701  and reach the sensing surface of the diaphragm  721 . As a response, the diaphragm  721  vibrates according to changes of sound pressure exerted on the diaphragm  721  and causes variations in capacitance value of the capacitor formed by the diaphragm  721  and the back plate  722 . The changes in capacitance value can be subsequently measured by auxiliary microelectronics of the device  700  and reflected in a current or voltage signal processed by the device. 
       FIG. 8  shows several diagrams illustrating structure variations of LRAPDs according to some examples. LRAPDs with these structures can be used in  FIG. 7  example. As shown, diagram (a) is a cross sectional view of a LRAPD  810  which includes an inlet  811  and a channel  812 . A channel formed by the inlet  811  and the channel  812  provides a path  814  for a sound wave to pass through the LRAPD  810  and enter the housing  710 . Diagram (b) shows a cross sectional view of another LRAPD  820  which includes an inlet  821 , a channel  821 , and an outlet  823 . A channel formed by the inlet  821 , the channel  821 , and the outlet  823  provides a path  824  for a sound wave entering the housing  710 . 
     Diagrams (d) and (e) show two different tunnel structures  830  and  850  in top view that can be used in either of the channels  812  and  822 . Accordingly, diagrams (c) and (f) shows cross sectional views of the two channel structures  830  and  850  respectively. As shown, the channel structure  830  has only one channel, while the channel structure  850  has multiple channels  851 . 
     It is noted that dimensions of the channel of the LRAPD  701  can be determined according to requirements of specific applications. For example, in applications of acoustic pressure sensing, such as microphones in a mobile device, the LRAPD  701  is used to expose sound wave to the sensing surface of the diaphragm  721 . In order to prevent water droplets from entering the housing  710 , the cross section size of a channel of the LRAPD  701  needs to be smaller than a certain dimension. On the other side, the dimension of the channels of the LRAPD  701  needs to be large enough to allow the sound wave going through without damping or distorting the sound wave. Accordingly, in one example, a LRAPD with the channel structure  850  having multiple channels  851  can be employed. The multiple channels  851  together provide a broad passage for the sound wave while each channel  851  has a size small enough for repelling water droplets. In another example, a LRAPD with the structure  840  having a single channel that has a narrow cross section is employed. In one example, the cross section area of a channel in the LRAPD is selected to be less than a circle with a diameter of 10 micrometers. In another example, the dimension of a channel in the LRAPD is determined to be in a range of 10-100 micrometers. In addition to numbers and size of channels in the LRAPD  701 , the length of the channels of the LRAPD  701  can be determined to be relatively short such that the sound wave can reach the sensing surface without being dampened. 
       FIG. 9  shows another pressure sensor device  900  using the waterproof packaging technique described above according to some examples. The device  900  has similar structure as the device  700  except that an opening  913  is positioned on a substrate  912  of the device  900 , and a LRAPD  901 , as illustrated for example in  FIGS. 2-8 , is disposed at the opening  913 . Accordingly, air pressure is exposed to a pressure sensor  920  through the LRAPD  901  in the substrate  912 . This design can further inhibit the entry of liquid into the chamber by being on an underside of the pressure sensor device  900 . 
       FIG. 10  shows a further pressure sensor device  1000  using the waterproof packaging technique described herein according to some examples. The device  1000  includes a housing  1010  formed by a cover  1011  and a substrate  1012 , and a pressure sensor  1020  enclosed in the housing  1010 . A chamber  1014  is formed between the cover  1011  and the pressure sensor  1020 . In addition, the substrate  1012  includes a first opening  1013 , a second opening  1025 , and a LRAPD  1001  disposed at the first opening  1013 . 
     In one example, the pressure sensor  1020  is a piezoresistive pressure sensor  1020 . In another example, the pressure sensor  1020  is a capacitive pressure sensor  1020 .  FIG. 10  shows a structure of the capacitive sensor  1020  that is used as an example to illustrate the waterproof packaging technique. As shown, the capacitive sensor  1020  includes a diaphragm  1021  and a perforated back plate  1022 . Two metal layers (not shown) each attached to the diaphragm  1021  or the back plate  1022  form two electrodes of a capacitor for sensing operation. A cavity  1023  is formed below the diaphragm  1021 , while the back plate  1022  is positioned above the diaphragm  1021 . In addition, the second opening  1025  in the substrate  1012  connects the cavity  1023  to the ambient atmosphere. In one example, at least part of the sensing surface (bottom surface in  FIG. 10 ) of the diaphragm  1021  and at least part of the surface of the sidewalls surrounding the cavity  1023  are coated with a SAM coating. 
