Method for manufacturing a device comprising a plurality of micromechanical elements

Abstract

The invention relates to a manufacturing method for manufacturing a device comprising a plurality of micromechanical elements, each element comprising a flexible membrane, and the elements are arranged in a planar manner with a determined topological structure, the design method comprising a step of defining the determined topological structure so as to have characteristics compatible with a common substrate (20) having cavities (24), the characteristics of which are predetermined, and each flexible membrane of the micromechanical elements is connected to a cavity (24) of the common substrate (20). The invention also relates to a manufacturing method for manufacturing a device comprising a plurality of micromechanical elements and such a device itself, only some of the pairs formed by a cavity (24) and a flexible membrane are configured to form a set of functional micromechanical elements.

Description

Technical Field

[0001] The present invention relates to devices comprising an array of micromechanical elements arranged in a planar manner. In particular, the present invention relates to any acoustic device implementing an array of micromechanical ultrasound transducers. These transducers may have piezoelectric or capacitive properties. The present invention may be applied, for example, in the field of ultrasound imaging. Background Art

[0002] Reference (D1) Lu, Yipeng and David A. Horsley, "Modeling, fabrication, and characterization of piezoelectric micromachined ultrasonic transducer arrays based on cavity SOI wafers", Journal of Microelectromechanical Systems 24.4 (2015): 1142-1149 introduces that a micromachined ultrasonic transducer (MUT) is formed by a membrane suspended above a cavity, which has low acoustic impedance and good coupling with air or another fluid for sound wave propagation. In receiver mode, the bending of the membrane under the action of sound waves is converted into electrical signals. In transmitter mode, the electrical signal causes the membrane to bend, thereby generating sound waves.

[0003] Documents US20130270967, US20140219063 and US20170128983 provide other illustrations of MUT devices and their manufacturing processes.

[0004] MUT technology enables the design of acoustic devices suitable, for example, for the field of imaging and has many advantages, notably its ease of integration with signal conditioning electronics, its low power consumption, its wide bandwidth, and the ease of fabricating large arrays of planarly arranged transducers.

[0005] The document also introduces that these transducers can be divided into two categories based on the activation mechanism of the membrane.

[0006] Capacitive MUT (a cross section of which is shown in Figure 1a ) has two electrodes 1a, 1b, one electrode being arranged on the membrane 2 and the other electrode being generally formed by the bottom of the cavity 3 from which the membrane is suspended. This second electrode can thus consist of a single contact 1b made on a carrier 4 carrying the cavity 3. The two electrodes form a capacitor whose capacitance depends on the degree of curvature of the membrane.

[0007] Piezoelectric MUT (a cross section of which is shown in Figure 1bThe diaphragm 2 (schematically shown in FIG. 2 ) also has two electrodes 1a, 1b arranged on the membrane 2 on either side of the piezoelectric material layer 5. In this type of device, the piezoelectric material layer 5 is coupled to the membrane 3 so as to cause it to bend or to follow its bending.

[0008] Regardless of the type of MUT used in the acoustic device, the MUT is usually arranged in a planar and defined topological structure, as described in detail in the document (D2) Ergun, Arif S., Goksen G. Yaralioglu and Butrus T, "Capacitive micromachined ultrasonic transducers: Theory and technology", Journal of aerospace engineering 16.2 (2003): 76-84.

[0009] In the context of this application, the term "topology" refers to the geometry of the individual transducers, in particular the shape and dimensions (width, length or diameter, depth) of the cavity and the associated membrane it comprises, as well as the planar distribution of the transducers forming the acoustic device.

[0010] Figure 1c An example topology is shown very schematically, comprising a plurality of transducer blocks b1, b2, which are spaced apart from each other by a block spacing distance p2 (measured between the centers of two transducers arranged facing opposite edges of the two blocks b1, b2). The device may comprise one such block, or indeed hundreds of such blocks, which are spaced apart from each other by a distance p2 of, for example, about 10 to 100 microns. For example, contacts, tracks or other elements enabling the transducers to be connected to a conditioning circuit are typically arranged in the available space between the individual blocks. Each block has a determined width and length (which can be expressed as the number of transducers of each of these dimensions) and depicts a two-dimensional array of transducers t that are generally regularly distributed. A block may comprise thousands of transducers. Each transducer is spaced apart from an adjacent transducer by a determined spacing distance p1, typically between 1 micron and 100 microns. Each transducer comprises a membrane suspended above a cavity, which in the example shown has a circular shape and a diameter d (a few microns to several hundred microns). The topology is also defined by the depth of the cavity, which can be below 1 micron and above tens of microns.

