Patent Publication Number: US-2011062535-A1

Title: Mems transducers

Description:
The present invention relates to transducers, and in particular to microelectromechanical systems (MEMS) ultrasonic transducers. 
     BACKGROUND 
     Volumetric ultrasound imaging, whereby a full set of data of all points in 3D space is acquired, is driven by next generation requirements to obtain and retrieve the complete information set in one operation and have it available for later review and analysis. These requirements are driven by various market segments, including military (sonar), industrial (non-destructive testing), automotive (collision avoidance) and medical (non-invasive imaging) markets. 
     In addition to the market drivers and need, there are clear technical issues fuelling developments. Real-time ultrasonic volumetric imaging has only now become a possibility due to increased digital processing power, which allows for real-time data analysis of a large number of parallel signals. However, this requires high-density 2D ultrasonic transducer arrays to provide sufficient spatial resolution in, for example, medical applications. Also, these high-density matrix configurations can allow electronic beam-steering to scan fast and accurately through a complete volume. To facilitate the huge amounts of data transfer to and from the 2D array, it is essential that pre and post data processing take place as close to the 2D array as possible. This is extremely difficult to achieve with current piezo crystal transducers. 
     There are also applications for lower-density concentrations of ultrasound transducers. For example, one area of development is that of gesture recognition in devices employing just a few transducers. Such transducers may transmit ultrasound waves and detect the reflected waves from a nearby user. The detected reflected waves may be processed to determine a gesture performed by, for example, the hand of the user, which is thereby used to control the device itself. This may comprise an application where the transducer is encapsulated. 
     Semiconductor technology is ideally suited to meet the requirements for volumetric imaging, as semiconductor fabrication techniques allow for relatively large array sizes in optimised configurations and also allow for the monolithic integration of the transducers with the processing electronics relatively close to the array. This is in contrast to the piezo crystal technology which is currently used for manufacturing of ultrasound probes. These are mechanically machined from bulk material in a sequential manufacturing process and require wire bonding of all individual pixels. Further, the frequency response of these piezo elements is not optimal for high frequency, mixed frequency and high bandwidth operation, which limits their use for some emerging advanced applications of ultrasound arrays. 
     Microelectromechanical systems (MEMS) ultrasound transducers are a new approach to ultrasound sensors. They are constructed using silicon micromachining technology which enables a plurality of small membranes in the order of microns in size suspended above submicron gaps to be constructed with greater accuracy than ever before. 
     There has been much interest and activity in this area from the academic and business communities, and consequently a number of manufacturing processes have been developed to produce MEMS ultrasonic transducers. The predominant method is the sacrificial release process. Although many variations of this process have been published they are all based an the same principle: a cavity or air-space is created below a suspended flexible membrane by growing/depositing a sacrificial layer and depositing the membrane over the sacrificial layer; the sacrificial layer is then removed, freeing the membrane and allowing it to move. 
       FIG. 1  shows this known manufacturing process. 
       FIG. 1   a  shows a substrate  10 , and an insulating layer  12  above the substrate  10 . In the first step of the process, an electrode  14  is deposited on the insulating layer  12 . 
     A portion  16  of sacrificial material is then deposited over the electrode ( FIG. 1   b ). An example of a suitable sacrificial material is polyimide. One method of depositing the sacrificial portion  16  in the required shape and location is to first deposit a layer of sacrificial material over the insulating layer  12 . The sacrificial layer is then cured at an elevated temperature, and patterned with photoresist. The final sacrificial portion  16  is achieved by etching with an anisotropic oxygen plasma. 
     A membrane layer  18  is then deposited over the insulating material  12  and the sacrificial portion  16  ( FIG. 1   c ). A suitable material for the membrane is silicon nitride. A second electrode  20  is deposited on the membrane layer  18  above the sacrificial portion  16  ( FIG. 1   d ). Release holes  22  are etched through the second electrode  20  and the membrane layer  18 , ( FIG. 1   e ). Finally, the sacrificial portion  16  is etched away in a wet-etch process, for example, the release holes  22  allowing etchant to access the sacrificial material beneath, and the etched material to flow out of the transducer. The membrane is therefore free to move relative to the substrate ( FIG. 1   f ). 
     In operation, the transducer may be used to generate pressure waves (e.g. acoustic or ultrasonic signals) by applying a potential difference between the two electrodes  14 ,  20 . The potential difference causes the membrane to displace, and thus a modulated potential difference can be used to generate waves of variable frequency. 
     Alternatively, the transducer can also be used to detect such pressure waves. An incoming wave will cause the membrane to displace, and the variation in capacitance this causes between the two electrodes  14 ,  20  can be measured to determine the frequency and amplitude of the incoming wave. 
     A paper by Ergun et al entitled (“Capacitive Micromachined Ultrasonic Transducers: Fabrication Technology”,  IEEE Trans. Ultra. Ferro. Freq. Control,  pp 2242-58, December 2005) describes the fabrication of a 2D array of ultrasonic transducers. However, a goal of this research is to produce an array of transducers which are as uniform as possible in shape, dimensions, etc. 
     SUMMARY OF INVENTION 
     According to a first aspect of the present invention, there is provided a microelectromechanical systems (MEMS) device, comprising: a substrate; and a plurality of transducers positioned on the substrate, said plurality of transducers comprising: at least a first transducer adapted to transmit pressure waves; and at least a second transducer adapted to detect pressure waves. 
     In one embodiment at least one of said first and second transducers comprises a cavity, said cavity being sealed from the outside of the transducer. 