     In some examples, the capacitive sensor  1020  performs gage pressure measurement. Accordingly, the chamber  1014  is exposed to the atmosphere pressure through the LRAPD  1001  such that back side (upper side in  FIG. 10 ) of the diaphragm  1021  is exposed to the atmosphere pressure. The sensing surface (lower side in  FIG. 10 ) of the diaphragm  1021  is directly exposed to the atmosphere pressure through the second opening  1025 . 
     In one example, the pressure sensor  1020  is an acoustic pressure sensor  1020  performing gage pressure measurement, and the device  1000  is configured to be a microphone, for example, used in a mobile device. Accordingly, sound wave pressure passing through the second opening  1025  is sensed by the diaphragm  1021 , while atmosphere pressure is vented to the chamber  1014  through the first opening  1013 . Because there is no need to allow sound waves to enter the chamber  1014 , the LRAPD  1001  proximate to the first opening  1013  can have a single channel with a small cross section area and a long length.  FIG. 11  shows structures of an example of the LRAPD  1001  in several diagrams (a) to (c). 
     Diagram (a) shows a cross sectional view of a LRAPD  1110  including an inlet  1111 , a channel  1112 , and an outlet  1113  which form a channel  1114 . The channel  1114  provides a path for sound waves to pass through. Diagrams (b) and (c) shows a top view  1120  and a sectional view  1130  of the channel  1114 , respectively. As shown, the channel  1114  includes a single channel  1121  which can be long such that sound waves may be prevented from entering the chamber  1014  while atmospheric pressure is vented into the chamber  1014 . 
       FIG. 12  shows another pressure sensor device  1200  using the waterproof packaging technique described above according to some examples. The device  1200  has similar structures as the device  1000  in  FIG. 10 . However, the device  1200  includes another LRAPD  1202  in addition to a LRAPD  1201 . Specifically, the device  1200  includes a housing  1210  enclosing a pressure sensor  1220 . A cavity  1223  is formed below a diaphragm  1221  of the pressure sensor  1220 , and an opening  1225  connects the cavity  1223  to the ambient environment. The LRAPD  1201  vents a chamber  1214  between the housing  1210  and the pressure sensor  1220  to ambient atmosphere. The LRAPD  1202  is disposed over the opening  1225  to prevent water droplets from entering the cavity  1223  while allowing air going into the cavity  1214 . 
     In one example, the device  1200  is configured to be a microphone and the pressure sensor  1220  is an acoustic pressure sensor for sensing sound waves. In order to expose the sensing surface of the diaphragm  1221  to sound waves without damping or distorting the sound waves, the LRAPD  1202  can have a structure similar to that in  FIG. 5  where multiple channels  501  are provided for exposing the sound waves to the diaphragm  1221 . 
     The pressure sensor devices described above can be applied to various applications, such as sound pressure sensing, barometric pressure measurement, manifold pressure sensing, and the like. However, it is noted that the water proof packaging techniques and the LRAPDs described above are not limited to pressure sensor devices or pressure sensor applications, and can be used for other devices and applications wherever ambient atmosphere is to be exposed to interior of the devices while liquid droplets are prevented from entering the interior of the devices. 
     Generally, an omnidirectional microphone is able to pick up sound from all directions, and may be unsuitable for certain applications where the environment is very-noisy. For example, when a driver sitting inside a vehicle speaks towards a hands-free mobile phone, an omnidirectional microphone in the mobile phone not only picks up his voice but also picks up all background sounds inside the vehicle thus reducing clarity of driver&#39;s voice at the other end of the line. For these applications, a cost effective directional microphone can be employed to mitigate effects of background noise. 
     Directional microphones can be based on different techniques. For example, one approach is to electronically cancel out the background noise. However, such type of devices requires at least two sound pressure sensors to realize the noise cancellation function, which enlarges device package footprint and increases cost due to an extra sensor element. In addition, auxiliary electronics performing electronic cancellation also do not effectively handle rapidly alternating environmental sound waves. 