[0011] The design of an acoustic device usually requires a precisely defined topology, i.e., the number and size of the transducer blocks, the spacing between the blocks, the arrangement of the transducers within the blocks (e.g., in the form of a matrix array with a specified spacing between two transducers), the size and shape of the cavities of each transducer, etc. These design parameters depend to some extent on the expected performance and function of the acoustic device. All transducers or all transducer blocks of an acoustic device do not necessarily have to be of the same size or have a single layout.

[0012] Document D1 also introduces several methods for manufacturing acoustic devices based on MUT, depending on whether the transducer is manufactured simultaneously with the analog and digital signal conditioning circuits, or manufactured afterwards, or manufactured separately. Therefore, the document proposes a simplified process for manufacturing acoustic devices, in which the transducer is manufactured separately from the conditioning circuit. The method uses a cavity formed in advance in the substrate to limit the number of photolithography mask layers that need to be used.

[0013] This approach is attractive, but it means that some of the work of designing the acoustic device must be undertaken by the manufacturer of the substrate. Specifically, the manufacturer of the substrate must be able to design and manufacture a substrate containing a cavity with a topology precisely chosen for the device. However, the device topology is often valuable sensitive information that the device manufacturer typically does not want to disclose.

[0014] Furthermore, substrate manufacturers must develop substrate manufacturing processes that are specific to each topology. The effort required for this makes rapid and economical development of acoustic devices more difficult.

[0015] Therefore, it is desirable to provide a substrate containing a cavity that can be used to manufacture a wide variety of acoustic devices and thus does not require a specific substrate manufacturing process to be developed specifically for each acoustic device. Summary of the invention

[0016] To achieve one of these aims, a subject of the invention is a method for designing a device comprising a plurality of micromechanical elements, each of which comprises a flexible membrane, and said elements being arranged in a planar manner with a determined topology. The method comprises a step of defining the determined topology so that it has characteristics compatible with a common substrate having cavities, the characteristics of which are predetermined, and each flexible membrane of the micromechanical elements being connected to a cavity of the common substrate.

[0017] Thus, the present invention defines design rules that set the topology of the device so that its membranes align with the cavities of a common substrate. Thus, it is not necessary to develop a specific substrate designed for the topology of the device.

[0018] Other advantageous and non-limiting features according to the invention which can be implemented individually or in any technically feasible combination:

[0019] - in the step of defining the topology, selecting a generic substrate from a set of generic substrates, the substrates having predetermined properties that distinguish them from each other;

[0020] - the defining step comprises selecting a generic substrate from a set of generic substrates such that the determined topology is defined to be as similar as possible to the desired topology;

[0021] - The design process also includes defining the thickness of the film;

[0022] - The step of defining a determined topology consists of the following steps:

[0023] o Selecting the size and shape of the element's membrane so that it corresponds to the shape of the cavity of the common substrate;

[0024] o The arrangement of the membranes of the micromechanical elements in the plane is chosen so that they are all perpendicular to the cavities of the common substrate when the device is aligned with this substrate;

[0025] - The arrangement of the cavities of the universal substrate is regular and covers all ranges;

[0026] - the micromechanical element is a micromirror or a micromechanical acoustic transducer or a micromechanical ultrasonic transducer;

[0027] - The cavities of the common substrate are not all identical in size and / or shape.

[0028] According to another aspect, the present invention proposes a manufacturing method for manufacturing a designed acoustic device, which comprises the step of providing a universal substrate.

