Patent Publication Number: US-9889472-B2

Title: CMUT device and manufacturing method

Description:
This application is the U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/EP2015/054801, filed on Mar. 9, 2015, which claims the benefit of European Patent Application No. 14161057.6, filed Mar. 21, 2014. These applications are hereby incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The present invention relates to a method of manufacturing a CMUT device comprising a plurality of CMUT cells and a plurality of interconnects. 
     The present invention further relates to a CMUT device comprising a plurality of CMUT cells and a plurality of interconnects. 
     BACKGROUND OF THE INVENTION 
     Capacitive micro-machined ultrasonic transducer (CMUT) devices are rapidly gaining popularity as the sensors in a range of sensing apparatuses such as imaging apparatuses. This is because CMUT devices can offer excellent bandwidth and acoustic impedance characteristics, which makes them the preferable over e.g. piezoelectric transducers. 
     Vibration of the CMUT membrane can be triggered by applying pressure (for example using ultrasound) or can be induced electrically. Electrical connection to the CMUT device, often by means of an integrated circuit (IC) such as an application specific integrated circuit (ASIC) facilitates both transmission and reception modes of the device. In reception mode, changes in the membrane position cause changes in electrical capacitance, which can be registered electronically. In transmission mode, applying an electrical signal causes vibration of the membrane. 
     CMUT devices generally operate with a biasing voltage applied. The CMUT can be operated in so called collapsed mode where the biasing voltage applied is increased above the collapse voltage to restrict the membrane and confine part of it against the substrate. The frequency of operation of the CMUT device is characterised by the material and physical properties of the membrane, such as the stiffness, and the size of the cavity. The bias voltage and application of the CMUT device also influence the operation mode. A CMUT device is often used in apparatuses for ultrasound imaging applications and in other applications where the CMUT device is used to detect fluid or air pressures. A pressure causes a deflection of the membrane that is electronically sensed as a change of capacitance. A pressure reading can then be derived. 
       FIG. 1  schematically depicts a top view and  FIG. 2  schematically depicts a cross-section along the line A-A′ in  FIG. 1  of a conventional CMUT device  1 . The CMUT device comprises a plurality of CMUT cells  100  in a CMUT region  10  of the device  1  and a plurality of interconnects  200 , which plurality may include routing lines  205 , in an interconnect region  20  of the device  1 . The boundary between the CMUT region  10  and the interconnect region  20  is indicated by the vertical dashed lines in  FIG. 2 . The interconnects  200  typically provide an interconnection to a conductive contact  210  such as a bond pad inside the CMUT device  1 . Such conductive contacts may provide a connection to the outside world or may be used to facilitate interconnections between different elements of the CMUT device  1 , e.g. between different CMUT cells  100 , between a CMUT cell  100  and a signal processing element, and so on. 
     Each CMUT cell  100  typically comprises a first electrode  110  separated from a second electrode  120  by a cavity  130 . The second electrode  120  is typically embedded in a membrane  140  made of one or more electrically insulating or dielectric layers. In some designs, the second electrode  120  is embedded in the membrane  140 , i.e. sandwiched in between a relatively thin dielectric layer portion  142  and a relatively thick dielectric layer portion  144  from the cavity  130  to prevent a short circuit between the first electrode  110  and the second electrode  120  upon deformation of the membrane  140  including the second electrode  120  towards the first electrode  110 . 
     Conventional CMUT designs have membrane  140  layer thicknesses in the order of 1-2 micron, which can be processed with common fabrication methods such as plasma-enhanced chemical vapour deposition (PECVD). However, where the CMUT cells  100  are required to operate at a low frequency, the membrane diameter D may need to be in excess of 100 micron, which can result in the thickness of the membrane  140  becoming larger than 3 micron. Consequently, the overall thickness of the dielectric layer stack in the interconnect region  20  on top of the conductive contacts  210 , e.g. bond pads, may exceed 6 micron. This overall thickness may further increase, for instance when the substrate  30  on which the CMUT cells  100  comprises planarization and/or encapsulation layers, with the conductive contacts  210 , e.g. bond pads, being formed underneath these additional layers, e.g. in the top metallization layer of a metallization stack. Such additional layers can add another 2-3 micron to the overall thickness of the dielectric layer stack. 
     In order to make an electrical connection to the CMUT and ASIC electrodes, the conductive contacts  210  are provided in the interconnect region  20  of the CMUT device  1 , which region for instance may be located at the perimeter of the die. The conductive contacts  210  are typically made of a conductive material, for instance a metal such as aluminium, and are initially covered by the electrically insulating layers of the dielectric layer stack. To allow electrical connection to the conductive contacts  210 , the conductive contacts  210  are typically etched open in one of the final phases of CMUT fabrication to form trenches  22 , which trenches  22  are subsequently lined or filled with a metal interconnect  200 . 