     According to a second aspect of the present invention, there is provided a method of manufacturing a microelectromechanical systems (MEMS) device, said MEMS device comprising a substrate, said substrate having at least a first site for a first transducer adapted to transmit pressure waves and at least a second site for a second transducer adapted to detect pressure waves, said method comprising: forming said first transducer on said first site, and said second transducer on said second site. 
     According to a further aspect of the invention, there is provided a microelectromechanical systems (MEMS) device, comprising: a substrate; and a plurality of transducers positioned on the substrate, said plurality of transducers comprising: at least a first transducer adapted to transmit or detect pressure waves having a first frequency; and at least a second transducer adapted to transmit or detect pressure waves having a second frequency, wherein said first frequency is different from said second frequency. 
     In one embodiment at least one of said first and second transducers comprises a cavity, said cavity being sealed from the outside of the transducer. 
     According to a further aspect of the invention, there is provided a method of manufacturing a microelectromechanical systems (MEMS) device, said MEMS device comprising a substrate, said substrate having at least a first site for a first transducer adapted to transmit or detect pressure waves having a first frequency and at least a second site for a second transducer adapted to transmit or detect pressure waves having a second frequency, said first frequency being different from said second frequency, said method comprising: forming said first transducer on said first site, and said second transducer on said second site. 
     According to a further aspect of the present invention, there is provided a method of manufacturing a microelectromechanical systems (MEMS) device, said MEMS device comprising a substrate, said substrate having at least a first site for a first transducer adapted to transmit or detect pressure waves, said method comprising: depositing a first portion of sacrificial material on said first site, depositing a first membrane layer over at least the first site, forming a release channel prior to the step of depositing the first portion of sacrificial material; etching away the first portion of sacrificial material via the release channel; and sealing the release channel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the following drawings, in which: 
         FIGS. 1   a  to  1   f  show a known process of manufacturing a MEMS transducer; 
         FIG. 2  is a graph comparing the frequency response of a membrane with a relatively high Q factor and a membrane with a relatively low Q factor; 
         FIG. 3  is a graph modelling the variation of the first resonant frequency of a transducer with membrane thickness; 
         FIG. 4  shows a 2D array according to the present invention; 
         FIGS. 5   a  and  5   b  both show a transducer adapted to transmit pressure waves and a transducer adapted to detect pressure waves according to aspects of the present invention; 
         FIGS. 6   a  and  6   b  both show a transducer adapted to transmit pressure waves and a transducer adapted to detect pressure waves according to other aspects of the present invention; 
         FIGS. 7   a  and  7   b  both show a transducer adapted to transmit pressure waves and a transducer adapted to detect pressure waves according to further aspects of the present invention; 
         FIGS. 8   a  to  8   k  show a process for manufacturing a MEMS device according to the present invention; and 
         FIGS. 9   a - 9   p  show alternative processes for manufacturing a MEMS device according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The inventors of the present invention found that it is possible to adapt MEMS transducers specifically to either transmit, or detect, pressure waves. In particular, it was found that, by varying various dimensions and parameters associated with the transducer, the Q factor of the transducer could be changed. A transducer with a relatively high Q factor is better suited to transmitting pressure waves, as it has a high response over a relatively narrow range of frequencies (i.e. it transmits pressure waves having a relatively well-defined frequency and high amplitude). Conversely, a transducer with a relatively low Q factor is better suited to detecting pressure waves, as it has a less strong, but more consistent, response over a relatively broad range of frequencies (i.e. it can detect incoming pressure waves which may have a broader range of frequencies). 
     Some of the various embodiments of the invention described below relate to a MEMS device that is sealed or closed from environmental parameters. By sealed it is meant that the transducer comprises at least one internal cavity that is closed from the outside. 
     It is noted that the sealed aspect of the invention is described in relation to embodiments comprising a plurality of transducers. However, it is noted that the sealed aspect of the invention also applies to just a single transducer. 
       FIG. 2  is a graph comparing the frequency response of a membrane with a relatively high Q factor and a membrane with a relatively low Q factor. As can be seen, the membrane with the relatively high Q factor has a high response over a narrow range of frequencies, in the illustrated example, around a central frequency of approximately 370 kHz; the response of this membrane at frequencies away from the central frequency is comparatively low. The membrane with the relatively low Q factor has the same central frequency of 370 kHz; the membrane&#39;s response at this central frequency is lower, but at frequencies away from the central frequency, the response is higher than the membrane with the high Q factor. That is, the response of the membrane with the low Q factor is relatively more consistent than that of the membrane with the high Q factor over a larger range of frequencies. 
     In  FIG. 2 , the two membranes have the same central, i.e. resonant, frequency. This can be achieved by appropriately adjusting parameters and dimensions of the transducer as described in more detail below. Further, however, there are advantages in forming transducers with differing resonant frequencies, and this will also be described in greater detail below. 
     One dimension that affects the performance of the transducer is the thickness of the membrane.  FIG. 3  is a graph modelling the variation of the first resonant frequency of a transducer with membrane thickness when all other dimensions and parameters are kept constant. In the illustrated example, the membrane diameter is 500 μm. It will be appreciated that corresponding models will apply to different membrane diameters, and are intended fall within the scope of the present invention. 
     As can be seen, the variation is a curve such that there are two solutions for each particular first resonant frequency. In the example shown, for a resonant frequency of approximately 240 kHz, membrane thicknesses of 0.2 and 1.2 μm are appropriate. Furthermore, a thicker membrane leads to a higher Q factor. Thus, a 0.2 μm thick membrane is suitable for detecting pressure waves at or around 240 kHz, and a 1.2 μm thick membrane is suitable for transmitting pressure waves at or close to 240 kHz. 