[0029] More generally, the invention proposes a method for manufacturing a device comprising a plurality of micromechanical elements, the method comprising:

[0030] - a step of providing a universal substrate comprising a surface layer arranged on a carrier, the main face of the carrier having an emergent cavity and a portion of the surface layer overhanging the cavity forming a flexible membrane connected to the cavity;

[0031] - at least one step of processing only part of a pair formed by a cavity and a membrane, so as to form at least one functional micromechanical element and at least one redundant pair not being able to convert the movement of its membrane into an electrical signal and vice versa.

[0032] Other advantageous and non-limiting features according to the manufacturing method which can be implemented individually or in any technically feasible combination:

[0033] - the processing step further comprises removing the membrane and / or the cavity;

[0034] - the treatment step also includes a step of neutralizing the membrane and / or the cavity;

[0035] - at least one processing step comprises the step of producing an array of conductive elements configured to connect only a portion of the cavity-membrane pairs;

[0036] - the step of producing the array of conductive elements comprises forming electrodes perpendicular to only a portion of the cavities;

[0037] - the method comprises the step of adjusting the thickness of the flexible film;

[0038] - the universal substrate comprises an intermediate layer between the main face of the carrier and the surface layer;

[0039] - the method comprises the step of depositing at least one first electrode on the surface layer;

[0040] - the method comprises the step of depositing a piezoelectric layer at least on the first electrode;

[0041] - The method comprises the step of depositing a second electrode on the piezoelectric layer.

[0042] Finally, the present invention proposes a device comprising a carrier and a surface layer arranged on the carrier, the main surface of the carrier having a concave cavity, and the part of the surface layer suspended above the cavity forming a flexible membrane connected to the cavity, only some of the pairs formed by a cavity and a flexible membrane are configured to form a set of functional micromechanical elements, and at least one redundant pair, which is unable to convert the movement of its membrane into electrical signals and vice versa.

[0043] According to other advantageous and non-limiting features of the device, which may be implemented individually or in any technically feasible combination:

[0044] - the device comprises an array of conductive elements configured to connect only a portion of the cavity-membrane pairs;

[0045] The micromechanical element is a micromirror or a micromechanical acoustic transducer or a micromechanical ultrasonic transducer. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Other features and advantages of the present invention will become apparent from the following detailed description of the invention, which is given with reference to the accompanying drawings, in which:

[0047] - Figures 1a to 1c The topological structures of the capacitive MUT, the piezoelectric MUT and the acoustic device of the prior art are shown respectively;

[0048] - Figures 2a to 2cA cross-sectional view and a top view schematically show an example of a universal substrate of the present invention. DETAILED DESCRIPTION

[0049] In general, the invention proposes, on the one hand, a universal substrate (or a set of such universal substrates) having a cavity in which a membrane is suspended. On the other hand, the purpose is to define design rules for devices, in particular acoustic devices, so that they can be manufactured from such a universal substrate. At least some of the membranes and the cavities of the universal substrate are intended to form functional micromechanical elements, such as transducers of acoustic devices. As a result, substrate manufacturers can provide substrates in large quantities and at a low cost without having to adjust the manufacturing process to specific customer needs. Device manufacturers, in particular manufacturers of acoustic devices, who comply with these design rules, can stock substrates without sharing the usually confidential nature of their products.

[0050] In the context of this application, a "cavity" refers to any void formed in a substrate. The void does not have to be completely enclosed or hermetically closed.

[0051] "Membrane" or "flexible membrane" refers to any structure suspended over a void in a substrate. It can have any shape, such as forming a partition or a beam. The perimeter of the suspended structure can be fully or partially bonded to the substrate. It can also be connected to the substrate by one or more legs. Regardless of the way the structure is connected to the substrate, at least a portion of the structure has at least one degree of freedom of movement, making it flexible.

[0052] Universal substrate

[0053] Therefore, according to one aspect, the present invention proposes to define a universal substrate 20 including a cavity. Such a universal substrate 20 may include a carrier 21 and a surface layer 23 arranged on the carrier. An example of the universal substrate 20 has been Figure 2a The cross-sectional view and Figure 2b The top view of the carrier 21 is schematically shown. It may also include an intermediate layer 22 between the main surface of the carrier and the surface layer. The carrier 21 includes a concave cavity 24 opened on its main surface. The portion of the surface layer 23 suspended above the cavity 24 of the carrier 21 forms a flexible membrane.