     However, the opening of the conductive contacts  210  in the interconnect region  20  of the CMUT devices  1  can become challenging if the overall thickness of the dielectric layer stack covering the conductive contacts  210  becomes too large, i.e. more than 2 micron. This typically requires an increase in the required etching time in order to etch through such thick dielectric layers. However, the maximum allowable etching time may be limited by selectivity and resist consumption issues. These conflicting requirements may make it impossible to fabricate CMUT devices  1  with thick membrane layers  140  in a conventional way. 
     A further challenge is that metallization of the conductive contacts  210  with thick metal interconnects  200  may become a challenge if the aspect ratio, i.e. the height H over the width W of the metal interconnects  200 , becomes unfavourable. This is especially important if the metal interconnects  200  are used for I/O routing, as such routing needs to carry high currents, and therefore needs to be implemented with a relatively thick metal interconnect  200  to reduce resistivity. For such I/O routing, conductive contacts  210  and the metal interconnects  200  require a low-resistive electrical connection between each other. This therefore requires the trenches in which the metal interconnects are to be formed to be effectively lined or filled with the metal. However, such an effective lining or filling can be challenging for trenches having steep and deep sidewalls, i.e. having high aspect ratios. 
     Consequently, the manufacturing of CMUT devices that meet design specifications is a non-trivial exercise. In order to obtain cost-effective devices, it is for instance desirable to manufacture the CMUT devices in existing manufacturing technologies. CMOS is a non-limiting example of such a technology. U.S. Pat. No. 8,309,428 B2 for instance discloses a CMOS manufacturing method of such a device. 
     However, it proves to be difficult to obtain high yields of acceptable CMUT devices from wafers manufactured in such technologies, in particular when the individual CMUT cells have a relatively large diameter, such as diameters in excess of 100 microns, which for instance is required when the CMUT device is to be operated in a low-frequency mode. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide a method of manufacturing a CMUT device comprising a plurality of CMUT cells and a plurality of interconnects that can produce such CMUT devices at good yields. 
     The present invention further seeks to provide such a CMUT device. 
     According to an aspect, there is provided a method of manufacturing a device comprising a plurality of micro-machined ultrasonic transducer cells in a first region on a substrate and a plurality of interconnects in a second region on said substrate the method comprising forming a dielectric layer stack over the substrate, said dielectric layer stack defining respective membranes of the micro-machined ultrasonic transducers in the first region; reducing the thickness of the dielectric layer stack in the second region by partially etching away the dielectric layer stack in the second region; etching a plurality of trenches in the reduced thickness portion of the dielectric layer stack, each of said trenches exposing a conductive contact in the second region; and lining or filling said trenches with a conductive material. 
     By reducing the height of the dielectric layer stack over the second region prior to the formation of the interconnects to the conductive contacts such as bond pads through this section of the dielectric layer stack, the aspect ratio (H/W) of the trenches in which the interconnects are to be formed is reduced, thereby mitigating the problems with effectively filling these trenches with the conductive material. This therefore improves the quality of interconnects, i.e. avoids resistance increases in the interconnects that are the result of partial lining or filling of the trenches only, and consequentially improves the yield of the manufacturing process due to the fact that fewer devices have to be rejected because of insufficient quality interconnects. 
     The etching step in which the thickness of the dielectric layer stack in the second region is reduced may be performed in any suitable manner, such as by depositing a resist layer over the dielectric layer stack developing the resist layer through a lithographic mask and removing the resist from said second region to facilitate selective removal of a part of the second region without affecting the first region because of the resist protection over the first region. 
     Alternatively, the step of reducing the thickness of the dielectric layer stack in the second region may comprise depositing a hard mask, e.g. a metal layer, over the dielectric layer stack and patterning the hard mask to expose the second region. 
     In order to improve the control over the etching step in which the dielectric layer stack is partially removed from the second region, the step of forming a dielectric layer stack over the substrate may further include forming an etch stop layer in said stack in the second region; and the step of partially etching away the dielectric layer stack in the second region may comprise terminating said etching step on the etch stop layer. 
     The etch stop layer may be removed prior to etching said plurality of trenches, for instance to further reduce the aspect ratio of the trenches to be formed. 
     In an example embodiment, the CMUT cells may be formed by forming a plurality of first electrodes in said first region on said substrate; forming a sacrificial material layer portion over each of the first electrodes; forming a first dielectric layer of the dielectric layer stack over the sacrificial material layer portions; forming a respective second electrode on the first dielectric layer over each of the sacrificial material layer portions; covering the resultant structure by at least one further dielectric layer of the dielectric layer stack; creating a plurality of openings in the first dielectric layer and the at least one further dielectric layer, each opening providing access to one of said sacrificial material portions; removing the sacrificial material portions through said openings; and forming at least a second further dielectric layer of the dielectric layer stack on the first dielectric layer, said second further dielectric layer sealing said openings. 