       FIG. 4  shows a 2D array  30  of MEMS transducers  34  according to an embodiment of the present invention. 
     The array  30  comprises a plurality of non-identical sub-arrays  32 . Each sub-array  32  comprises a plurality of MEMS transducers  34 , for example as described above with respect to  FIG. 1 . According to the present invention, however, some of the sub-arrays  32   a  (unshaded elements in  FIG. 4 ) comprise MEMS transducers specifically adapted to detect pressure waves. Others of the sub-arrays  32   b  (shaded elements in  FIG. 4 ), interleaved with the “detecting” sub-arrays  32   a,  comprise MEMS transducers specifically adapted to transmit pressure waves. 
     In this application, “pressure waves” are any waves generated by oscillation of the membrane of the MEMS transducers, regardless of the frequency of those oscillations. Therefore, the term includes ultrasonic waves, as well as lower frequency, acoustic waves. 
     Thus, the individual MEMS transducers  34  in the plurality of sub-arrays  32   a  adapted to detect pressure waves may have a relatively low Q factor; the individual MEMS transducers  34  in the plurality of sub-arrays  32   b  adapted to transmit pressure waves may have a relatively high Q factor. 
     Of course, it will be apparent to those skilled in the art that the embodiment illustrated in  FIG. 4  is just one possible arrangement, and that alternative arrangements of transducers are possible within the scope of the invention. In particular, sub-arrays  32  may take any shape. However, hexagonal sub-arrays  32  are advantageous because they minimize the amount of wasted space on a given substrate. Further, each sub-array  32  may not comprise exclusively transmitting or detecting transducers; rather, each sub-array  32  may comprise both transmitting and detecting transducers. In an alternative embodiment, the individual transducers  34  may not be arranged in sub-arrays as described, but in a single array. 
     In a still further embodiment, rather than a first plurality of substantially identical transducers for transmitting pressure waves, and a second plurality of substantially identical transducers for detecting pressure waves, a plurality of transducers may be provided having a range of transmitting or detecting properties. That is, a plurality of transducers may be provided for transmitting pressure waves, each transducer having different dimensions, Q factor, etc, such that each transducer primarily transmits at a particular, different, resonant frequency. Similarly, a plurality of transducers may be provided for detecting pressure waves, each transducer having different dimensions, Q factor, etc, such that each transducer primarily detects a particular, different, resonant frequency. 
     A MEMS device comprising transmitting and detecting transducers having a range of resonant frequencies is far more sensitive to different frequencies, and is capable of transmitting over a broader range of frequencies. 
     As previously mentioned, various dimensions, parameters, etc, may be modified in order to adapt the transducer for either transmitting or detecting pressure waves, or for adjusting the resonant frequency of the transducer. In the description of various embodiments hereinafter, references to two transducers respectively adapted to transmit and to detect pressure waves will be taken to further include two transducers adapted to transmit or to detect pressure waves at different respective frequencies. 
       FIG. 5   a  illustrates a MEMS device  40  according to one embodiment of the present invention. 
     The MEMS device  40  comprises a first transducer  42  optimized for transmitting pressure waves, having a diameter DM 1 , and a second transducer  44  optimized for detecting pressure waves, having a diameter DM 2 . It can be seen that the diameter DM 2  of the membrane of the second transducer  44  is greater than the diameter DM 1  of the first transducer  42 , meaning that it is more sensitive to incoming pressure waves, and therefore more suited to detecting pressure waves. The smaller diameter DM 1  of the membrane of the first transducer  42  means that it can generate pressure waves having greater amplitudes, i.e. it can generate a greater variation in pressure, and is therefore more suited to transmitting pressure waves. 
     The embodiment shown in  FIG. 5   b  is similar, and thus like numerals are used to indicate like components, but both transducers are sealed. The first transducer  42  comprises a first cavity  45 , and the second transducer  44  a second cavity  46 . The cavity  45  is formed by removal of sacrificial material via a release channel  47 , while the second cavity  46  is formed by removal of sacrificial material via a release channel  48 . The cavities  45 ,  46  are sealed after removal of the sacrificial material by plugging release holes  47   a  and  48   a,  respectively. 
       FIG. 6   a  illustrates a MEMS device  50  according to a further embodiment of the present invention and  FIG. 6   b  illustrates a sealed embodiment. 
     In each case the MEMS device  50  comprises a first transducer  52  optimized for transmitting pressure waves, and a second transducer  54  optimized for detecting pressure waves. The diameter DE 1  of the electrodes  53   a,    53   b  of the first transducer  52  are greater than the diameter DE 2  of the electrodes  55   a,    55   b  of the second transducer  54 . The force between the two electrodes  53   a,    53   b  is proportional to their area, so a greater area means that a greater force can be generated by the transducer  52 , making it more suitable for transmitting pressure waves because a higher amplitude can be attained. The smaller diameter of the electrodes  55   a,    55   b  of the second transducer  54  makes the membrane more flexible, and therefore more sensitive to incoming pressure waves. 
     In an alternative embodiment, the mass of the electrodes may be adjusted instead of altering their diameter. A transducer with an electrode having a relatively high mass is more suitable for transmitting pressure waves, as it can generate waves with relatively higher amplitude. Likewise, a transducer with an electrode having a relatively low mass is more suitable for detecting pressure waves as the membrane is more easily deflected by the incoming wave. This may be achieved by utilizing a heavier conductor as the material for the electrode, for example, or by making the electrodes thicker. 