     Aspects of the disclosure provide a packaging technique for making a directional microphone device. The packaging technique employs a certain mechanical structure to cancel environmental noise in contrast to electronic noise cancellation method. The packaging technique does not require an extra sensor or electronic chip to realize directional functions and serves the purpose of a cost effective directional microphone. 
     A microphone is a sensing device which includes a sound pressure sensor for measuring changes of sound pressure propagating in the air or other types of media. In some applications, the sound pressure is small in terms of pressure scale and therefore a sound pressure sensor is categorized as an ultra-low pressure sensor. A sound pressure sensor can be based on various transduction mechanisms, such as capacitive sensing, piezoresistive sensing, and the like, and can be fabricated with various technologies, such as MEMS technology or non-MEMS technology. 
       FIGS. 13A and 13B  show an example of a sound pressure sensor  1300  based on capacitive sensing. The capacitive sensor  1300  includes two electrodes (not shown) each attached to a diaphragm (a moving plate)  1311  and a back plate (a Fixed plate)  1312  with a small gap  1313  between both electrodes, thus forming a capacitor. In  FIG. 13A , no sound pressure is exerted on the diaphragm  1311 , and the two plates  1311 / 1312  of the sensor  1300  are parallel to each other. In  FIG. 13B , a varying sound pressure  1321  is exerted on the diaphragm  1311  and the diaphragm  1311  moves accordingly towards or away from the fixed plate  1312 , thus causing deflections  1314  of the diaphragm  1311  and changing capacitance value of the capacitor. Changes of the capacitance value can be indicated, for example, by an output current of a circuit connected with the capacitor. In this way, sound pressure variations can be measured using the capacitive sensor  1300 . 
       FIG. 14  shows an example of an omnidirectional microphone device  1400 . The microphone device  1400  includes a sound pressure sensor  1420 , and a housing  1410  enclosing the pressure sensor  1420 . The housing  1410  includes an opening  1411  for allowing sound waves entering the housing  1410  and reaching a sensing surface  1423  of the sound pressure sensor  1420 . In one example, the sound pressure sensor  1420  is a capacitive sensor  1420  which includes a diaphragm  1421  and a perforated back plate  1422 . As shown, as the microphone device  1400  is omnidirectional, the microphone  1400  picks up sounds  1431  entering the opening  1411  from all directions including background noise, which reduces the clarity of a speaker&#39;s voices. 
       FIGS. 15A and 15B  shows how sound pressures of two synchronized sound waves are cancelled at a capacitive pressure sensor  1500 . The capacitive pressure sensor  1500  includes a diaphragm  1511 , and a fixed plate  1512  with a gap  1513  between the diaphragm  1511  and the fixed plat  1522 , thus forming a capacitor. In  FIG. 15A , when sound pressures  1521  are exerted on one side (upper side) of the diaphragm  1511 , the diaphragm  1511  deflects to deflecting positions  1514  accordingly. In  FIG. 15B , however, when a pair of synchronized sound pressures  1521 / 1522  approaches both sides of the diaphragm  1511  simultaneously, the pressures  1521 / 1522  exerted on both sides cancel each other out. Accordingly, the diaphragm  1511  does not move. The pair of synchronized sound pressures  1521 / 1522  can be generated by two synchronized sound waves each exerting a varying sound pressure  1521  or  1522  on opposite side of the diaphragm  1511  while keeping the two varying sound pressures  1521 / 1522  equal to each other. 
       FIG. 16  shows an example capacitive pressure sensor  1600  for illustrating a principle for making a directional microphone. The capacitive pressure sensor  1600  includes a diaphragm  1611  and a fixed plate  1612  which forms a capacitor, and the diaphragm  1611  moves as a response to a sound pressure exerted on the diaphragm  1611  causing capacitance variation of the capacitor. Changing of the capacitance incurs a varying output current or voltage signal indicating variations of the sound pressure. 