     The step of forming said plurality of first electrodes may comprise depositing a first metal layer on said substrate and patterning the first metal layer, said patterning step further comprising forming the conductive contacts in said second region. 
     The step of forming said second electrodes may comprise depositing a second metal layer on said first dielectric layer and patterning the second metal layer, said patterning step further comprising forming the conductive contacts in said second region. 
     The substrate may carry at least one of a planarization stack and a passivation stack, wherein the conductive contacts are located below the planarization stack and/or passivation stack. 
     Each membrane may have a diameter of at least 100 micron such that the CMUT cells are suitable for use at low frequencies. In this embodiment, each membrane as defined by part of the dielectric layer stack preferably has a thickness of at least 3 micron. 
     According to another aspect, there is provided a device comprising a plurality of micro-machined ultrasonic transducer cells in a first region on a substrate, and a plurality of interconnects in a second region on said substrate for providing connections to respective bond pads on the substrate, the device further comprising a dielectric layer stack over said substrate defining the respective membranes of the micro-machined ultrasonic transducer cells in said first region, wherein the plurality of interconnects extend through said dielectric layer stack in said second region, a portion of said dielectric layer stack being removed from the second region such that the dielectric layer stack having has a stepped profile in which said stack is recessed in said second region. 
     As previously explained, the provision of such a device having a stepped profile in the dielectric layer stack, i.e. the dielectric layer stack has a reduced thickness in the second region comprising the interconnects facilitates the generation of sufficient quality interconnects due to the reduced aspect ratio of the trenches in which the interconnects are formed. This improves the yield of the device manufacturing process and reduces the cost of the individual devices. 
     In an embodiment, said stack is recessed by at least 2 micron in said second region to significantly reduce the aspect ratio of the trenches. 
     Each of the membranes may have a diameter of at least 100 micron and optionally have a thickness of at least 3 micron in order to facilitate use of the device at low frequencies. 
     The second region may be a peripheral region of the device, for instance the peripheral region of a wafer or a die, which peripheral region envelopes the first region. 
     In an example embodiment, each micro-machined ultrasonic transducer cell comprises a first electrode separated from a second electrode by a cavity, and wherein each second electrode is embedded in the membrane such that the second electrode is separated from the cavity by a first dielectric layer of said dielectric layer stack and the membrane further comprises at least one further dielectric layer of the dielectric layer stack on the second electrode, wherein the at least one further dielectric layer is thicker than the first dielectric layer. It has been found that this arrangement yields CMUT cells having membranes that are resistant to buckling during the release of the cavity, thereby further improving the yield of the manufacturing process and consequently reducing the cost of the device. 
     In accordance with yet another aspect, there is provided an apparatus comprising the device according to an embodiment of the present invention. Such an apparatus may for instance be an ultrasound imaging device or a pressure sensing device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are described in more detail and by way of non-limiting examples with reference to the accompanying drawings, wherein: 
         FIG. 1  schematically depicts a top view of a prior art CMUT device; 
         FIG. 2  schematically depicts a cross-section of a prior art CMUT device along the line A-A′ in  FIG. 1 ; 
         FIG. 3  schematically depicts a cross-section of a CMUT device according to an embodiment of the present invention; 
         FIG. 4  schematically depicts a cross-section of various aspects of a CMUT device according to alternative embodiments of the present invention; and 
         FIG. 5  schematically depicts a method of manufacturing a CMUT device according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts. 
     In the context of the description and claims of the present application, where a layered structure is described, it should be understood that where a first layer is described as being on or over a second layer, this is intended to include embodiments in which the first layer is directly on, i.e. in contact with, the second layer as well as embodiments in which one or more intermediate layers are present in between the first layer and the second layer. Similarly, where the present application described a method of manufacturing such a layered structure, it should be understood that where a first layer is formed on or over a second layer, this may mean that the first layer is formed directly on or over the second layer, i.e. in contact therewith, or may mean that one or more intermediate layers are formed on the second layer prior to the formation of the first layer thereon or over. 
       FIG. 3  schematically depicts a cross-section of a device  1  comprising a plurality of CMUT cells  100  according to an embodiment of the present invention, in which the prior art device as shown in  FIG. 2  has been altered as follows. Compared to the prior art device shown in  FIG. 2 , the device  1  of the present invention comprises a stepped dielectric layer stack. The stepped dielectric layer stack comprises two steps; an upper step in a first region  10  of the dielectric layer stack and a lower step in a second region  20  of the dielectric layer stack. The first region  10  corresponds to the region in which the membranes  140  of the various CMUT cells  100  are located and the second region  20  corresponds to the region in which interconnects  200  to the conductive contacts  210  such as bond pads are located. The first region  10  will also be referred to as the CMUT region and the second region  20  will also be referred to as the interconnect region. 
