Abstract:
A micro-electrical-mechanical system (MEMS) transducer comprises a layer of dielectric material having an electrode formed in the layer of dielectric material. A region of the layer of the dielectric material is adapted to provide a leakage path which, in use, removes unwanted charge from the layer of dielectric material.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a MEMS device and process, and in particular to a MEMS device and process relating to a transducer, and in particular a capacitive transducer such as a MEMS capacitive microphone. 
       BACKGROUND OF THE INVENTION 
       [0002]    Consumer electronics devices are continually getting smaller and, with advances in technology, are gaining ever increasing performance and functionality. This is clearly evident in the technology used in consumer electronic products such as, for example, mobile phones, laptop computers, MP3 players and personal digital assistants (PDAs). Requirements of the mobile phone industry, for example, are driving components to become smaller with higher functionality and reduced cost. For example, some mobile phones now require multiple microphones for noise cancelling, or accelerometers to allow inertial navigation, while maintaining or reducing the small form factor and aiming at a similar total cost to previous generation phones. 
         [0003]    This has encouraged the emergence of miniature transducers. For example, in respect to speech applications, initially electret microphones were used to capture speech, but more recently micro-electrical-mechanical (MEMS) transducers have been introduced. MEMS transducers may be used in a variety of applications including, but not limited to, pressure sensing, ultrasonic scanning, acceleration monitoring and signal generation. Traditionally such MEMS transducers are capacitive transducers some of which comprise one or more membranes with electrodes for read-out/drive deposited on the membranes and/or a substrate. Relative movement of these electrodes modulates the capacitance between them, which then has to be detected by associated electronic circuitry such as sensitive electronic amplifiers. 
         [0004]      FIG. 1  shows a schematic diagram of a capacitive microphone device. The capacitive microphone device comprises a flexible membrane  11  that is free to move in response to pressure differences generated by sound waves. A first electrode  13  is mechanically coupled to the flexible membrane  11 , and together they form a first capacitive plate of the capacitive microphone device. A second electrode  15  is mechanically coupled to a generally rigid structural layer or back-plate  17 , which together form a second  2  capacitive plate of the capacitive microphone device. The membrane  11  and back-plate  17  are formed from a dielectric material, for example silicon nitride. 
         [0005]    The flexible membrane  11  is free to move in response to a stimulus, for example sound or pressure waves. This movement of the flexible membrane causes the capacitance between the first and second electrodes  13 ,  15  to change. In use the capacitive microphone device is typically arranged to drive into a high impedance circuit, such that the charge on the capacitor does not change significantly. Therefore, the change in the capacitance of the transducer due to the stimulus results in a change 11V in the voltage across the transducer capacitance. This change in voltage can be detected and processed to provide an electrical output signal. 
         [0006]      FIG. 2  shows the membrane  11  and back-plate  17  of  FIG. 1  in greater detail, together with their respective electrodes  13  and  15 . The first and second electrodes  13 , are biased by a voltage source  19 , for example a charge pump providing a bias voltage of 12v. 
         [0007]    As can be seen from  FIG. 2 , the second electrode  15  is formed within the back-plate  17  such that it is surrounded by dielectric material. The electrode is encapsulated within the layer of dielectric material, i.e. the back-plate  17 , for a number of reasons. 
         [0008]    Firstly, by forming the electrode 15  within the back-plate  17  the dielectric material acts to protect the electrode  15  from harmful environmental effects. For example, the dielectric material prevents the metal electrode  15  from corroding or oxidising. 
         [0009]    Secondly, by forming the electrode  15  within the back-plate  17  the fabrication process is improved. This is because it is easier to deposit a metal electrode onto silicon nitride than it is to deposit a metal electrode onto polyimide (polyimide being typically used as a sacrificial layer during the fabrication process for forming the air gap that exists between the back-plate  17  and the membrane  11 ). 
         [0010]    Thirdly, encapsulating the electrode  15  within the back-plate  17  has the effect of lowering the stiction forces that exist when the membrane  11  is deflected towards the back-plate  17 . 
         [0011]    A disadvantage of the arrangement shown in  FIG. 2  is that the dielectric layer under the electrode  15  of the back-plate  17  is susceptible to charging, especially for a dielectric layer comprising silicon nitride. This is particularly the case when the resistance of the air gap between the first and second electrodes  13 ,  15  is reduced, for example in a humid atmosphere. This causes ions to drift through the air gap and accumulate onto the lower surface of the back-plate  17 , causing a charge accumulation on the lower surface of the dielectric layer. As can be seen from  FIG. 3 , a positive charge accumulates on the upper surface of the second electrode  15 , while a negative charge accumulates on the lower surface of the first electrode  13 . 
         [0012]    The charge on the silicon nitride layer can build up to the maximum of the applied voltage (e.g. 12V). This has the disadvantage of reducing the sensitivity of the MEMS device. 
         [0013]    In the example shown in  FIG. 3  a 2v charge in the silicon nitride layer (i.e. between the underside of the second electrode  15  and the lower surface of the back-plate  17 ) leads to a sensitivity loss of: 
         [0000]      20 log(10/12)=1.6 dB 
         [0014]    Similarly, a 6V charge in the silicon nitride layer gives an attenuation of 6 dB, and a 9V charge an attenuation of 12 dB. 