     As shown, a sound wave  1620  is generated from a sound source A. A sound wave  1621 , which is a portion of the sound wave  1620 , approaches an upper side of the diaphragm  1611  from the direction perpendicular to the diaphragm  1611 . The sound wave exerts a pressure on the upper side of the diaphragm  1611 . Another sound wave  1622 , which is another portion of the sound wave  1620 , propagates along a route  1623  and approaches a lower side of the diaphragm  1611 . Due to a delay caused by passing along the route  1623 , the sound waves  1621  and  1622  are not synchronized when reaching opposite sides of the diaphragm  1611 . Accordingly, sound pressures of the two sound waves  1621  and exerted on opposite sides of the diaphragm  1611  will not cancel each other, and the sound pressure of the sound wave  1621  can thus be detected. 
     As shown, a sound wave  1630   a  is generated from a sound source B, and two sound waves  1631   a / 1631   b,  each of which is a portion of the sound wave  1630 , approaches opposite sides of the diaphragm  1611 . As the two sound waves  1631   a / 1631   b  are generated from the same source B, and reaches opposite sides of the diaphragm  1611  at the same time via paths having the same length, the two sound waves  1631   a / 1631   b  synchronizes with each other and will cancel each other. As a result, pressures imposed on the two opposite sides of the diaphragm  1611  by the sound waves  1631   a / 163  lb will not contribute to deflections of the diaphragm  1611 . Similarly, sound waves  1632   a / 1632   b,  which are portions of a sound wave  1630   a  generated from a third sound source C, will not be reflected in the varying output signal. 
     As a result, the sound wave  1621  from the perpendicular direction deflects the diaphragm  1611  and can be sensed by the capacitive pressure sensor  1600  while sound waves from other directions, such as the sound waves  1630   a / 1630   b,  will not be sensed due to the cancellation effect. When a microphone is configured in a way showed in  FIG. 16  that sound waves are allowed to approach a sensing surface from a direction perpendicular to the sensing surface, or approach opposite sides of the sensing surface via two equal-length routes from other directions, the microphone can have a directional sensing ability which picks up sound from the perpendicular direction while canceling sound from other directions. 
     The principle illustrated in  FIG. 16  can be employed to create a directional microphone. Accordingly, a packaging technique based on the principle is described herein to make a cost effective unidirectional microphone. The packaging technique is illustrated with examples shown in  FIGS. 17-21 . The examples in  FIGS. 17-21  are based on MEMS technologies, but the packaging technique is not exclusive to MEMS microphone and it can also be used with other types of microphone fabricated with non-MEMS technologies. 
     It is also noted that although capacitive pressure sensors are used as examples for illustrating the principle and the packaging technique for making a directional microphone in  FIGS. 16-21 , the principle and the packaging technique are not limited to capacitive pressures sensors. Any sensors based on other transduction mechanisms with a sensing diaphragm capable of receiving a sound wave at both sides can be used for making directional microphones based on the principle and packaging technique described herein. For example, a directional microphone can be made of a piezoresistive pressure sensor. 
       FIG. 17  shows a sectional view of an example unidirectional microphone device  1700  according to some examples. The device  1700  includes a cover  1712  and a first substrate  1730  which form a housing  1724  enclosing an acoustic pressure sensor  1716 . A chamber  1746  is formed between the cover  1724  and the acoustic pressure sensor  1716 . In one example, the first substrate  1730  is a part of the acoustic pressure sensor  1716 . For example, the first substrate  1730  is a portion of a silicon wafer which is used for fabricating one or more parts of the acoustic pressure sensor  1716 , and the cover  1712  is later bonded and sealed to the first substrate  1730  to form the housing  1724 . In another example, the acoustic pressure sensor  1716  is fabricated on a silicon die that is bonded to the first substrate  1730  that is a part of a packaging housing, and the cover  1712  is then sealed to the packaging substrate  1730  to form the housing  1724 . In the above two examples, the cover  1712  or the package housing  1724  can be made of any suitable materials, such as plastics, metals, ceramics, glass, and the like. In some examples, the first substrate  1730  can include one or more electrical contacts  1702  for establishing connections, for example, with a print circuit board (PCB). 
     The acoustic pressure sensor  1716  can be a capacitive pressure sensor, a piezoresistive sensor, and the like, and can be a sensor based on MEMS technology or non-MEMS technology. In one example, the acoustic pressure sensor  1716  includes a sensing diaphragm  1722  which can move as a response to a sound pressure exerted on the sensing diaphragm  1722 . The acoustic pressure sensor  1716  may further include a fixed plate in other examples, such as the capacitive sensors in  FIGS. 7, 9, 10 and 12 , although not shown in  FIG. 17 , for performing capacitive sensing. 