[0054] The universal substrate 20 can be manufactured very easily using a process of the "bond and thin" type. Such a method comprises providing a carrier 21 and forming a cavity 24 on a main face of the carrier 21, for example by wet or dry etching, and if necessary patterning a resist previously deposited on the main face of the carrier 21 using a photolithography step, thereby masking areas to be protected from corrosion or opening areas to be etched.

[0055] The method may also comprise providing a donor substrate and bonding it to the carrier 21 to cover the recessed cavities 24. The carrier 21 and / or the donor substrate may be previously provided with a layer, such as a dielectric layer, on the surface to be bonded, thereby forming a buried intermediate layer 22 of the substrate 20. The layer or layers formed on the carrier 21 may be deposited before or after etching the recessed cavities 24 into the surface of the carrier. Thus, according to a variant, it is possible to coat the walls defining these cavities.

[0056] In a subsequent step, the donor substrate is thinned, which can be done mechanically, chemically or by a fracture level with a preformed weakening plane in the donor substrate, for example by injecting gaseous substances such as helium and / or hydrogen. The thickness of the surface layer 23 can be from 100 nm to tens of micrometers, or even hundreds of micrometers.

[0057] In some cases, the bonding step can be carried out in a controlled atmosphere bonding chamber, for example so as to have a defined vacuum pressure and / or a specific gas state. The atmosphere present in the cavity, in particular its vacuum level and / or the nature of the gas filling the cavity, is thereby controlled.

[0058] Preferably, the bonding step uses molecular bonding, i.e., the clean and smooth bonding surfaces of the carrier 24 and the donor substrate adhere to each other by van der Waals forces or covalent forces without applying adhesives. Prior to their assembly, the adhesion is enhanced by heat treatment at a temperature of 100° C. to 1000° C. or above, or by plasma or chemical treatment of the surfaces required for contact to promote adhesion.

[0059] Variants of this process can be envisaged. By way of example and known per se, the cavities 24 can be formed after the step of bonding the donor substrate and the carrier and after the step of thinning the donor substrate. A common approach used in the field of manufacturing micromechanical devices is based on the use of a silicon-on-insulator (SOI) substrate in which these cavities 24 are etched. An exploitable property is that silicon and the buried insulator can be selectively etched, thereby creating cavities 24 in the buried insulator 22 of the SOI substrate. Alternatively, the holes enabling the selective etching of the cavities 24 can be plugged after their formation, thereby making them in particular sealed.

[0060] Advantageously, for reasons of availability and cost, the carrier 21 and the donor substrate consist of silicon wafers. Thus, they can be round wafers, the diameter of which can be 200 mm, 300 mm or more. However, they can equally be wafers of other materials, or any form of material other than wafers. The material of the donor substrate will likely be specifically selected so that the surface layer 23, in particular the portion of the layer suspended over the cavity 24 and forming the membrane, has a determined stiffness, thereby allowing these membranes to bend when they are placed in a controlled manner under pressure. In the same way, it can be provided to select the properties of any intermediate layer 22 to vary the stiffness of the membrane. It can also be selected to form the intermediate layer from a dielectric (e.g. silicon dioxide or silicon nitride) so that the surface layer 23 is electrically insulated from the carrier 21.

[0061] Provision may be made for alignment marks to be arranged in the surface layer 23 or on the side of the universal substrate 20 so that the position of the cavities is precisely known and the electrodes and other layers required to complete the manufacture of the transducer (more generally a micromechanical element) can be placed in perfect alignment with these cavities 24, thereby making the element functional.

[0062] The general feature of the substrate 20 is that the shape, size and arrangement of the cavities within the substrate are not specific to one particular product or one particular application.

[0063] Therefore, if Figure 2b As shown in the example of , a universal substrate may include a plurality of cavities 24 which are uniformly distributed over the entire extent of the substrate 20 and which all have the same shape and the same size. The expression "the entire extent of the substrate" is understood to mean at least one central area of ​​the main surface of the substrate, which may not include areas peripheral to this area. In this example, all cavities have a circular shape in the plane of the substrate, with a diameter equal to 40 μm and a depth of 0.2 μm, and are uniformly distributed at the intersections of a grid, the generatrix of which is mutually perpendicular. The distance between the center of a cavity and the center of other adjacent cavities is equal to 100 μm.