     In the embodiment shown in  FIG. 6   b  the first transducer  52  comprises a first cavity  51 , and the second transducer  54  a second cavity  56 . The cavity  51  is formed by removal of sacrificial material via a release channel  57 , while the second cavity  56  is formed by removal of sacrificial material via a release channel  58 . The cavities  51 ,  56  are sealed after removal of the sacrificial material by plugging release holes  57   a  and  58   a,  respectively. 
       FIG. 7   a  illustrates a MEMS device  60  according to a yet further embodiment of the present invention. 
     The MEMS device  60  comprises a first transducer  62  optimized for transmitting pressure waves, having a first membrane thickness T 1 , and a second transducer  64  optimized for detecting pressure waves, having a second thickness T 2 . The membrane thickness T 2  of the second transducer  64  is less than the membrane thickness T 1  of the first transducer  62 , meaning that the second transducer  64  is more sensitive to incoming pressure waves, and therefore more suited to detecting pressure waves. The greater thickness of the membrane of the first transducer  62  means that it can generate pressure waves having greater amplitudes, i.e. it can generate a greater variation in pressure, and is therefore more suited to transmitting pressure waves. 
       FIG. 7   b  illustrates a similar embodiment having sealed cavities. The first transducer  62  comprises a first cavity  65 , and the second transducer  64  a second cavity  66 . The cavity  65  is formed by removal of sacrificial material via a release channel  67 , while the second cavity  66  is formed by removal of sacrificial material via a release channel  68 . The cavities  65 ,  66  are sealed after removal of the sacrificial material by plugging release holes  67   a  and  68   a,  respectively. 
       FIGS. 8   a  to  8   k  illustrate one method of manufacturing MEMS devices according to the present invention, and in particular the embodiment described with respect to  FIG. 7   a . However, the figures will also be used to describe a possible manufacturing process of other embodiments of the present invention. 
     It will be further appreciated by those skilled in the art that some of the steps of the illustrated method need not be performed in the order stated herein. However, as will also be apparent, some steps must be performed before or after others as may be, in order that the desired structure is generated. 
       FIG. 8   a  shows a starting point of the manufacturing process. A substrate  100  is provided, with an insulating layer  102  on top of the substrate. In this example, for compatibility with CMOS processing techniques the substrate  100  is a silicon wafer, but it will be appreciated that other substrate materials and electronic fabrication techniques could be used instead. The insulating layer  102  may be formed by thermal oxidation of the silicon wafer, forming an oxide layer, or by deposition of an insulating material using any one of numerous known techniques, such as plasma enhanced chemical vapour deposition (PECVD). 
     A base layer  104  of silicon nitride is then deposited on top of the insulating layer  102  ( FIG. 8   b ). The base layer  104  may be deposited using PECVD. However, it will be appreciated that other dielectric layers and/or processes may be used. For example, the layer might not be pure silica; borophosphosilicate glass (BPSG) may also be used. 
     Next, referring to  FIG. 8   c , electrodes  106 ,  108  are deposited at the sites of a transmitting transducer and a detecting transducer, respectively. The electrodes  106 ,  108  may be formed by sputtering or depositing a conducting material, for example aluminium, on the surface of the base layer  104 . In the present example, the electrodes  106 ,  108  are the same size and shape. However, when forming transducers  52 ,  54  as described with reference to  FIG. 6 , the size and/or shape of the electrodes  106 ,  108  may be varied at this stage. For example, the electrode  106  for the transmitting transducer may have a greater diameter, or a greater mass, than the electrode  108  for the detecting transducer. 
     Depositing the electrodes  106 ,  108  by sputtering is preferable to other methods such as thermal evaporation due to the low substrate temperatures used. This ensures compatibility with CMOS fabrication processes. In addition, where materials other than aluminium are deposited, this method benefits from the ability to accurately control the composition of the thin film that is deposited. Sputtering deposits material equally over all surfaces so the deposited thin film has to be patterned by resist application and dry etching with a Cl 2 /BCl 3  gas mix to define the shape of the electrodes  106 ,  108  as well as to define the interconnect points (not shown in the Figures) that allow interconnection to the circuit regions (i.e. either the underlying CMOS circuit or the off-chip circuits, neither illustrated). 
     Next, referring to  FIG. 8   d , sacrificial layers  110 ,  112  are deposited over the electrodes  106 ,  108 , respectively. To ensure compatibility with CMOS fabrication techniques, for example, the sacrificial layers  110 ,  112  can be made of a number of materials which can be removed using either a dry release or a wet release process. Using a dry release process is advantageous in that no additional process steps or drying are required after the sacrificial layer is released. Polyimide is preferable as the sacrificial layer as it can be spun onto the substrate easily and removed with an oxygen plasma clean. The polyimide coating is spun on the wafer to form a conformal coating, using parameters and techniques that will be familiar to those skilled in the art. A primer may be used for the polyimide layer. The polyimide layer is then patterned with photoresist and etched in an anisotropic oxygen plasma, thus leaving the sacrificial layers  110 ,  112  as shown in  FIG. 8   d . It will appreciated by a person skilled in the art that alternative methods of depositing the sacrificial layers  110 ,  112  may be used, for example applying and etching a photosensitive polyimide. 
     The sacrificial layers  110 ,  112  define the dimensions and shape of the cavities or spaces underneath the membranes that will be left when the sacrificial layers  110 ,  112  are removed as discussed below. 