     In one example, the acoustic pressure sensor  1716  further includes a cavity  1736  below the sensing diaphragm  1722 . For example, in the  FIG. 17  example, the acoustic pressure sensor  1716  includes one or more sidewalls  1732  below the sensing diaphragm  1722  that support the sensing diaphragm  1722  at the edge of the sensing diaphragm  1722 , and enclosing the cavity  1736  below the sensing diaphragm  1722 . In addition, lower edges of the sidewalls  1732  are attached to a second substrate  1726  which includes an opening  1738  connected to the cavity  1736 . Then, the second substrate  1726  is attached to the first substrate  1730 . Thus, the sensing diaphragm  1722 , the sidewalls  1732 , the second substrate  1726 , and the first substrate  1730  enclose the cavity  1736 . 
     It is to be appreciated that in various designs, the cavity  1736  can be formed with various structures. For example, the acoustic pressure sensor  1716  may not include structure of the sidewalls  1732  and the sensing diaphragm  1722  may be disposed directly over the opening  1738  of the second substrate  1726 . For another example, the second substrate  1726  may not include an opening  1738 , thus enclosing the cavity  1736  from below. For a further example, the second and third substrate  1726  and  1730  may be one substrate, for example, fabricated from one silicon wafer. 
     In one example, the acoustic pressure sensor  1716  includes the second substrate  1726 . The second substrate  1726  includes one or more channels  1734   a / 1734   b  connecting the cavity  1736  with exterior of the acoustic pressure sensor  1716 . In one example, an outlet at one end of each channel is connected with the cavity  1736  and an outlet at the other end of each channel is opened at the edge of the second substrate  1726  as shown in  FIG. 17 . Each of the one or more channels  1734   a / 1734   b  provides a path for sound waves to reach the cavity, and subsequently the lower side of the sensing diaphragm  1722 , from exterior of the acoustic sensor  1716 . 
     It is noted that although only two channels  1734   a / 1734   b  are shown in  FIG. 17 , there can be more than two channels crossing the second substrate  1726 , for example, 6, 8, 20, and any other suitable numbers of channels. In addition, in one example, the multiple channels can be evenly distributed and extend from the cavity  1736  or the opening  1738  to the edge of the second substrate  1726 . 
     In one example, the acoustic pressure sensor  1716  further includes an auxiliary integrated circuit  1714 . The auxiliary integrated circuit  1714  can be used for conditioning or controlling operations of the acoustic pressure sensor  1716 , or the auxiliary integrated circuit  1714  can include circuitry, such as a pre-amplifier, for processing a signal generated at the acoustic pressure sensor  1716 . 
     As shown in  FIG. 17 , the cover  1712  can include two types of openings for allowing sound pressures entering the housing  1724  and reaching the sensing diaphragm  1722 . A first opening  1720  is positioned above the sensing diaphragm  1722  such that sound waves from the first opening  1720  reach the sensing diaphragm  1722  from above the sensing diaphragm  1722 . One or more second openings  1708   a / 1708   b  are positioned at the edge of the cover  1712  where the cover  1712  is bonded to the first substrate  1730  in one example. In addition, in one example, second openings  1708   a / 1708   b  are positioned adjacent to entrances  1735   a / 1735   b  of channels  1734   a / 1734   b.  For example, the second opening  1708   a  is adjacent to the entrance  1735   a  of the channel  1734   a,  while the second opening  1708   b  is adjacent to the entrance  1735   b  of the channel  1734   b.  Accordingly, when sound waves enter the housing  1724  from a second openings  1708   a / 1708   b,  the sound waves can have two separate paths to reach opposite sides of the sensing diaphragm  1722 : one path is through a channel  1734   a / 1734   b  to reach the lower side of the sensing diaphragm  1722 , and another path is through the chamber  1746  to reach the upper side of the sensing diaphragm  1722 . 