[0064] Figure 2c A top view of a second example of a universal substrate 20 is shown. In this second example, the cavities 24a, 24b, 24c are arranged in groups, and the cavity groups are evenly distributed on the surface of the substrate 20. In the example shown, each group of cavities includes three circular cavities 24a, 24b, 24c arranged adjacent to each other, and their diameters are 20μm, 40μm and 60μm respectively. The other parameters of the substrate 20 are the same as those of the first example.

[0065] More generally, a common substrate may have multiple groups of cavities, each group of cavities having different characteristics (shape, size, etc.). The groups of cavities may be arranged in spaced areas of the substrate surface, or, conversely, each group of cavities may be arranged side by side with each other. Each group of cavities may be used to form a micromechanical element having a determined function. For example, a first group of cavities may have cavity characteristics that make its cavities particularly suitable for forming a transmitting transducer, while a second group of cavities may have cavity characteristics that make its cavities particularly suitable for forming a receiving transducer. In a second example embodiment, a first group of cavities may have cavity characteristics that make its cavities particularly suitable for forming a low-frequency transducer, and a second group of cavities may have cavity characteristics that make its cavities particularly suitable for forming a high-frequency transducer.

[0066] In the context of the present invention, it is conceivable to provide a plurality of types of common substrates 20. For example, it may be provided to create Figure 2a and 2b An example of is a set of types of generic substrates of the model, in which the diameters of the cavities 24 are respectively selected to be 5 μm, 20 μm, 40 μm and 60 μm.

[0067] More generally, the set of substrate types have pre-established characteristics that distinguish them from one another.

[0068] Each type of universal substrate 20 requires a specific manufacturing method that must be adjusted to take into account the size of the cavity 24 and the relative extent of the area where the contacts are placed. However, since the number of universal substrates that form a collection is limited, it is relatively easy to predefine these methods and implement them according to the type of substrate required.

[0069] Of course, the invention is by no means limited to the example of a universal substrate 20 just given. It is possible to envisage cavities with different shapes, for example square, rectangular or even hexagonal, or cavities gathered with different depths or arranged differently on the carrier surface. Although this is particularly advantageous and can limit constraints during the design of the device, in particular, the arrangement does not have to be regular or cover the entire extent of the universal substrate.

[0070] In any case, the invention provides for the creation of a limited number (eg 1, 3, 10 or 20) of generic substrate types, the characteristics of which are also precisely predefined.

[0071] Design methods for acoustic components

[0072] According to another aspect, the invention proposes a design process for designing a device comprising a micromechanical element. In the example described below, the functional micromechanical element is a micromechanical ultrasonic transducer and the device is an acoustic device. The method aims to build a model of the device so that it can then be manufactured on a universal substrate. As a supplement, it can also establish the various steps that must be performed to manufacture an acoustic device from a universal substrate. Of course, the design process will take into account the selected capacitive or piezoelectric properties of the transducer.

[0073] As described above, such a transducer comprises: a flexible membrane suspended over a cavity, which can be hermetically closed and contains a controlled atmosphere; and two electrodes, which are placed on either side of the membrane and / or the cavity. Moreover, the electrodes of the transducer can be arranged in other ways. In particular, a group of pairs consisting of a cavity / a connected flexible membrane, arranged side by side and forming one or more transducers can be addressed by a single pair of electrodes placed at the ends of the group. It is also conceivable to arrange these electrodes differently, for example to cross them on a given face of the membrane, in particular when the membrane comprises piezoelectric material. The transducers are all distributed on a plane and in a determined topological structure. For the sake of clarity, it is assumed that the planar shape and size of the transducer correspond exactly to the shape and size of the membrane forming the transducer.

[0074] The design of an acoustic device comprises the following steps: In this step, the topology of a plurality of transducers, i.e. micromechanical elements each comprising a flexible membrane, is defined. Thus, there may be the following problems: the number and size of the transducer blocks, the distribution of the transducers in each block, the distance separating two adjacent transducers, and the shape and size of each transducer inside each block are determined according to the target application field and the expected performance level of the device.