     The sacrificial layers  110 ,  112  are provided for a number of reasons. These include supporting and protecting the membrane of the MEMS device during the manufacturing process. The sacrificial layers  110 ,  112  are also provided for defining the diameter of the membranes, such that the size of the membranes can be altered by altering the diameter of the sacrificial layers  110 ,  112 . In the present example, the sacrificial layers  110 ,  112  are substantially identical in shape and size. However, when manufacturing transducers  42 ,  44  as described with respect to  FIG. 5 , the sacrificial layers  110 ,  112  may have different diameters. In particular, the sacrificial layer  110  for the transmitting transducer may have a smaller diameter that the sacrificial layer  112  for the detecting transducer. 
     Next, referring to  FIG. 8   e , a membrane layer  114  is deposited over the base layer  104  and the sacrificial layers  110 ,  112 . The membrane layer  114  may be formed from silicon nitride deposited by PECVD, as before, although alternatively polysilicon may be used. In addition, titanium adhesive layers may be used between the aluminium and the silicon nitride. 
     Although not shown in  FIGS. 8   d  and  8   e , the upper surface of the sacrificial layers  110 ,  112  may be formed with one or more dimples (in the form of small cavities) in their outer area (i.e. near the periphery of the sacrificial layers  110 ,  112 ). As a result, the depositing of the membrane layer  114  causes one or more dimples (in the form of protrusions) to be formed in the outer area or periphery of the membrane. These dimples in the outer area of the membrane  114  reduce the contact area of the membrane with the underlying substrate in the event of overpressure or membrane pull-in, whereby the surface of the membrane comes into contact with another surface of the MEMS device. The dimples reduce the stiction forces such that they are below the restoring forces (i.e. the membrane tension), thereby allowing the membrane to release itself. 
     Next, referring to  FIG. 8   f , second electrodes  116 ,  118  are deposited substantially over the sacrificial layers  110 ,  112 , respectively. In general, for simplicity of the manufacturing process, the second electrodes  116 ,  118  have substantially the same size and shape as their respective counterpart electrodes  106 ,  108 ; however, this is not a strict requirement. For example, when manufacturing transducers  52 ,  54  such as described with respect to  FIG. 6 , the electrode  116  for the transmitting transducer  52  may have a greater mass and/or diameter and/or thickness than the electrode  118  for the detecting transducer  54 . 
     The second electrodes  116 ,  118  are deposited in substantially the same way as the first electrodes  106 ,  108 . 
     Next, referring to  FIG. 8   g , release holes  120  are etched through the electrode  116  and the membrane layer  114  to allow access to the sacrificial layer  110 , and release holes  122  are etched through the electrode  118  and the membrane layer  114  to allow access to the sacrificial layer  112 . In the illustrated embodiment, the release holes  120 ,  122  are formed through both the membrane layer  114  and the electrodes  116 ,  118 ; however, where the electrode diameter is less than the diameter of the membrane, for example, the release holes may be positioned substantially around the periphery of the membrane, such that they do not pass through the electrodes themselves. It will be appreciated that the formation of the release holes  120 ,  122  through the respective electrodes  116 ,  118  and membrane layer  114  may be formed in one process step or several process steps depending on the materials involved, and the etching process or processes used. 
     It is to be noted that, when manufacturing a MEMS device  60  as described with respect to  FIG. 7 , release holes  120  in the transducer for transmitting pressure waves are not necessary at this stage. 
     At this stage, the method for manufacture of MEMS devices  40 ,  50  is substantially complete (i.e. membranes with differing diameters, or differing electrode diameter or size). The sacrificial layers  110 ,  112  are preferably removed using a dry etch process, such as an oxygen plasma system, so that the membrane is free to move in both transducers. 
       FIGS. 8   h  to  8   k  describe the further steps of a method for manufacturing a MEMS device  60  as described with respect to  FIG. 7  (i.e. a device having transducers with differing membrane thickness). 
     With reference to  FIG. 8   h , a further sacrificial layer  124  is deposited over the electrode  118 , connecting with the sacrificial layer  112  through the release holes  122 . The further sacrificial layer  124  may again be formed from silicon nitride, or one of the alternative materials mentioned previously. Again, any one of the techniques previously mentioned may be used to deposit the sacrificial layer  124 . 
     Next, referring to  FIG. 8   i , a further membrane layer  126  is deposited over the first membrane layer  114 , the electrode  116 , and the further sacrificial layer  124 . In a preferred embodiment, the second membrane layer  126  is formed from the same material as the first membrane layer  114 , such that the two layers  114 ,  126  substantially bond together to form a single layer of material. The second membrane layer  126  may be formed from any of the alternatives for the first membrane layer  114 . 
     In  FIG. 8   j , release holes  128  are etched through the thickened membrane of the transmitting transducer (i.e. first and second membrane layers  114 ,  126 ). As before, the release holes  128  may pass through the electrode  116 , or around the periphery of the electrode  116 . 
     Further, the second membrane layer  126  is removed from above the sacrificial layer  124  in the detecting transducer, to create an opening  130  in the membrane layer  126 . 
     Finally, as shown in  FIG. 8   k , the completed device  60  is created by removing the sacrificial layers  110 ,  112 ,  124 . The sacrificial layers  110 ,  112 ,  124  are preferably removed using a dry etch process, such as an oxygen plasma system, so that the membrane is free to move in both transducers. 