     In operation, the device  1700  can pick up a sound  1718  entering the first opening  1720  while a sound  1728  entering second openings  1708   a / 1708   b  is being cancelled. Specifically, as an example shown in  FIG. 17 , a first portion of the sound  1718  entering the first opening  1720  approaches the upper side of the sensing diaphragm  1722  from a direction perpendicular to the sensing diaphragm  1722 . At the same time, a second portion of the sound  1718  entering the first opening  1720  propagates along the route  1710  and reaches the lower side of the sensing diaphragm  1722 . The route  1710  passes the channel  1734   a  as illustrated in  FIG. 17 . The second portion of the sound  1718  is delayed due to the long sound travel route  1710 . Accordingly, the first and second portion of the sound  1718  are not synchronized and do not cancel each other out. Thus, the sound  1718  can be detected by the sensor. 
     In contrast, the sound  1728  entering the second opening  1708   b  can include a first portion and a second portion which reach the upper side and lower side of the sensing diaphragm  1722  via two different routes  1744 / 1742  as shown in  FIG. 17 . The path  1744  is between the cover  1712  and the acoustic sensor  1716 , and the path  1742  is via the channel  1734   b  through the second substrate  1726 . As shown, the two different routes  1744 / 1742  can have approximately the same distance, and no delay or small delay is introduced between the first and second portion of the sound  1728 . Accordingly, the first and second portion of the sound  1728  will cancel each other out, and thus the sound  1728  cannot be detected by the sensor. 
     In one example, the first substrate  1730  includes a first barrier wail structure  1704  and second barrier wall structure  1706  for blocking sound waves inside the housing  1724  from leaving the housing. As a result, interferences caused by sound waves leaving the housing  1724  and subsequently reentering the housing  1724  can be avoided. As shown, in one example, the barrier wall structures  1704 / 1706  is disposed proximate the second openings  1708   a / 1708   b  such that sound waves from inside the housing  1724  will be reflected back to the housing  1724 . It also reduces sounds from various angles from entering the housing  1724 . 
     It is noted that, in order to cancel pressures generated by sound waves entering the housing  1724  through a second opening  1708   a  in the cover  1712 , a length of a channel  1734   a  can be determined in such a way that the length of the channel  1734   a  is equal to or approximately equal to a length of a route from the second opening  1708   a  to the upper side of the sensing diaphragm  1722 . Under such a configuration, two portions of a sound wave entering the second opening  1708   a  will reach the upper side and lower side of the sensing diaphragm  1722 , respectively, at a same time thus being synchronized to each other. As a result, sound pressures of the two portions are cancelled by each other. 
       FIG. 18  shows a sectional view of an example substrate structure  1800 . The substrate structure  1800  corresponds to a combination of the first substrate  1726  and the second substrate  1730  shown in  FIG. 17 . As shown, the substrate structure  1800  includes a first substrate  1810  and a second substrate  1820  corresponding to the first substrate  1726  and the second substrate  1730  shown in  FIG. 17 . The second substrate  1820  can include an opening  1821  connected with the cavity  1736  in  FIG. 17 . The second substrate  1820  can further include two channels  1822   a  and  1822   b  extending from the edge of the second substrate  1820  to the opening  1821 , thus each providing a path  1823   a  or  1823   b  for sound waves reaching the opening  1821 . 
     In addition, the first and second substrate  1810  and  1820  can be bonded together, for example, by some adhesive materials  1841 . In one example, a gap  1840  is formed between the two substrates  1810  and  1820 . However, in another example, no gap exists between the two substrates  1810  and  1820 . In various examples, the first substrate  1810  can be a print circuit board (PCB), a part of a packing housing, a silicon chip, and the like, and the second substrate  1820  can be a silicon die, or a chip made of suitable materials. 
       FIG. 19  shows a perspective view of an example unidirectional microphone device  1900 . As shown, a sound pressure sensor  1901  is packaged within a housing  1910 . The housing  1910  includes a cover  1912  attached to a substrate  1914 . A first opening  1916  is positioned over the sensor  1901  at the top of the cover  1912 . In one example, the position of the first opening  1916  is close to one side of the microphone and away from the middle of the cover  1912 . In addition, multiple second openings  1918  are positioned along the edge of the cover  1912  where the cover  1912  and the substrate  1814  adjoin to each other. In operation, sounds entering the first opening  1916  can be picked up by the sensor  1901 , while sounds entering the second openings  1918  will not be sensed by the sensor  1901  due to the cancellation effect discussed previously. 