[0075] According to the invention, the step of defining is performed so that the defined topology has properties that are compatible with the universal substrate, i.e. a topology in which the flexible membrane of the micromechanical element can be connected to the cavity of the universal substrate. In other words, the step of defining requires that the selected topology complies with a set of design rules to ensure the compatibility of the topology with the universal substrate. It is thus ensured that at the end of the method, the designed acoustic device can be manufactured from this universal substrate.

[0076] Therefore, care will be taken in this step to ensure that the shape and dimensions (diameter, length, width and / or depth) of the membrane of the transducer do correspond to the shape and dimensions of the cavity of the common substrate.

[0077] Care should also be taken to ensure that the arrangement of the transducers in the topological plane is such that each transducer can be placed perpendicular to the cavity when projected onto the surface of the common substrate. However, not all cavities of the common substrate necessarily correspond to a transducer, and some cavities of the common substrate may not be used for acoustic devices.

[0078] Therefore, when the common substrate includes groups of cavities of various sizes, as is the case in the second example of the common substrate ( Figure 2c ), the topology of the acoustic device can be limited to using only cavities of one specific size. Alternatively, as described in the previous example, multiple groups of cavities may be used to form transducers with different characteristics.

[0079] For example, the latter rule might require the following elements:

[0080] - In each block of the transducers, the transducers are evenly distributed in a plane and arranged at nodes of a grid whose generatrixes are perpendicular to each other;

[0081] - The distance separating two adjacent blocks corresponds to a multiple of the distance separating two transducers.

[0082] It is thus ensured that the distribution is not incompatible with a generic substrate of the type described with reference to FIG. 2 , ie a substrate whose cavities are uniformly distributed in the plane of the substrate.

[0083] The design method is of course not limited to this topology definition step. It includes other steps, such as the definition of the membrane thickness, the definition of the geometry and routing of the electrodes in the various blocks, the definition of the electrical contact areas around these blocks, and any other design steps conventionally performed to make the micromechanical elements functional and to produce a complete model of the acoustic device.

[0084] The determined topology may differ from the desired topology, which may result directly from the functional analysis of the device being designed. For example, the cavities of the transducer need to have a size and shape such that these cavities correspond to the shape and size of the cavities of the generic substrate, which may result in a difference between the resonant frequency of the membrane and the frequency desired in the first step of the design method.

[0085] In order to limit the impact of such differences, the method of the invention may include a step of adjusting certain parameters of the acoustic device being designed. Of course, this adjustment step may only concern those parameters that do not cause compatibility problems of the topology with the common substrate. The method of the invention may also provide for the introduction of a supplementary step in the manufacture of the acoustic device for adjusting certain parameters of the device during its manufacture.

[0086] For example, it is conceivable to vary the thickness of the membrane during the manufacture of the device, thereby adjusting the resonant frequency of the transducer. It is thus possible to compensate for any frequency differences related to the shape and / or size of the cavity that has to be used due to a common substrate. This adjustment can be made to the membrane of a specific transducer, a group of transducers or all the transducers of an acoustic device, by thickening or thinning. The material of the thickening layer can be chosen according to the effect on the dynamic behavior of the membrane.

[0087] In order to limit the impact of the differences, it may also be provided that a sufficient number of different generic substrates are provided, which are well distributed over the design-selection space. In this case, for any desired topology, there may be the following generic substrates, which require that the determined topology is close enough to the desired topology to minimize the impact on the performance and function of the acoustic device.

[0088] The design process for designing an acoustic device is naturally implemented by means of computing devices. These devices may be configured to facilitate the definition of the determined topology, for example by automatically determining the determined topology from the desired topology. To this end, the method must access preset characteristics of one or more generic substrates through available computing devices.

[0089] In case of a collection or group of available generic substrates, the method can search for and select the most suitable generic substrate in the group and propose the determined topology that is most similar to the desired topology, i.e., the topology that has the least impact on the function and / or performance of the device.

[0090] Manufacturing method for manufacturing acoustic device

[0091] According to another aspect, the invention also relates to a manufacturing method for manufacturing an acoustic device, and a device manufactured using the method. As mentioned above, at least some steps of the manufacturing method may have been defined in the design process itself, at least in terms of its main parameters.