     In the illustrated embodiment, the first and second membrane layers  114 ,  126  substantially encase the electrode  116  of the transmitting transducer. The formation of a sandwich structure has the advantage of reducing unwanted deformation in the membrane. In other words, if the electrode is placed between two layers of nitride, or vice versa, then the stress is more equalised, and results in the membrane moving with less unwanted deformation. However, it will be apparent to one skilled in the art that the deposition of the electrode  116  may take place at a later stage, such that the electrode  116  is positioned on top of the thickened membrane. 
       FIGS. 9   a - 9   p  illustrate a process for forming MEMS transducers according to the present invention having sealed cavities. The method may use several of the same steps and provide the same structures as describe above in relation to  FIGS. 8   a - 8   k  and therefore similar reference numerals will be used. 
       FIG. 9   a  shows a starting point of the manufacturing process. A substrate  100  is provided, with an insulating layer  102  on top of the substrate. In this example, for compatibility with CMOS processing techniques the substrate  100  is a silicon wafer, but it will be appreciated that other substrate materials and electronic fabrication techniques could be used instead. The insulating layer  102  may be formed by thermal oxidation of the silicon wafer, forming an oxide layer, or by deposition of an insulating material using any one of numerous known techniques, such as plasma enhanced chemical vapour deposition (PECVD). 
     A base layer  104  of silicon nitride is then deposited on top of the insulating layer  102  ( FIG. 9   b ). The base layer  104  may be deposited using PECVD. However, it will be appreciated that other dielectric layers and/or processes may be used. For example, the layer might not be pure silica; borophosphosilicate glass (BPSG) may also be used. 
     Next, referring to  FIG. 9   c , electrodes  106 ,  108  are deposited at the sites of a transmitting transducer and a detecting transducer, respectively. The electrodes  106 ,  108  may be formed by sputtering or depositing a conducting material, for example aluminium, on the surface of the base layer  104 . In the present example, the electrodes  106 ,  108  are the same size and shape. However, when forming transducers  52 ,  54  as described with reference to  FIG. 6   b , the size and/or shape of the electrodes  106 ,  108  may be varied at this stage. For example, the electrode  106  for the transmitting transducer may have a greater diameter, or a greater mass, than the electrode  108  for the detecting transducer. 
     Depositing the electrodes  106 ,  108  by sputtering is preferable to other methods such as thermal evaporation due to the low substrate temperatures used. This ensures compatibility with CMOS fabrication processes. In addition, where materials other than aluminium are deposited, this method benefits from the ability to accurately control the composition of the thin film that is deposited. Sputtering deposits material equally over all surfaces so the deposited thin film has to be patterned by resist application and dry etching with a Cl 2 /BCl 3  gas mix to define the shape of the electrodes  106 ,  108  as well as to define the interconnect points (not shown in the Figures) that allow interconnection to the circuit regions (i.e. either the underlying CMOS circuit or the off-chip circuits, neither illustrated). 
     Next, referring to  FIG. 9   d , release channels  107 ,  109  are formed in the base layer  104  and insulating layer  102 . The release channels  107 ,  109  are provided in order to enable an etching path to be formed with the sacrificial material that is to be deposited in subsequent steps, as will be explained below. Although the release channels  107 ,  109  are shown as penetrating into the base layer  104  and insulating layer  102 , it is noted that the release channels could also be formed such that they penetrate into the base layer  104  only. Furthermore, in an embodiment where a base layer  104  is not provided, the release channels  107 ,  109  will penetrate the insulating layer  102  only. Furthermore, although not shown, the release channels may form part of the substrate  100 . 
     There are numerous possibilities for realising the release channels  107 ,  109 . For example, the release channels  107 ,  109  can be formed as one continuous channel that is fabricated around the periphery of the MEMS transducer. In other words, the release channels  107 ,  109  shown in  FIG. 9   d  form part of a continuous trough or ring around the MEMS transducer. According to another embodiment, each release channel  107 ,  109  can be formed as a discrete channel that creates a tunnel like structure for allowing the etching material to reach the sacrificial material. In the latter embodiment, a plurality of separate release channels  107 ,  109  may be formed around the periphery of the MEMS transducer. 
     It is noted that steps  9   c  and  9   d  may be reversed, if desired, so that the release channels  107 ,  109  are formed prior to depositing the electrodes  106 ,  108 . In such a method, sacrificial material may be deposited within the formed release channels  107 ,  109  prior to depositing the electrodes  106 ,  108 . 
     Next, referring to  FIG. 9   e , sacrificial layers  110 ,  112  are deposited over the electrodes  106 ,  108 , respectively. The sacrificial material used for depositing the sacrificial layers  110 ,  112  may also be deposited within the release channels  107 ,  109 , assuming that the release channels  107 ,  109  have not been previously filled, as described in the preceding paragraph. To ensure compatibility with CMOS fabrication techniques, the sacrificial layers  110 ,  112  can be made of a number of materials which can be removed using either a dry release or a wet release process. Using a dry release process is advantageous in that no additional process steps or drying are required after the sacrificial layer is released. Polyimide is preferable as the sacrificial layer as it can be spun onto the substrate easily and removed with an oxygen plasma clean. The polyimide coating is spun on the wafer to form a conformal coating, using parameters and techniques that will be familiar to those skilled in the art. A primer may be used for the polyimide layer. The polyimide layer is then patterned with photoresist and etched in an anisotropic oxygen plasma, thus leaving the sacrificial layers  110 ,  112 , plus sacrificial material in the release channels  107 ,  109 , as shown in  FIG. 9   e . It will appreciated by a person skilled in the art that alternative methods may be used for depositing the sacrificial layers  110 ,  112  and sacrificial material in the release channels  107 ,  109 , for example applying and etching a photosensitive polyimide. 