       FIGS. 20A / 20 B/ 20 C show an example of a unidirectional microphone device  2000  in sectional view, perspective sectional view, and perspective view, respectively. An anechoic chip  2040  is mounted on top of a cover  2012  of the device  2000  in order to reduce interference to a sensing operation of the device  2000 . 
     The device  2000  includes a first substrate  2011  the cover  2012  which encloses an acoustic pressure sensor  2020 . The sensor  2020  includes a sensing diaphragm  2021 , a cavity  2023  below the sensing diaphragm  2021  surrounded be sidewalls  2022 . The sensor  2020  also includes a second substrate  2030 , and auxiliary electronics  2024 . The second substrate  2030  includes an opening  2032  below the cavity  2023 . The second substrate  2030  further includes multiple channels  2031   a  - 203  Id, each of which has an inlet at the outer edge of the second substrate  2030  and an outlet connected with the cavity  2023  below the sensing diaphragm  2021 . 
     In addition, the device  2000  includes a first opening  2014  in the cover  2012  above the sensing diaphragm  2021 , and multiple second openings  2013  at the edge of the cover  2012  where the cover  2012  adjoins to the first substrate  2011 . 
     As shown, the anechoic chip  2040  is mounted on top of the cover  2012 , and includes an opening  2043  over the opening  2014  in the cover  2012 . In one example, the anechoic chip  2040  is coated with a porous layer  2041  for deadening sound. The porous layer  2041  can include minute cavities or holes  2042  on its surface for trapping incoming sound. The structure of the minute spaces or holes can be various in various examples. For example, the minute cavities  2042  can be formed by rectangular studs densely arranged in orthogonal directions, densely arranged cones, or combination of different structures. 
     It is noted that mounting anechoic chips is optional when packaging a unidirectional microphone. In addition, an anechoic chip can be mounted beneath the surface of top side of a cover where the top surface has an opening to hold and expose the anechoic chip. 
       FIG. 21  shows an example directional microphone device  2100  with LRAPDs according to some examples. The device  2100  has similar structure as the device  1700  in  FIG. 17 . However, multiple LRAPDs are employed to protect the sensor  1716  from water droplet damaging. Specifically, a LRAPD  2110  is disposed proximate to the first opening  1720 , while multiple LRAPDs  2120   a / 2120   b  are disposed proximate to the second openings  1708   a / 1708   b.  In one example, these LRAPDs  2110 / 2120   a / 2120   b  are configured to allow sound pressures to reach the sensing diaphragm without dampening the corresponding sound waves. Thus, short and broad channels may be employed in these LRAPDs  2110 / 2120   a / 2120   b.    
     As describes, aspects of the disclosure provide a waterproof packaging technique which can be used for fabricating waterproof microphones in mobile devices. The waterproof packaging technique employs a liquid-resistant air inlet passive device (LRAPD) which can include a liquid-repellant channel and can be attached to an opening in a housing enclosing an acoustic pressure sensor. In one example, the inner surface of the LRAPD is coated with a self-assembled monolayer (SAM) to realize the waterproof function. 
     A device based on the waterproof packaging technique can include a microelectromechanical system (MEMS) device, a housing enclosing the MEMS device, and a liquid-resistant air inlet passive device (LRAPD) on the housing. The LRAPD can include at least one channel connecting an exterior of the housing with a chamber formed between the housing and the MEMS device. An inside surface of the channel can be coated with a liquid-repellant coating. In some examples, the liquid-repellant coating can be a self-assembled monolayer (SAM) coating. The LRAPD can be attached to an inner side of the housing with an inlet of the channel connected to an opening in the housing, or attached to an outer side of the housing with an outlet of the channel connected to an opening in the housing. Alternatively, the LRAPD can be disposed in an opening of the housing. 
     In one example, the LRAPD includes multiple channels connecting an exterior of the housing with the chamber, and surfaces of the multiple channels are coated with a liquid-repellant coating. In some examples, the housing can include a cover over a substrate supporting the MEMS device, and the LRAPD can be disposed on the substrate. 