     The partial removal of the dielectric layer stack in the second region  20  reduces the height of the dielectric layer stack in the second region  20  a height H′. Consequently, the aspect ratio of interconnects  200  in the trenches  22  reduces to H′/W. In at least some embodiments, H′ is at least 2 micron, e.g. about 3 micron, smaller than H, wherein H is the maximum thickness of the dielectric layer stack as for instance is shown in  FIG. 2 . This therefore facilitates a more effective lining or filling of the trenches  22  through the dielectric layer stack in the region  20  that expose the conductive contacts  210  such as bond pads with a suitable metal to form interconnects  200 . Consequently, the reduced aspect ratio of these trenches facilitates the formation of interconnects  200  having excellent conductive properties and reduces the risk of the formation of interconnects  200  that suffer from high resistivity caused by partial lining or filling only of these trenches  22 . This therefore facilitates the formation of CMUT membranes  140  having relatively high thicknesses, e.g. thicknesses in excess of 3 micron, without complicating the formation of interconnects  200  due to increased aspect ratios resulting from such relatively thick CMUT membranes  140 . 
     At this point it is noted that the interconnects  200  may provide any suitable interconnection. Non-limiting examples of such interconnections include the provision of an external contact to a CMUT cell  100  or to another circuit in or on the substrate  10 , in which case the conductive contact  210  may be implemented as a bond pad, an I/O routing connection between one or more CMUT cells  100  and one or more external bond pad(s), an I/O routing connection to other CMUT cells or to a metal layer of a metallization stack on the substrate  30  below the CMUT cells  100 , and so on. 
     In  FIG. 3 , the conductive contact  210  is shown as forming part of the metallization layer from which the first electrodes  110  of the CMUT cells  100  have been formed, e.g. by patterning a deposited metallization layer as will be explained in more detail later. However, it should be understood that this location of the conductive contacts  210  is by way of non-limiting example only. The conductive contacts  210  may be located in any suitable location, a few non-limiting examples of which are shown in  FIG. 4 , which schematically depicts a cross-section of a section of a device  1  in which conductive contacts  210 ,  210 ′ and  210 ″ are shown in three different suitable locations. Each conductive contact is connected to a respective interconnect  200 ,  200 ′ and  200 ″. 
     As previously explained, the conductive contact  210  may be formed in the metallization layer of the first electrode  110 . It is however equally feasible to form conductive contact  210 ′ in the metallization layer of the second electrode  120  or to form conductive contact  210 ″ in an upper metallization layer of the metallization stack (not shown) below the layer stack  35  carried by the substrate  30 . The layer stack  35  may be a passivation stack and/or a planarization stack, e.g. of an IC such as an ASIC formed in and on the substrate  30 . It is noted that the trenches to expose the respective conductive contacts  210 ,  210 ′ and  210 ″ may be formed by sub-trenches in some embodiments. In particular, in case of the conductive contact  210 ″ below the passivation and/or planarization stack of the device, the interconnect  200 ″ may comprise a first portion formed in a trench  22  extending to the layer stack  35  and a second portion, for example formed in a separate processing step, extending through the layer stack  35 , as will be explained in more detail below. 
     In case of the conductive contacts  210 ,  210 ′ being located above the layer stack  35 , further interconnects extending from these conductive contacts through the layer stack  35  and connecting conductive contacts  210 ,  210 ′ to the metallization stack on the substrate  30  may be present as will be apparent to the person skilled in the art. Such additional interconnects for instance would allow for the signals generated by the CMUT cells  100  to be processed by circuit elements (not shown) on the substrate  30 , e.g. in the case of the substrate  30  forming part of an ASIC. 
     It is noted for the avoidance of doubt that the conductive contacts, e.g. bond pads, provide contacts to the various parts of the device  1 , such as to the respective first electrodes  110  and the respective second electrodes  120  such that the signals generated by these electrodes can be processed either by circuitry on- or off-chip. This function of such conductive contacts is of course well-known to the skilled person and will not be explained in further detail for the sake of brevity only. 
       FIG. 5  schematically depicts an embodiment of a CMUT manufacturing method including processing steps for reducing the height of the interconnect region  20  of the CMUT device as explained above. The method proceeds in step (a) with the provision of a substrate  30 , which may be any suitable substrate such as a silicon substrate, a silicon-on-insulator substrate, a silicon germanium substrate, a gallium nitride substrate and so on. A silicon-based substrate may for instance be used in a CMOS manufacturing process. The substrate  30  may comprise several structures, such as semiconductor devices, a metallization stack interconnecting the semiconductor devices and/or the CMUT cells, a passivation stack  35  over the metallization stack and so on. The substrate  30  may for instance be the substrate of an application specific integrated circuit (ASIC) including the CMUT cells  100  on its layer stack  35 , e.g. passivation and/or planarization stack, wherein the CMUT cells  100  may be connected to signal processing circuitry on the substrate  30  by the metallization stack. The provision of such substrates  30  is well-known per se and belongs to the routine skills of the skilled artisan such that the provision of suitable substrates  30  will not be discussed in further detail for the sake of brevity only. 