[0092] The manufacturing method includes providing an example of a universal substrate, which is selected from a universal substrate group during the design process. Generally, multiple devices can be manufactured on the universal substrate using wafer scale processing. It is conceivable that the universal substrate has cavities with pre-established characteristics, and the cavities are respectively connected to the flexible membranes suspended thereon.

[0093] According to this aspect of the present description, the manufacturing method includes at least one processing step in which only certain pairs of pairs formed by a membrane and a cavity are activated or deactivated to form at least one functional micromechanical element, and at least one non-functional redundant pair, i.e., the redundant pair cannot convert the movement of its membrane into an electrical signal, and vice versa.

[0094] Some of these processing steps are performed only on those membranes that are designated in the topology as being required to form the common substrate of the transducer. Thus, as described above, the processing steps do not necessarily result in the formation of transducers from all cavities present in the common substrate. Therefore, again referring to Figure 1c , the regions separating two adjacent transducer tiles will not be processed in some of these processing steps, even though they are overhanging the cavity of the common substrate.

[0095] In other words, in the manufacturing method, some of the pairs formed by a cavity and a membrane of a common substrate are activated to form functional micromechanical elements, i.e. elements capable of converting the movement of the membrane into electrical signals or vice versa. Thus, the acoustic device will not use one or more redundant cavity-membrane pairs that are not activated or switched off.

[0096] Thus, in one embodiment, redundant cavity-membrane pairs of the common substrate are removed, ie pairs that were not selected at the design stage. This removal can be achieved by a processing step in which the relevant cavity-membrane assembly is completely or partially etched away.

[0097] In another embodiment, redundant cavity-membrane pairs of the common substrate, i.e. pairs not selected at the design stage, are cancelled without having to eliminate any trace of them. This cancellation can be achieved through a processing step and can take many forms: the flexible membrane can be blocked, the interconnecting wiring can be cut, the operating point can be shifted relative to the resonant frequency, or even the membrane can be punctured, etc.

[0098] A processing step may also include a step of adjusting the thickness of the surface layer so as to adjust the thickness of the film suspended over the cavity. The thickness of the adjusted film of the film can be obtained by thickening or thinning the surface layer. Using conventional photolithography techniques that can define the area to be thinned or thickened, this thinning or thickening can be performed locally on the film of a specific transducer or a group of transducers, or on all films of the device. Thinning is preferably achieved by the following etching steps: solution chemical etching, gaseous or plasma chemical etching, sacrificial oxidation or physical etching by ion beam sputtering. Thickening is preferably achieved by the deposition of the same material (usually silicon) as the material that originally formed the film. When it is an epitaxial case, the deposited layer can be single crystalline. However, it can also be polycrystalline or amorphous. Other materials can also be used to ultimately produce a heterogeneous suspended structure: for example, SiO2, Si3N4 or metal is deposited on an initial film made of single crystal silicon. Thickening can also be achieved by converting part of the thickness of the original silicon film into SiO2 by oxidation, which oxidation is accompanied by the effect of swelling the layer, which effect is well known and well characterized by those skilled in the art. Other variations in thickness and adjustments of thickness are easy to carry out in several steps of the production process for manufacturing an acoustic device: for example during the formation of the piezoelectric layer, or even during the manufacture of the electrodes.

[0099] The processing step may further comprise the step of producing an array of conductive elements configured to electrically connect certain cavity-membrane pairs and enable the corresponding micromechanical elements to function. The production of the array of conductive elements may comprise forming electrodes, in particular electrodes perpendicular to a portion of the cavity.

[0100] Depending on whether the transducer employed is capacitive or piezoelectric, the manufacturing process will include additional processing steps required to complete the fabrication of the transducer.

[0101] Thus, it may comprise the step of depositing at least one first electrode on the surface layer, and in the case of a piezoelectric transducer, may comprise the step of depositing a piezoelectric layer on the first electrode, and the step of depositing a second electrode on the piezoelectric layer.

[0102] In the case of a capacitive transducer, an aperture may be required to allow placement of the second electrode on the carrier.