         [0015]    Any charge accumulated in the silicon nitride layer has a very long decay time, possibly days or weeks, which is obviously much longer than the response time of the MEMS device. 
         [0016]      FIG. 4  is a graph illustrating the sensitivity reduction on two microphones after temperature humidity bias testing (at 85 C./85° A). 
       SUMMARY OF THE INVENTION 
       [0017]    It is an aim of the present invention to provide a MEMS device, and a method of fabricating a MEMS device, that reduces or removes the undesired effect mentioned above. 
         [0018]    According to a further aspect of the invention, there is provided a micro-electrical-mechanical system (MEMS) transducer comprising: a layer of dielectric material; an electrode formed in the layer of dielectric material; wherein a region of the layer of the dielectric material is adapted to provide a leakage path which, in use, removes unwanted charge from the layer of dielectric material. 
         [0019]    According to a first aspect of the invention, there is provided a method of forming a micro-electrical-mechanical system (MEMS) transducer, the method comprising the steps of: depositing a first layer of dielectric material; depositing an electrode; depositing a second layer of dielectric material; wherein the step of depositing the first layer of dielectric material comprises forming a leakage path which, in use, removes unwanted charge from the first layer of dielectric material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    For a better understanding of the invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings in which: 
           [0021]      FIG. 1  is schematic cross-sectional view of a MEMS microphone according to the prior art; 
           [0022]      FIG. 2  shows the back-plate, membrane and associated electrodes of  FIG. 1  in greater detail; 
           [0023]      FIG. 3  illustrates the accumulation of charge in the device of  FIGS. 1 and 2 ; 
           [0024]      FIG. 4  is a graph illustrating the sensitivity reduction on two microphones after temperature humidity bias testing; 
           [0025]      FIG. 5  shows a MEMS device according to an embodiment of the invention; 
           [0026]      FIG. 6  shows the electrical properties of silicon nitride, and a Fowler-Nordheim plot; 
           [0027]      FIG. 7  illustrates measured conductivities through 50 nm of silicon nitride using different materials; 
           [0028]      FIG. 8  illustrates the charge decay of silicon nitride having helium depletion; 
           [0029]      FIG. 9  illustrates the charge decay of silicon nitride having silicon enrichment; and 
           [0030]      FIG. 10  shows a further embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0031]    The invention will be described in relation to a MEMS transducer in the form of a MEMS capacitive microphone. It will be appreciated, however, that the invention is equally applicable to any form of capacitive transducer including, but not limited to pressure sensors, ultrasonic transducers, acceleration monitoring and signal generation transducers. 
         [0032]    Furthermore, although the invention will be described in relation to a back-plate of a MEMS device, it will be appreciated that the invention is equally applicable to any other dielectric layer of a MEMS device having an associated electrode, for example a membrane having a layer of dielectric material between its electrode and the other electrode of the capacitive transducer, for example if the electrode  13  was provided on the underside of the membrane  11 , or encapsulated therein. 
         [0033]    It is also noted that, although the embodiments of the invention are described in relation to a back-plate formed from silicon nitride, it will be appreciated that the invention is applicable with other dielectric materials. 
         [0034]    According to the invention a leakage path is provided in a region of the back-plate for preventing or removing any unwanted charge from accumulating in the back-plate. 
         [0035]    Referring to  FIG. 5 , according to one embodiment the thickness of the back-plate is reduced on the underside of the electrode  15 , Le. in a region  17   a  of the back-plate. It will be appreciated that the term “underside” refers to the side of the electrode which faces the other electrode of the capacitive transducer, i.e. the side that faces electrode  13 . Reducing the thickness of the back-plate  17  in this region  17   a  has the effect of lowering the impedance of the dielectric layer between the electrodes  15 ,  13 . In other words, the impedance of the dielectric layer under the electrode  15  is reduced in a plane that lies along an axis X-X, this axis X-X being an axis that runs in the general direction from one electrode  15  to another electrode  13 . 
         [0036]    It is noted that the back-plate in a MEMS device such as that shown in  FIG. 5  may be formed by depositing a first layer of silicon nitride, depositing the electrode  15  on a portion of the first layer of silicon nitride, and depositing a second layer of silicon nitride over the electrode  15  and first layer of silicon nitride, thereby encapsulating the electrode with silicon nitride to form the back-plate  17 . The first layer of silicon nitride may be deposited over a sacrificial layer (not shown), which is deposited during the fabrication process in order to support the formation of the back-plate  17 , but then removed to provide the air gap between the back-plate  17  and the membrane  11 . Therefore, the layer on the underside of the back-plate  17  can be reduced in thickness by depositing the first layer with reduced thickness compared to the second layer. 
         [0037]    According to one embodiment, the thickness of the dielectric layer in the region  17   a  is reduced such that the dielectric layer behaves in a tunneling mode of operation, such that when the capacitive transducer is biased during use any unwanted charge stored in the region  17   a  of the back-plate is removed. 