     A first electrode  110  is formed on the substrate  30 , which electrode may be formed from any suitable electrically conductive material, e.g. metals or metal alloys, doped semiconductor materials such as doped poly-silicon, (semi)conducting oxides and so on. It is for instance particularly advantageous to use metals that are readily available in the manufacturing technology of choice, as this requires minimal redesign of the manufacturing flow, which is attractive from a cost perspective. For example, in a CMOS process, conductive materials such as Al, W, Cu, Ti, TiN and so on, as well as combinations of such materials, may be used to form the first electrode  110 . As the formation of such an electrode is well-known per se, this will not be explained in further detail for the sake of brevity. 
     At the same time, a conductive contact  210  may be formed on the substrate  30 , which may be made of the same material as the first electrode  110 , e.g. by patterning a deposited metal or metal alloy layer to form the one or more first electrodes  110  and one or more bond pads. It is reiterated that the conductive contact  210  is not necessarily a bond pad but may take any suitable shape, e.g. if the conductive contact  210  is to provide a contact for (internal) I/O routing purposes as previously mentioned. The formation of the conductive contact  210  may be omitted from this step if the bond pad  210  is to be provided in a different layer as also shown in  FIG. 4 , e.g. in a top layer of a metallization stack (not shown) of the substrate  30 , in the layer defining the second electrode  120  as will be explained in more detail later, and so on. 
     The first electrode  110 , the (optional) conductive contact  210  and the substrate  30  may subsequently optionally be covered by an electrically insulating material layer  11 . This is shown in step (b). Electrically insulating layers will also be referred to as dielectric layers in the present application. Such a dielectric layer  11  for instance may be used to electrically insulate the first electrode  110  from its counter electrode (see below) to prevent short-circuits between the electrodes during the operation of the CMUT device. In addition, the dielectric layer  11  may be used to protect the first electrode  110  and the substrate  30  from damage during the removal of the sacrificial material to form the cavity over the first electrode  110 . 
     Although the dielectric layer  11  is shown to cover the entire substrate surface  30 , it is equally feasible to provide a patterned dielectric layer  11  in which only certain parts of the substrate  30  together with the first electrode  110  are covered by the dielectric layer  11 . Any suitable dielectric material may be used for the protection of the first electrode  110  and the substrate  30 , e.g. one or more materials selected from silicon nitride (Si 3 N 4 ), silicon oxide (SiO 2 ) or the like, although it is emphasized that the suitable dielectric materials are not limited to these example materials. As such a dielectric layer  11  may be formed in any suitable manner, e.g. using suitable deposition techniques such as CVD and PECVD, its formation will not be explained in further detail for the sake of brevity. 
     In step (c), a sacrificial material is formed, e.g. through a suitable deposition technique, on the dielectric layer  11 . The sacrificial material is patterned to form a first portion  12  from which the cavity is formed and may further comprise a second portion  12 ′ acting as a channel through which the sacrificial material is removed. 
     The height of the first portion  12  and the second portion  12 ′ of the sacrificial material corresponding to the gap height of the cavity to be formed is typically in the range of 100-1000 nm although it should be understood that values outside this range may also be contemplated. 
     In an embodiment, the first portion  12  is deposited as a circular portion having a few teeth-like protrusions as the second portion  12 ′, e.g. 2-8 of such protrusions. A top-view of such a sacrificial material portion is shown in step (c′), in which four of such protrusions are shown by way of non-limiting example only. The teeth-like second portions  12 ′ are typically used as cavity access platforms outside the membrane to be formed through which access to the first portion  12  can be provided for opening or releasing the cavity. It should be understood that the first portion  12  and the second portion  12 ′ are typically formed to the same thickness or height, with the membrane to be formed extending towards the substrate  30  in between the teeth-like second portions  12 ′. In the various drawings of the present application, the second portions  12 ′ are shown to have a different thickness in order to indicate this aspect, i.e. the aspect that the membrane of the CMUT device extends towards the substrate  30  in between the teeth-like second portions  12 ′. This should not be construed as the first portion  12  and the second portion  12 ′ having different thicknesses in reality. 
     In principle, any suitable sacrificial material may be used, although for device performance reasons it is preferable to use sacrificial materials that can be effectively removed in a subsequent etching step. For instance, the use of metals such as Al, Cr and Mo or non-metals such as amorphous silicon or silicon oxide may be contemplated. Materials such as Al, amorphous silicon and silicon oxide are for instance readily available in CMOS processes, and of these materials Al can be particularly effectively removed by etching. The patterned sacrificial material may be formed in any suitable manner, e.g. using suitable deposition and patterning techniques and its formation will not be explained in further detail for the sake of brevity. 
