GB2469410A

GB2469410A - MEMS ultrasonic transducer array

Info

Publication number: GB2469410A
Application number: GB1012727A
Authority: GB
Inventors: Robert Errol Mcmullen; Richard Ian Laming; Anthony Bernard Traynor; Tsjerk Hoekstra
Original assignee: Wolfson Microelectronics PLC
Current assignee: Cirrus Logic International UK Ltd
Priority date: 2008-05-07
Filing date: 2008-05-07
Publication date: 2010-10-13
Anticipated expiration: 2028-05-07
Also published as: GB2469410B; GB2459863B; GB201012727D0; GB201012728D0; GB2469411A; GB0808294D0; GB2469412A; GB2459863A; GB201012729D0; GB2469411B; GB2469412B

Abstract

A MEMS device 60 comprises a substrate having at least a first transducer 62 optimized for transmitting pressure waves, and at least a second transducer 64 optimized for detecting pressure waves. The transducer array is formed using a sacrificial layer release method. The transducers can be optimised for transmitting or receiving by varying the thickness of the membrane T1,T2 and the diameter or mass of the electrode and/or the mass of the membrane of each respective transducer. The transmitting transducer is provided with a higher Q factor than the receiving transducer. Also disclosed is an array of transmitting transducers and an array of receiving transducers, wherein elements in the array of transmitting and /or receiving transducers are arranged to have different resonant frequencies. The array is useful in medical ultrasound imagers and sonar systems, and also for gesture recognition in PDAs, MP3 players and laptops.

Description

MEMS TRANSDUCERS

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 on 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.

Figure 1 shows this known manufacturing process.

Figure la 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 (Figure 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 (Figure 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 (Figure id). Release holes 22 are etched through the second electrode 20 and the membrane layer 18, (Figure le). 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 (Figure if).

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.

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.

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.

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: Figures la to if show a known process of manufacturing a MEMS transducer; Figure 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; Figure 3 is a graph modelling the variation of the first resonant frequency of a transducer with membrane thickness; iO Figure 4 shows a 2D array according to the present invention; Figure 5 shows a transducer adapted to transmit pressure waves and a transducer adapted to detect pressure waves according to an aspect of the present invention; iS Figure 6 shows a transducer adapted to transmit pressure waves and a transducer adapted to detect pressure waves according to another aspect of the present invention; Figure 7 shows a transducer adapted to transmit pressure waves and a transducer adapted to detect pressure waves according to a further aspect of the present invention; and Figures 8a to 8k show a process 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).

Figure 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'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 Figure 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. Figure 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 pm. 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 pm are appropriate.

Furthermore, a thicker membrane leads to a higher Q factor. Thus, a 0.2 pm thick membrane is suitable for detecting pressure waves at or around 240 kHz, and a 1.2 pm thick membrane is suitable for transmitting pressure waves at or close to 240 kHz.

Figure 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 Figure 1. According to the present invention, however, some of the sub-arrays 32a (unshaded elements in Figure 4) comprise MEMS transducers specifically adapted to detect pressure waves. Others of the sub-arrays 32b (shaded elements in Figure 4), interleaved with the "detecting" sub-arrays 32a, 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 32a adapted to detect pressure waves may have a relatively low Q factor; the individual MEMS transducers 34 in the plurality of sub-arrays 32b 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 Figure 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.

Figure 5 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 DM1, and a second transducer 44 optimized for detecting pressure waves, having a diameter DM2. It can be seen that the diameter DM2 of the membrane of the second transducer 44 is greater than the diameter DM1 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 DM1 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.

Figure 6 illustrates a MEMS device 50 according to a further embodiment of the present invention.

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 DEl of the electrodes 53a, 53b of the first transducer 52 are greater than the diameter DE2 of the electrodes 55a, 55b of the second transducer 54. The force between the two electrodes 53a, 53b 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 55a, 55b 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.

Figure 7 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 Ti, and a second transducer 64 optimized for detecting pressure waves, having a second thickness T2. The membrane thickness T2 of the second transducer 64 is less than the membrane thickness Ti 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.

Figures 8a to 8k illustrate a method of manufacturing MEMS devices according to the present invention, and in particular the embodiment described with respect to Figure 7.

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.

Figure 8a 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 (Figure 8b). 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 Figure 8c, 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 Figure 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 C12/BCI3 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 Figure 8d, 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 Figure 8d. 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 Figure 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 Figure 8e, 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 Figures 8d and 8e, 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 Figure 8f, 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 Figure 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 Figure 8g, 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 Figure 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.

Figures 8h to 8k describe the further steps of a method for manufacturing a MEMS device 60 as described with respect to Figure 7 (i.e. a device having transducers with differing membrane thickness).

With reference to Figure 8h, 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 Figure 8i, 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 Figure 8j, 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 Figure 8k, 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.

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 in this aspect the present invention provides a method 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.