     As can be seen from  FIG. 9   e , the sacrificial layers  110 ,  112  are formed such that a portion of each sacrificial layer  110 ,  112 , overlaps a portion of the respective release channels  107 ,  109 . 
     The sacrificial layers  110 ,  112  define the dimensions and shape of the cavities underneath the membranes that will be left when the sacrificial layers  110 ,  112  are removed as discussed below. 
     The sacrificial layers  110 ,  112  are provided for a number of reasons. These include supporting and protecting the membrane of the MEMS device during the manufacturing process. The sacrificial layers  110 ,  112  are also provided for defining the diameter of the membranes, such that the size of the membranes can be altered by altering the diameter of the sacrificial layers  110 ,  112 . In the present example, the sacrificial layers  110 ,  112  are substantially identical in shape and size, However, when manufacturing transducers  42 ,  44  as described with respect to  FIG. 5   b , the sacrificial layers  110 ,  112  may have different diameters. In particular, the sacrificial layer  110  for the transmitting transducer may have a narrower diameter that the sacrificial layer  112  for the detecting transducer. 
     Next, referring to  FIG. 9   f , a membrane layer  114  is deposited over the sacrificial layers  110 ,  112 , over at least a portion of the base layer  104 , and over a portion of the release channels  107 ,  119 . The membrane layer  114  may be formed from silicon nitride deposited by PECVD, as before, although alternatively polysilicon may be used. In addition, titanium adhesive layers may be used between the aluminium and the silicon nitride. 
     Although not shown in  FIGS. 9   e  and  9   f , the upper surface of the sacrificial layers  110 ,  112  may be formed with one or more dimples (in the form of small cavities) in their outer area (i.e. near the periphery of the sacrificial layers  110 ,  112 ). As a result, the depositing of the membrane layer  114  causes one or more dimples (in the form of protrusions) to be formed in the outer area or periphery of the membrane. These dimples in the outer area of the membrane  114  reduce the contact area of the membrane with the underlying substrate in the event of overpressure or membrane pull-in, whereby the surface of the membrane comes into contact with another surface of the MEMS device. The dimples reduce the stiction forces such that they are below the restoring forces (i.e. the membrane tension), thereby allowing the membrane to release itself. 
     Next, referring to  FIG. 9   g , second electrodes  116 ,  118  are deposited substantially over the sacrificial layers  110 ,  112 , respectively. In general, for simplicity of the manufacturing process, the second electrodes  116 ,  118  have substantially the same size and shape as their respective counterpart electrodes  106 ,  108 ; however, this is not a strict requirement. For example, when manufacturing transducers  52 ,  54  such as described with respect to  FIG. 6   b , the electrode  116  for the transmitting transducer  52  may have a greater mass and/or diameter and/or thickness than the electrode  118  for the detecting transducer  54 . 
     The second electrodes  116 ,  118  are deposited in substantially the same way as the first electrodes  106 ,  108 . 
     At this stage, the method for manufacture of MEMS devices  40 ,  50  is substantially complete (i.e. membranes with differing diameters, or differing electrode diameter or size), apart from the removal of the sacrificial layers  110 ,  112 , are will be described below. 
     Next, referring to  FIG. 9   h , a release hole  117  is etched through the membrane layer  114  to allow access to the sacrificial material in the release channel  107 , which in turn is connected to the sacrificial layer  110 . In a similar manner, a release hole  119  is etched in the membrane layer  114  to allow access to the sacrificial material in the release channel  109 , which in turn is connected to the sacrificial layer  112 . As can be seen, the first and second release holes  117 ,  119  are formed through the membrane layer  114  in areas which correspond to second portions of the respective release channels  107 ,  109 , the second portions of the respective release channels  107 ,  109  being outside the respective areas defined by the first and second sacrificial layers  110 ,  112 . 
     The sacrificial material, both in the release channels  107 ,  109  and the sacrificial layers  110 ,  112 , is preferably removed using a dry etch process, such as an oxygen plasma system, so that the membrane is free to move in both transducers. 
     Referring to  FIG. 9   i , after removal of the sacrificial material from the release channels  107 ,  109  and sacrificial layers  110 ,  112 , the release holes  117 ,  119  are sealed or plugged with a suitable sealant, thus preventing moisture or other environmental parameters from penetrating the MEMS transducer. 
       FIGS. 9   j  to  90  describe alternative steps to those shown in  FIGS. 9   h  to  9   i , for manufacturing a MEMS device  60  as described with respect to  FIG. 7   b  (i.e. a device having transducers with differing membrane thickness). 
     Thus, according to this embodiment, once the MEMS device has been fabricated up to step  9   g,  the following steps are followed in order to fabricate a MEMS device  60  as described with respect to  FIG. 7   b . Referring to  FIG. 9   j , release holes  122  are etched through the electrode  118  and the membrane layer  114  to allow access to the sacrificial layer  112 . In the illustrated embodiment, the release holes  122  are formed through both the membrane layer  114  and the electrode  118 ; however, where the electrode diameter is less than the diameter of the membrane, for example, the release holes may be positioned substantially around the periphery of the membrane, such that they do not pass through the electrode itself. It will be appreciated that the formation of the release holes  122  through the electrode  118  and membrane layer  114  may be formed in one process step or several process steps depending on the materials involved, and the etching process or processes used. 