     It will be understood that the diameter of the first portion  12  defines the diameter of the cavity of the CMUT device to be formed. In an embodiment, the diameter is selected in a range of 20-500 micron, more preferably in a range of 50-300 micron, although it should be understood that larger diameters may also be contemplated, e.g. diameters up to 1,000 micron. 
     In step (d), a first dielectric layer  13  of the membrane to be formed is deposited over the first portion  12  and the second portion  12 ′ of the sacrificial material and the exposed portions of the dielectric layer  11 . As the first dielectric layer  13  and the dielectric layer  11  are both exposed to the etch recipe for removing the sacrificial layer, the first dielectric layer  13  and the dielectric layer  11  may be of the same material, although it is of course also plausible to use different materials for the first dielectric layer  13  and the dielectric layer  11  respectively. In an embodiment, the first dielectric layer  13  and the dielectric layer  11  each comprise at least one layer formed of any suitable dielectric material, such as a silicon oxide layer, e.g., SiO 2 , a silicon nitride layer, e.g., Si 3 N 4  or the like. Many other suitable dielectric layer materials will be apparent to the skilled person. The first dielectric layer  13  may be formed as a layer stack, e.g. an oxide-nitride stack or an oxide-nitride-oxide stack. Similarly, the dielectric layer  11  may be formed as such a stack. It is reiterated that any suitable dielectric material may be used for the dielectric layer  11  and the first dielectric layer  13 . 
     After the formation of the first dielectric layer  13 , an etch stop layer  14  may be formed in the interconnect region  20  over the bond pad  210  to facilitate the reduction of the thickness of the dielectric stack in the interconnect region  20  as will be explained in more detail later. Any suitable etch stop material such as silicon nitride or a suitable metal may be selected. As will be apparent to the skilled person, the etch stop layer preferably can be selectively removed relative to the underlying first dielectric layer  13  at a later stage in the manufacturing process. If the etch stop layer  14  has to be removed at a later stage, a material should therefore be selected for the etch stop layer  14  that can be selectively removed without significantly affecting the underlying first dielectric layer  13 . 
     The purpose of the etch stop layer  14  is to provide a well-defined point over the bond pads  210  at which an etch process to reduce the thickness of the dielectric stack in the interconnect region  20  can be terminated in order to reduce the thickness of the dielectric stack in the interconnect region  20  to reduce the aspect ratio (H/W) of the interconnects  200  as previously explained. This will be explained in more detail below. It should however be understood that the location of the etch stop layer  14  in the dielectric stack can be varied, i.e. the etch stop layer  14  does not necessarily have to be formed on top of the first dielectric layer  13  but may be formed on any suitable intermediate layer in the manufacturing process of the CMUT device  1 . Moreover, the etch stop layer  14  may be omitted if a highly selective etching process can be provided that can terminate on a desired layer of the dielectric layer stack. For instance, one of the intermediate layers of the dielectric layer stack may be chosen such that it can act as an etch stop layer as well. It will be understood that in this scenario this intermediate layer must have a composition that is sufficiently different to its immediately overlaying layer of the dielectric layer stack to ensure that the etch recipe used to remove the layers of the dielectric layer stack over this intermediate layer does not affect this intermediate layer acting as etch stop layer. 
     Next, the second electrode  120  is formed on the first dielectric layer  13  as shown in step (e) such that the second electrode  120  is oriented opposite the first electrode  110 . The second electrode  120  preferably is formed of the same electrically conductive material as the first electrode  110 , although it should be understood that the second electrode  120  and the first electrode  110  alternatively may be formed of different materials. The second electrode  120  may for instance be formed from any suitable electrically conductive material such as Al, W, Cu, Ti, TiN and so on, as well as combinations of such materials. The second electrode  120  may be formed using well-known techniques that are not further explained for the sake of brevity only. The first electrode  110  and the second electrode  120  may be formed to any suitable thickness, e.g. 200-700 nm thickness. 
     After the formation of the second electrode  120 , the method proceeds as shown in step (f), in which the second dielectric layer  15  is formed. In an optional embodiment, the second dielectric layer  15  is formed to a first thickness t 1 , which exceeds the thickness of the first portion  12  of the sacrificial material in between the first electrode  110  and the second electrode  120  such that upon formation of the cavity  130  the height g of the cavity gap is substantially smaller than the thickness t 1 , i.e. g/t 1 &lt;&lt;1. Preferably t 1 ≧5g. This ensures that during the release of the cavity  130  in step (g), i.e. by formation of the access or via  16  and the subsequent removal of the first portion  12  and the second portion  12 ′ of the sacrificial material, the membrane exhibits excellent membrane robustness during the cavity release step as g&lt;&lt;t 1  at the stage of removal of the sacrificial material to form the cavity  130 . Moreover, because the second dielectric layer  15  is formed, e.g. deposited, prior to the release of the cavity  130 , a membrane with excellent flatness characteristics is obtained as the presence of the sacrificial material prevents deformation of the first dielectric layer  13  during the formation of the second dielectric layer  15 . 