     The MEMS device can be a pressure sensor, such as a piezoresistive pressure sensor, a capacitive pressure sensor, and the like in various examples. In one example, the MEMS device is an acoustic pressure sensor with a sensing surface facing the chamber for sensing an acoustic wave, and the LRAPD is formed in a direction of the sensing surface to allow the acoustic wave to reach the sensing surface without dampening the acoustic wave. In another example, a surface of a diaphragm opposite the sensing surface faces the chamber, and the LRAPD is configured to provide an air pressure in the chamber that is equal to atmospheric pressure. Accordingly, in one example, the LRAPD includes a zigzag channel. 
     In a further example, the acoustic pressure sensor includes a cavity between the sensing surface and a housing with an opening in the housing connecting the cavity with the exterior of the housing, and the LRAPD covers the opening. 
     In one example, the LRAPD includes a zigzag channel. In another example, the LRAPD includes a cavity proximate the channel to collect liquid. In a further example, the MEMS device is an acoustic pressure sensor with a sensing surface facing the chamber, and the channel of the LRAPD includes a portion sloping from one end to the other end with respect to the sensing surface to allow an acoustic wave to reach the sensing surface. In one example, the MEMS device is an acoustic pressure sensor with a sensing surface facing the chamber, and the channel of the LRAPD includes a longest portion running parallel from a first end to a second end with respect to the sensing surface to allow an acoustic wave to reach the sensing surface. 
     As described, aspects of the disclosure provide another packaging technique for making a directional microphone. The packaging technique employs mechanical structures to cancel undesired background noise to realize directional picking up functions instead of requiring an extra sensor in electronic noise-cancelling techniques. Accordingly, the packaging technique enables a directional microphone with reduced a footprint and cost. 
     A directional microphone device based on the packaging technique can include an acoustic sensor and a housing enclosing the acoustic sensor. The acoustic sensor can include a sensing diaphragm for sensing sound pressure, a cavity below the sensing diaphragm, and a first substrate. The directional microphone device can further includes a channel with an inlet open at an edge of the first substrate and an outlet connected with the cavity. The housing can include a cover attached to a second substrate supporting the first substrate. The cover can include a first opening over the sensing diaphragm and a second opening at a side of the cover. The second opening can be disposed adjacent to the inlet of the channel. 
     In some examples, a first distance of a first path from the second opening to the sensing diaphragm via the channel is configured to be equal to a second distance of a second path from the second opening to the sensing diaphragm via a chamber between the cover and the acoustic sensor. 
     In some examples, the directional microphone device includes multiple channels each having an inlet open at the edge of the first substrate and an outlet connected with the cavity. The multiple channels can extend from the cavity to the edge of the first substrate, and can be evenly distributed from each other. In some examples, the cover includes multiple second openings at sides of the cover. The multiple second openings can be evenly distributed along the edge of the cover. In addition, in some examples, the multiple second openings are positioned adjacent to respective inlets of the multiple channels. 
     The acoustic sensor can be fabricated with MEMS technology. The acoustic sensor can be a capacitive pressure sensor, or a piezoresistive pressure sensor, and the like. In some examples, the microphone device can include an anechoic chip disposed over the cover and configured to absorb sound waves reaching the anechoic chip. 
     In one example, the first substrate is bonded to the second substrate. In another example, the channel is formed between the first substrate and the second substrate. In a further example, the first substrate and the second substrate are a same substrate made from a silicon wafer. 
     In one example, the second substrate further includes a barrier wall disposed outside the housing at an edge of the second substrate and adjacent to the second opening outside the housing. In one example, the barrier wall is configured to block sound waves inside the housing from leaving the housing, and to block sound waves outside the housing from entering the housing. 
     In one example, the acoustic sensor includes sidewalls attached to the sensing diaphragm and the first substrate to form the cavity. In another example, the first substrate includes an opening below the cavity. In a further example, the sensing diaphragm is attached to the first substrate, and the cavity is positioned within the first substrate. 
     While aspects of the present disclosure have been described in conjunction with the specific embodiments thereof that are proposed as examples, alternatives, modifications, and variations to the examples may be made. Accordingly, embodiments as set forth herein are intended to be illustrative and not limiting. There are changes that may be made without departing from the scope of the claims set forth below.