[0103] At the end of the manufacturing process, an acoustic device is obtained, which comprises a carrier and a surface layer placed on a main face of the carrier. The carrier has a concave cavity, and the part of the surface layer overhanging the cavity forms a flexible membrane connected to the cavity. Only a part of the pair formed by a cavity of a common substrate and a flexible membrane is configured to form a set of functional micromechanical elements, i.e. elements capable of converting the movement of the membrane and electrical signals to each other.

[0104] As seen above, non-activated cavities and / or membranes in the common substrate, such as those arranged in the area separating the two transducer blocks, can be preserved, offset or removed, for example by wet or dry etching of the surface layer, the intermediate layer (if present) or even part of the carrier. Thus, the non-activated flexible membrane and the cavity suspended thereon are removed.

[0105] It is also possible to use certain unused areas of the substrate that still contain cavities as channels for cutting the substrate to separate devices manufactured by wafer-scale processing methods of common substrates.

[0106] Of course, the invention is not limited to the embodiments and examples described and variant embodiments may be derived therefrom without departing from the scope of the invention as defined in the claims.

[0107] The invention is in particular not limited to the design or manufacture of acoustic components comprising micromachined ultrasound transducers.

[0108] For example, there may be a problem of transducers being able to operate in a frequency range other than the ultrasonic frequency range. It is therefore conceivable to form a loudspeaker or microphone array. Each transducer can then be addressed individually or in packs, which is particularly advantageous in the field of high-fidelity acoustics or directional acoustics.

[0109] The invention is more generally applicable to any device comprising a plurality of micromechanical elements using membranes or other suspended structures arranged in an array in a planar and specific topology and capable of being processed to form a functional micromechanical element.

[0110] Thus, for example, there can be the problem of a micromechanical element that defines a reflective area in order to form a controllable mobile array of micromirrors. Such a microsystem can be produced for the purpose of forming a light beam, enabling the realization of new projection systems, screens and remote identification systems. In this case, a flexible film of a universal substrate forms the carrier of the micromirrors, which becomes movable and controllable after being functionalized in a topology determined at the design stage. In this case, the cavity corresponds to the void created in the substrate so that the micromirrors can perform a reciprocating movement. The micromirrors can be fixed in several ways: by their entire or partial perimeter, by a single central foot, or on the contrary by a plurality of distributed feet. The activation or on the contrary deactivation of only part of the micromirrors can be achieved in many ways: simple removal by selective etching; releasing or on the contrary blocking the micromirrors by removing or forming blocking points, respectively; forming or on the contrary removing the reflective layer by the micromirrors; forming or on the contrary removing the absorbing layer by the micromirrors.

Claims

1. A method for manufacturing a device comprising a plurality of micromechanical elements, the method comprising: The step of providing a universal substrate (20), the universal substrate comprising a surface layer (23) arranged on a carrier, the main surface of the carrier (21) having a concave cavity (24), and a portion of the surface layer suspended above the cavity forming a flexible membrane connected to the cavity; At least one processing step for processing only some of the pairs formed by a membrane and a cavity to form at least one functional micromechanical element and at least one redundant pair that is unable to convert the movement of its membrane into an electrical signal and vice versa, wherein the processing step includes removing or offsetting the membrane and / or cavity of the at least one redundant pair.

2. The manufacturing method according to claim 1, wherein: At least one processing step includes the step of producing an array of conductive elements configured to connect only a portion of the cavity-membrane pairs.

3. The manufacturing method according to claim 2, wherein: The step of creating an array of conductive elements includes forming electrodes perpendicular to only a portion of the cavities.

4. The manufacturing method according to claim 1, comprising the step of adjusting the thickness of the flexible film.

5. The manufacturing method according to claim 1, wherein: The universal substrate (20) further comprises an intermediate layer (22) between the main surface of the carrier (21) and the surface layer (23).

6. A manufacturing method as claimed in claim 1, comprising the step of depositing at least one first electrode on the surface layer (23).

7. A manufacturing method as claimed in claim 6, comprising the step of depositing a piezoelectric layer at least on the first electrode.

8. The manufacturing method of claim 7, comprising the step of depositing a second electrode on the piezoelectric layer.

Images