Claims

CLAIMS1. 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 located at a first site on the substrate and comprising a first membrane and being adapted to transmit pressure waves; and at least a second transducer located at a second, different site on the substrate and comprising a second membrane and being adapted to detect pressure waves; wherein said first membrane has a first thickness and said second membrane has a second thickness and said first thickness is greater than said second thickness.
2. A MEMS device as claimed in claim 1, wherein said first transducer has a first Q factor, and wherein said second transducer has a second Q factor, said first Q factor being higher than said second Q factor.
3. A MEMS device as claimed in any preceding claim, wherein said first transducer comprises a first electrode positioned on the first membrane, said first electrode having a first mass, and wherein said second transducer comprises a second electrode positioned on the second membrane, said second electrode having a second mass, said first mass being different from said second mass.
4. A MEMS device as claimed in claim 3, wherein said first mass is greater than said second mass.
5. A MEMS device as claimed in any preceding claim, wherein said first transducer comprises a first electrode positioned on the first membrane, said first electrode having a first diameter, and wherein said second transducer comprises a second electrode positioned on the second membrane, said second electrode having a second diameter, said first diameter being different from said second diameter.
6. A MEMS device as claimed in claim 5, wherein said first diameter is greater than said second diameter.
7. A MEMS device as claimed in any one of the preceding claims, wherein the plurality of transducers further comprises a first plurality of transducers adapted to detect pressure waves.
8. A MEMS device as claimed in claim 7, wherein each transducer of said first plurality of transducers is adapted to primarily detect a pressure wave having a different respective frequency.
9. A MEMS device as claimed in any one of the preceding claims, wherein the plurality of transducers further comprises a second plurality of transducers adapted to transmit pressure waves.
10. A MEMS device as claimed in claim 9, wherein each transducer of said second plurality of transducers is adapted to primarily transmit a pressure wave having a different respective frequency.
11. A MEMS device as claimed in any one of claims 7 to 10, wherein each transducer of said first plurality of transducers, or each transducer of said second plurality of transducers, has a different respective Q factor.
12. A MEMS device as claimed in any one of claims 7to 11, wherein each transducer of said first plurality of transducers, or each transducer of said second plurality of transducers, comprises a respective membrane.
13. A MEMS device as claimed in claim 12, wherein each respective membrane has a different respective thickness.
14. A MEMS device as claimed in claim 12 or 13, wherein each respective membrane has a different respective diameter.
15. A MEMS device as claimed in any one of claims 12 to 14, wherein each respective membrane comprises a respective electrode, each respective electrode having a different respective mass.
16. A MEMS device as claimed in any one of claims 12 to 14, wherein each respective membrane comprises a respective electrode, each respective electrode having a different respective diameter.
17. 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 adapted to transmit pressure waves on said first site, and said second transducer adapted to detect pressure waves on said second site; wherein said forming step comprises forming a membrane over the first site and a membrane over the second site wherein the membrane is thicker at the first site than the second site.
18. A method as claimed in claim 17 wherein the step of forming a membrane over the first site and a membrane over the second site comprises: depositing a first portion of sacrificial material at the first site; depositing a second portion of sacrificial material at the second site; and forming a first membrane layer over at least the first site and the second site; depositing a third portion of sacrificial material at the second site; depositing a second membrane layer over at least the first site and the second site; and etching away said second membrane layer from the second site, such that the overall membrane is thicker at said first site than at said second site.
19. A method as claimed in claim 17 or 18, further comprising: depositing a first electrode at the first site; and depositing a second electrode at the second site, wherein a mass of said first electrode is different from a mass of said second electrode.
20. A method as claimed in claim 19, wherein the mass of said first electrode is greater than the mass of said second electrode.
21. A method as claimed in any of claims 17 to 20, further comprising: depositing a first electrode at the first site; and depositing a second electrode at the second site, wherein a diameter of said first electrode is different from a diameter of said second electrode.
22. A method as claimed in claim 21, wherein the diameter of said first electrode is greater than the diameter of said second electrode.
23. An ultrasound imager, comprising: a MEMS device as claimed in any one of claims 1 to 16.
24. A sonar transmitter, comprising: a MEMS device as claimed in any one of claims 1 to 16.
25. A sonar receiver, comprising: a MEMS device as claimed in any one of claims 1 to 16.
26. A mobile phone, comprising: a MEMS device as claimed in any one of claims 1 to 16.
27. A personal desktop assistant, comprising: a MEMS device as claimed in any one of claims 1 to 16.
28. An MP3 player, comprising: a MEMS device as claimed in any one of claims 1 to 16.
29. A laptop, comprising: a MEMS device as claimed in any one of claims 1 to 16.
30. An imaging device comprising a housing, wherein a MEMS device as claimed in any one of claims 1 to 16 is provided within the housing.
31. An imaging device as claimed in claim 52, further comprising a fluid within said housing.