     With reference to  FIG. 9   k , a further sacrificial layer  124  is deposited over the electrode  118 , connecting with the sacrificial layer  112  through the release holes  122 . The further sacrificial layer  124  may again be formed from silicon nitride, or one of the alternative materials mentioned previously. Again, any one of the techniques previously mentioned may be used to deposit the sacrificial layer  124 . 
     Next, referring to  FIG. 9   l , a further membrane layer  126  is deposited over the first membrane layer  114 , the electrode  116 , and the further sacrificial layer  124 . In a preferred embodiment, the second membrane layer  126  is formed from the same material as the first membrane layer  114 , such that the two layers  114 ,  126  substantially bond together to form a single layer of material. The second membrane layer  126  may be formed from any of the alternatives for the first membrane layer  114 . 
     In  FIG. 9   m , a release hole  127  is etched through the membrane layer  114  to allow access to the sacrificial material in the release channel  107 , which in turn is connected to the sacrificial layer  110 . In a similar manner, a release hole  129  is etched in the membrane layer  114  to allow access to the sacrificial material in the release channel  109 , which in turn is connected to the sacrificial layer  112 , and to the sacrificial layer  124  via the release holes  122 . 
     Next, as shown in  FIG. 9   n , the completed device  60  is created by removing the sacrificial material from the release channels  107 ,  109  and the sacrificial layers  110 ,  112 ,  124 . The sacrificial material from the release channels  107 ,  109  and the sacrificial layers  110 ,  112 ,  124  is preferably removed using a dry etch process, such as an oxygen plasma system, so that the membrane is free to move in both transducers. 
     Finally, as shown in  FIG. 9   o , the MEMS device is sealed and protected from environmental parameters by sealing the holes  127 ,  129 . 
     The resulting MEMS device  60  comprises a first transducer having a membrane with a first thickness T 1 , and a second transducer having an effective membrane with a second thickness T 2 . The transducer having the membrane with the first thickness T 1  is particularly suited for use as a transmitter, while the transducer having the membrane with the second thickness T 2 , where T 2 &lt;T 1 , is particularly suited for use as a receiver. 
     In  FIGS. 9   j  to  9   o  the fabrication of the second transducer is shown as having release holes  122  for enabling the sacrificial material  124  to be etched by first etching away the sacrificial material from the release channel  109  and the sacrificial layer  112 . However, according to a further embodiment, the step of etching release holes in  FIG. 9   j  can be omitted, and instead the sacrificial layer  124  removed as follows. The steps shown in  FIGS. 9   k - 9   o  would be followed as above. However, the absence of release holes  122  would result in the sacrificial layer  124  being inaccessible using the release channel  109  and the sacrificial layer  112 . As such, the sacrificial layer  124  is removed by first removing a portion of the membrane  126 , and then etching away the sacrificial layer  124  from above. This would result in a device as shown in  FIG. 9   p . The resulting device is still sealed, in so far as the cavity created by the removal of the sacrificial layer  112  is sealed from the environment. 
     Although the method of fabricating a sealed transducer has been described in relation to a device having first and second transducers on the same substrate, it is noted that the method is also applicable to the fabrication of a single transducer. 
     In the illustrated embodiment, the first and second membrane layers  114 ,  126  substantially encase the electrode  116  of the transmitting transducer. The formation of a sandwich structure has the advantage of reducing unwanted deformation in the membrane. In other words, if the electrode is placed between two layers of nitride, or vice versa, then the stress is more equalised, and results in the membrane moving with less unwanted deformation. However, it will be apparent to one skilled in the art that the deposition of the electrode  116  may take place at a later stage, such that the electrode  116  is positioned on top of the thickened membrane. 
     A person skilled in the art will further appreciate that not described in the methods above are steps for depositing connection pads for the electrodes. However, it will be apparent that these may be deposited and connected to the electrodes at various stages throughout the method. Further, future technology may allow the direct integration of electronics within the transducers themselves; such developments may of course still be considered as falling within the scope of the present invention, as defined by the claims appended hereto. 
     It can be seen, therefore, that the present invention provides methods for manufacturing first and second transducers  62 ,  64  having differing membrane thicknesses on the same substrate and in the same process. 
     It will be appreciated that various combinations of the embodiments described above may be combined in a particular transducer or transducer array. That is, although the illustrated embodiments describe transducers with only one differing parameter/dimension on a single substrate, it will be appreciated that transducers on a single substrate may have any combination of different membrane thickness, different membrane diameter, and different electrode diameter, thickness or mass. Any or all of the above parameters may be varied in order to obtain a particular resonant frequency or frequency response characteristic for a transducer. 
     Further, although the description has been primarily directed towards a substrate with a first transducer adapted for transmitting pressure waves and a second transducer adapted for detecting pressure waves, it will be appreciated that the present invention also provides a substrate with two or more transducers adapted to transmit or to receive pressure waves, wherein the two or more transducers have different respective resonant frequencies. 
     In addition, it is noted that, although not shown in any of the embodiments, the transducers may be provided with a back volume. 
     The invention may also be used in an application whereby the MEMS device is formed in a housing or structure, and whereby a fluid for enhancing the transmission of ultrasonic waves is provided in said housing, for example between the MEMS device and a surface of the housing or structure. The housing may be used in an imaging application. 
     The present invention may be embodied in a number of systems and devices, including, for example, medical ultrasound imagers and sonar receivers and transmitters, as well as mobile phones, PDAs, MP3 players and laptops for gesture recognition purposes. 
     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 processor 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. A method claim reciting a plurality of steps in a certain order does not exclude a method comprising that plurality of steps in an alternative order, except where expressly stated.