     The first portion  12  and the second portions  12 ′ of the sacrificial material are subsequently removed as shown in step (g) by the formation of the access or via  16  using a suitable etch recipe to form the cavity  130  in between the first electrode  110  and the second electrode  120  embedded in between the first dielectric layer  13  and the second dielectric layer  15  of the membrane  140  of the CMUT device. Suitable etch recipes for such conventional sacrificial materials are well-known per se and the skilled person will have no difficulty selecting an appropriate etch recipe using his common general knowledge. 
     The thickness of the dielectric layer stack including the membrane  140  is further increased during the sealing of the access or via  16  in step (h) by the formation of the further dielectric layer  17  including the plug  18  in the access or via  16 . The further dielectric layer  17  may be substantially thinner than the second dielectric layer  15 . The further dielectric layer  17  may be formed to a thickness of at least twice the height of the cavity  130  to effectively seal the access or via  16 . 
     At this point it is emphasized that steps (a)-(h) schematically depict an advantageous but non-limiting example of forming one or more CMUT cells  100  on a substrate  30 . Many alternative routes will be apparent to the skilled person. In particular, it is noted that the following steps are not specific to the embodiment depicted in steps (a)-(h) and may be applied to any CMUT manufacturing process in which a dielectric layer stack includes a CMUT region  10  in which the membranes  140  of the CMUT device  100  are defined and an interconnect region  20  covering the bond pads  200  of the CMUT device  1 . 
     A notable process variation is that the access or via  16  may be sealed in any suitable manner using any suitable material, e.g. by depositing and patterning a dedicated sealing layer such as a metal layer to form the plug  18 . It is furthermore noted that the cavity  130  may be released at any suitable point in the CMUT manufacturing process, e.g. prior to the formation of the second dielectric layer  15 . Another notable process variation is that the first electrode  110  and/or the second electrode  120  may or may not be separated from the cavity  130  by a dielectric layer, as this is a typical design choice. As previously mentioned, a dielectric layer, i.e. an electrically insulating layer, may be provided over the first electrode  110  and/or the second electrode  120  to prevent direct contact between the first electrode  110  and the second electrode  120  during operation of the CMUT cell  100 . The membrane  140  may be formed in any suitable manner, e.g. by a single dielectric layer rather than a stack of dielectric layers, and so on. Such process choices fall within the routine skills of the skilled person and will therefore not explicitly mentioned in detail for the sake of brevity only. 
     In step (i), a resist layer  19  is deposited over the dielectric stack and the region of the resist layer  19  over the interconnect region  20  is selectively developed through a mask  300  as indicated by the vertical arrows, and the developed resist is subsequently removed (not shown), e.g. using a suitable solvent to selectively expose the interconnect region  20  of the dielectric layer stack. The formation and development of such resist layers is well-known per se and many suitable resist materials will be apparent to the skilled person such that this will not be explained in further detail for the sake of brevity only. It is furthermore noted that although  FIG. 5  schematically depicts the use of a positive resist, it is equally feasible to use a negative resist, in which case an inverse of the mask  300  may be used to expose the portion of the resist to be removed. It will furthermore be understood that instead of a resist layer  19 , a hard mask may be deposited over the exposed surfaces of the device in progress and patterned to expose the interconnect region  20 . Other suitable masking techniques will be apparent to the skilled person. 
     The thickness of the dielectric layer stack in the interconnect region  20  is reduced in step (j) by exposing the exposed interconnect region  20  of the dielectric layer stack to a suitable etch recipe (or a combination of suitable etch recipes in case of the dielectric layer stack being formed of different materials) and terminating the etch step at the desired depth, e.g. by terminating the etch step on the etch stop layer portion  14  as previously explained. As the etching of dielectric layers is well-known per se in the field of for instance semiconductor processing, this will not be explained in further detail for the sake of brevity only. It suffices to say that any suitable etch recipe may be used to selectively remove the desired portion of the dielectric stack in the interconnect region  20 . 
     In step (k), the etch stop layer portion  14  may be removed if necessary, e.g. using a suitable etch recipe. Additionally or alternatively the resist layer  19  may be removed at this stage, e.g. using a suitable solvent. In case a hard mask is used instead of the resist layer  19 , the hard mask may be removed at this stage using a suitable etch recipe, as is well-known per se. A further resist layer or hard mask layer  21  may be subsequently formed and patterned as previously explained for the resist layer  19  to expose a region  21 ′ over the conductive contact  210  as shown in step (l). 
     Next, as shown in step (m) a trench  22  may be etched in the exposed portion of the recessed interconnect region  20 . Depending on the location of the conductive contact  210 , the formation of the trench  22  may include extending the trench  22  through the passivation stack  35  on the substrate  30 , for instance in case of a bond pad  210  being formed in an upper layer of the metallization stack of the substrate  30  as previously explained. Alternatively, in a scenario in which the conductive contact  210  forms part of the metallization stack, a via (not shown) may previously have been formed through the passivation and/or planarization stack  35 , in which case the trench  22  terminates on the passivation stack  35 , thereby exposing the previously formed via. In some embodiments, the preformed via through the passivation (and/or planarization) stack  35  may be filled, e.g. lined, with a suitable conductive material, e.g. metal, in a process step in which the metal layer for the first electrode  110  is simultaneously deposited. Many more feasible process variations will be apparent to the skilled person. 
     The manufacturing process of the CMUT device  1  may subsequently be completed by filling the trench  22  with a suitable conductive material, e.g. a metal such as aluminium, a metal alloy such as an aluminium alloy, any of the metals or metal alloys previously mentioned in the context of the formation of the electrodes  110  and  120 , and so on, to form the interconnect  200 . To this end, as shown in step (n), the further resist layer or hard mask  21  (and any underlying resist and/or hard mask layers if still present) is removed as previously explained for the resist layer or hard mask  19 , after which a metal layer  24  including the interconnect  200  is deposited over the resultant structure. As is shown in step (n), the trench  22  is lined by the metal layer  24  although it should be understood that in some embodiments the trench  22  may be (substantially) filled by the metal layer  24 , e.g. by extending the duration of the metal deposition step. 
     Next, the metal layer  24  is patterned to remove the metal layer  24  from the regions in which the metal should not be present. To this end, a second further resist layer (or hard mask)  25  may be deposited over the metal layer  24  and subsequently patterned to protect the interconnect  200  from being removed. This is shown in step (o). The exposed portions of the metal layer  24  are subsequently removed, e.g. using a suitable etch recipe followed by the removal of the patterned second further resist layer (or hard mask)  25  to yield the device  1  including the interconnect  200  as shown in step (p). As previously explained, by reducing the thickness of the dielectric stack in the interconnect region  20  of the CMUT device  1 , i.e. providing a recessed interconnect region  20 , the interconnect  200  has a reduced height, i.e. a reduced aspect ratio, which facilitates the formation of high quality interconnects  200  and therefore improves the yield of the manufacturing process compared to manufacturing processes in which the interconnects  200  have to extend downwardly through a dielectric layer stack portion in the interconnect region  20  that has the same thickness as the dielectric layer stack portion in the CMUT region  10 . 
     At this point it is noted that although not shown in the various embodiments, it should be understood that the CMUT devices  1  manufactured in accordance with embodiments may comprise additional circuit elements, which may be integrated on the substrate  30  or may be provided on a separate substrate and integrated into a single package with one or more of the CMUT devices from a wafer manufactured in accordance with embodiments of the present invention. Such additional circuitry may be instance be an IC, e.g. an ASIC, for controlling the one or more CMUT devices  1  and/or processing the signals generated by the one or more CMUT devices  1 , e.g. to control transmission and/or reception modes of the one or more CMUT devices  1  as previously explained. 
     It is furthermore noted that in the aforementioned manufacturing process, a wafer processed during the manufacturing process may contain a single die, i.e. a single device, in which case the substrate  30  corresponds to the wafer, or a plurality of dies that may be singulated in any suitable manner, e.g. diced, after the completion of the device manufacturing process, in which case the substrate  30  corresponds to a part of the wafer. The interconnection region  20  may be peripheral to a substrate and/or to the wafer as a whole. 
     One or more CMUT devices  1  according to embodiments of the present invention may be advantageously incorporated into sensing apparatuses such as a pressure sensing apparatus and in particular in a medical imaging apparatus, e.g. an ultrasound imaging apparatus, where the integration of CMUT-based sensing elements can significantly improve the imaging resolution of the apparatus, which for instance improves the detectability of small size objects, e.g. anomalies such as tumours, in the body of a subject under investigation, e.g. a mammalian body such as a human body. In an embodiment, such an apparatus may comprise a CMUT device  1  according to embodiments of the present invention comprising a plurality of CMUT cells  100 . Each of the CMUT cells  100  may be individually addressable. Alternatively, appropriate groups of CMUT cells  100 , which together form an acoustic element, may be individually addressable at the group level. The CMUT cells  100  or groups of CMUT cells  100  may be arranged in a matrix. In a further embodiment, several CMUT devices  1 , e.g. CMUT dies may be mounted, e.g. tiled, on a carrier, wherein the multiple CMUT devices  1  together define a large sensor area. Such an apparatus may for instance comprise several hundreds or thousands of (individually addressable) CMUT cells  100  distributed over one or more CMUT devices  1 . 
     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. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The invention can be implemented by means of hardware comprising several distinct elements. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.