         [0038]      FIG. 6  shows a Fowler-Nordheim plot illustrating the electrical properties of silicon nitride, including an ohmic region and a tunneling region. The silicon nitride layer on the underside of the electrode  15  is deposited such that the silicon nitride layer is adapted to operate in the tunneling mode. 
         [0039]    This enables a current to flow through the silicon nitride layer which discharges any accumulated charge, thus making the air gap the key insulator in the capacitive circuit. 
         [0040]    It will be appreciated that reducing the thickness of the back-plate on the underside of the electrode to a bare minimum thickness can have an undesired effect of reducing the insulating effect of the dielectric of the capacitive microphone device, which could lead to device collapse and short circuit. 
         [0041]    Therefore, according to another embodiment, one or more parameters of the silicon nitride layer and/or one or more parameters associated with the deposition of the silicon nitride layer are altered to provide a region that has improved charge removal properties for a given thickness of silicon nitride. Preferably this is done in addition to reducing the thickness as mentioned above. However, it is noted that the reduction in the thickness of the silicon nitride can be altered in relation to what other parameters are changed in the composition of the silicon nitride, or its deposition, thus enabling the thickness to be reduced by less than would otherwise be needed. 
         [0042]    The parameters are selected such that the dielectric layer underneath the electrode has a first resistance in a first direction X-X (i.e. between the surface of the electrode and the corresponding outer surface of the back-plate), and a second resistance in a second direction Y-Y (Le. along the body of the silicon nitride layer). A very high sheet resistance is maintained in the nitride layer in direction Y-Y in order to prevent undesirable leakage currents from the 12V charge pump input, (which would otherwise cause noise at the microphone output). 
         [0043]    A key parameter is controlling the layer thickness and separation of the conductive features such that the nitride layer appears to be resistive through its thickness, but is a near perfect insulator over the sheet resistance. 
         [0044]      FIG. 7  shows current conduction through 50 mm thick nitride layers of different compositions. An ohmic conductive region can be seen for all the materials where the current is greater than 1e-5A. The material should be controlled so that a measure of conductivity is always present through the sheet (for example, material 4C1 in  FIG. 7 ), but where the conductivity across the sheet is minimised through aspect ratio control and material choice. 
         [0045]    Silicon Nitrides with different deposition properties have been measured for charge decay using microphone die with 300 nm nitride below the back-plate electrode. Standard silicon nitride has been found to have very long charge decay times extending to weeks or months. Silicon nitrides deposited using helium depletion have been seen to decay to 2.5V in about 100 hours, as shown in  FIG. 8 . Silicon nitrides deposited which are 1.3% silicon rich have seen charge decay to 2.5V in about 20 minutes, as shown in  FIG. 9 . 
         [0046]    The electrodes  15 ,  13  are preferably placed in a central region of the back-plate and membrane, i.e. rather than over the entire surface of the membrane or back-plate, thus having the advantage of maximising the leakage path via the sidewalls of the device. 
         [0047]    In the embodiments described above the sound ports are shown passing through the dielectric material and the electrode. It will be appreciated, however, that the invention is equally applicable to a device where the sound ports do not pass through the electrode itself. 
         [0048]    In the embodiment of  FIG. 5 , i.e. having sound ports passing through the electrode, it can be seen that the electrode is encapsulated within the dielectric material. In other words, the edges of the electrode  15  are not exposed on the inner surfaces of the sound ports. This is achieved by etching holes having a first diameter in the electrode  15  prior to the dielectric layer being formed over the electrode  15 , such that when the smaller diameter sound ports are subsequently etched through the back-plate, the electrode  15  is not exposed on any of the inner surfaces of the sound ports. 
         [0049]    However, according to another embodiment of the invention, the electrode is not etched prior to forming the layer of dielectric over the electrode. Thus, when the sound ports are etched through the dielectric material and the electrode, the electrode will be exposed on the inner surface of the sound ports. Although this has the disadvantage of not providing an environmental barrier, it has the advantage of providing a surface leakage path  101  between the edge of the exposed electrode and the lower surface of the back-plate. 
         [0050]    According to another embodiment, the device can be configured such that certain sound ports have the electrode exposed, while other sound ports have the sound port encapsulated. This has the advantage of enabling the effect of the leakage path to be controlled more accurately. The variation in the number of sound ports having the electrode exposed can be determined during the step of etching holes in the electrode, i.e. prior to the top layer of the back-plate being formed. For example, some holes in the electrode can be configured to have diameters that are larger than the sound ports (such that the electrode does not become exposed when the sound ports are subsequently etched), while other holes in the electrode can be etched to have diameters that match the diameters of the sound ports (or not etched at all). 
         [0051]    It is noted that the invention may be used in a number of applications. These include, but are not limited to, consumer applications, medical applications, industrial applications and automotive applications. For example, typical consumer applications include laptops, mobile phones, PDAs and personal computers. Typical medical applications include hearing aids. Typical industrial applications include active noise cancellation. Typical automotive applications include hands-free sets, acoustic crash sensors and active noise cancellation. 
         [0052]    It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single feature or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope.