CMUT transducer and manufacturing process of a CMUT transducer

Abstract

CMUT Transducer and Method for Manufacturing a CMUT Transducer This description relates to a method for manufacturing a CMUT transducer, comprising the following steps: a) forming a first layer of silicon oxide (103) on one face of a first silicon layer (101) defining a first electrode of the transducer; b) forming a second layer of silicon oxide (111) on one face of a second silicon layer (113); c) after step a), forming, on the side of said face of the first silicon layer (101), by localized oxidation of the silicon of the first silicon layer (101), walls of silicon oxide (107); and d) after steps b) and c), transfer and fix the assembly comprising the second silicon layer (113) and the second silicon oxide layer (111) onto the assembly comprising the first silicon layer (101), the first silicon oxide layer (103), and the silicon oxide walls (107). Figure for the abbreviation: Fig. 11.

Description

Title of the invention: CMUT transducer and method of manufacturing a CMUT transducer Technical field

[0001] The present description relates generally to the field of ultrasonic transducers, and, more particularly, to that of capacitive ultrasonic membrane transducers, also called CMUT transducers (from the English "Capacitive Micromachined Ultrasonic Transducer"). Prior art

[0002] Conventionally, a CMUT transducer comprises a flexible membrane suspended above a cavity, a first electrode, called the lower electrode, located on the side of the cavity opposite the membrane, and a second electrode, called the upper electrode, located on the side of the cavity opposite the first electrode and mechanically secured to the flexible membrane. In operation, a direct current (DC) bias voltage is applied between the electrodes. When an appropriate alternating current excitation voltage, superimposed on the DC bias voltage, is applied between the electrodes, the flexible membrane vibrates under the effect of the variation in the electrostatic force exerted between the electrodes, resulting in the emission of an ultrasonic acoustic wave.Conversely, when the transducer receives an ultrasonic acoustic wave, the flexible membrane begins to vibrate under the effect of the variation in mechanical pressure, leading to the appearance, between the lower and upper electrodes of the transducer, of an alternating voltage superimposed on the direct polarization voltage, due to the variation in capacitance between the electrodes.

[0003] A CMUT transducer is conventionally coupled to an electronic control circuit configured to, during a transmission phase, apply between the electrodes of the transducer an alternating excitation voltage superimposed on a direct bias voltage, so as to cause the emission of an ultrasonic acoustic wave by the transducer, and, during a reception phase, apply between the electrodes of the transducer a direct bias voltage and read between said electrodes an alternating voltage generated under the effect of a received ultrasonic acoustic wave.

[0004] The emission frequency of a CMUT transducer is generally linked to its resonance frequency, which depends on various parameters and in particular on the geometric and mechanical characteristics of the membrane and the cavity, as well as on the external environment.

[0005] It would be desirable to have a CMUT transducer and a method for manufacturing such a transducer, overcoming all or part of the disadvantages of trans- CMUT transducers and known CMUT transducer manufacturing methods. Summary of the invention

[0006] One embodiment provides a method of manufacturing a CMUT transducer, comprising the following steps: a) forming a first layer of silicon oxide on one face of a first layer of silicon defining a first electrode of the transducer; b) forming a second layer of silicon oxide on one face of a second layer of silicon; c) after step a), forming, on the side of said face of the first silicon layer, by localized oxidation of the silicon of the first silicon layer, silicon oxide walls having a height greater than the thickness of the first silicon oxide layer, said walls laterally delimiting a cavity of the transducer; and d) after steps b) and c), transferring and fixing the assembly comprising the second silicon layer and the second silicon oxide layer onto the assembly comprising the first silicon layer, the first silicon oxide layer and the silicon oxide walls, so as to close the cavity of the transducer, said cavity extending vertically from the face of the first silicon oxide layer opposite the first silicon layer to the face of the second silicon oxide layer opposite the second silicon layer.

[0007] According to one embodiment, in step a), the first layer of silicon oxide is formed by thermal oxidation in dry growth of said face of the first layer of silicon and, in step b), the second layer of silicon oxide is formed by thermal oxidation in dry growth of said face of the second layer of silicon.

[0008] According to one embodiment, step c) comprises a step of depositing a layer of silicon nitride on the face of the first layer of silicon oxide opposite the first layer of silicon, followed by a step of localized etching of the layer of silicon nitride and the first layer of silicon oxide at the desired locations of the silicon oxide walls, followed by a step of thermal oxidation to form the walls of silicon oxide, followed by a step of removing the layer of silicon nitride.

[0009] According to one embodiment, the removal of the silicon nitride layer is carried out by wet etching.

[0010] According to one embodiment, in step d), the assembly comprising the second silicon layer and the second silicon oxide layer is fixed to the assembly comprising the first silicon layer, the first silicon oxide layer and silicon oxide walls by direct bonding.

[0011] According to one embodiment, the direct bonding implemented in step d) comprises annealing at a temperature between 700 and 1100°C.

[0012] According to one embodiment, the direct bonding implemented in step d) is a bonding of the face of the second silicon oxide layer opposite the second silicon layer to the face of the silicon oxide walls opposite the first silicon oxide layer.

[0013] According to one embodiment, the first silicon layer is a fixed substrate and the second silicon layer is a flexible membrane of the transducer.

[0014] According to one embodiment, the thickness of the first layer of silicon oxide is substantially equal to the thickness of the second layer of silicon oxide.

[0015] According to one embodiment, the method further comprises steps of forming, on the face of the first silicon layer opposite the first silicon oxide layer, contact metallizations of the transducer, and a step of connecting said contact metallizations to an integrated circuit for controlling the transducer.

[0016] According to one embodiment, the first silicon layer is doped.

[0017] According to one embodiment, the method comprises a training step, on a face of the second silicon layer opposite the second silicon oxide layer, a metal layer defining a second electrode of the transducer.

[0018] Another embodiment provides a CMUT transducer comprising: - a first silicon layer defining a first electrode of the transducer - a first layer of silicon oxide arranged on and in contact with the upper face of the first layer of silicon; - localized walls of silicon oxide extending vertically higher than the upper face of the first layer of silicon oxide and partially penetrating into the first layer of silicon, said walls laterally delimiting a cavity of the transducer; - a second layer of silicon oxide closing the cavity by its upper face, the cavity extending vertically from the upper face of the first layer of silicon oxide to the lower face of the second layer of silicon oxide; and - a second layer of silicon arranged on and in contact with the upper face of the second layer of silicon oxide. Brief description of the drawings

[0019] These characteristics and advantages, as well as others, will be explained in detail in the following description of particular embodiments given without limitation in relation to the attached figures among which:

[0020] [Fig.lA], [Fig.lB], [Fig.lC], [Fig.lD], [Fig.lE], [Fig.lF], [Fig.lG], [Fig.lH] and [Fig.II] are sectional views illustrating successive steps of an example of a method for manufacturing CMUT transducers according to one embodiment;

[0021] [Fig.2A], [Fig.2B], [Fig.2C], [Fig.2D] and [Fig.2E] are sectional views illustrating successive steps of another example of a method of manufacturing CMUT transducers according to one embodiment;

[0022] [Fig.2F] is a sectional view illustrating an alternative embodiment of the device of [Fig.2E]; and

[0023] [Fig.3] [Fig.3] is a sectional view illustrating another alternative embodiment of the device of [Fig.2E]. Description of the embodiments

[0024] The same elements have been designated by the same references in the different figures. In particular, the structural and / or functional elements common to the different embodiments may have the same references and may have identical structural, dimensional and material properties.

[0025] For the sake of clarity, only the steps and elements useful for understanding the described embodiments have been shown and are detailed. In particular, the various applications that the described transducers may have have not been detailed, the described embodiments being compatible with the usual applications of ultrasonic transducers, in particular in ultrasonic imaging devices. Furthermore, the control circuits of the described transducers have not been detailed, the described embodiments being compatible with all or most of the known CMUT transducer control circuits.

[0026] In the present description, unless otherwise specified, a CMUT transducer is a device composed of one or more CMUT transduction elements arranged according to the requirements of the application. Each CMUT transduction element consists of one or more CMUT transduction elementary cells electrically connected to each other, for example in parallel. Each CMUT elementary cell comprises, for example, a single flexible membrane suspended above a cavity, and two opposite electrodes adapted to receive an electrical excitation signal to vibrate the membrane and / or to generate an electrical response signal under the effect of a vibration of the membrane.

[0027] Unless otherwise specified, when referring to two elements connected to each other, this means directly connected without intermediate elements other than conductors, and when referring to two elements connected (in English "coupled") to each other, this means that these two elements can be connected or be connected by through one or more other elements.

[0028] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "upper", "lower", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made unless otherwise specified to the orientation of the figures.

[0029] Unless otherwise specified, the expressions "about", "approximately", "substantially", and "of the order of" mean to within 10%, preferably to within 5%.

[0030] Figures 1A to 11I are sectional views illustrating successive steps of an example of a method for manufacturing CMUT transducers according to one embodiment. In Figures 1A to 11I, the production of a single elementary CMUT transducer cell has been shown. In practice, a large number of cells can be produced simultaneously from the same starting substrate.

[0031] [Fig.1A] is a sectional view illustrating the structure obtained at the end of a step of forming, on a starting silicon substrate 101, a layer of silicon oxide 103 then a layer of silicon nitride 105. The substrate 101 is preferably relatively heavily doped. For example, the substrate 101 is a silicon substrate with a doping level of between 1013 and 1018 atoms / cm3. The thickness of the substrate 101 is for example between 30 μm and 1 mm, for example between 400 and 800 μm. The substrate 101 corresponds for example to a silicon wafer or to a portion of a silicon wafer. It will be noted that the substrate 101 may optionally be thinned at the end of the process.

[0032] The silicon oxide layer 103 is formed on and in contact with the upper face of the substrate 101. The layer 103 is for example formed by dry growth thermal oxidation so as to obtain a high-quality oxide. The layer 103 extends for example continuously and with a substantially uniform thickness over the entire upper surface of the substrate 101. The thickness of the layer 103 is for example between 20 and 300 nm, for example between 100 and 150 nm, for example of the order of 125 nm.

[0033] The silicon nitride layer 105 is for example deposited on and in contact with the upper face of the layer 103. The layer 105 extends for example continuously and with a substantially uniform thickness over the entire surface of the substrate 101. The layer 105 is for example deposited by chemical vapor deposition, for example by LPCVD (Low Pressure Chemical Vapor Deposition), which has the advantage of not degrading the quality of the underlying silicon oxide layer 103. As a variant, the layer 105 can be deposited by PECVD (Plasma Enhanced Chemical Vapor Deposition), or by any other suitable deposition method. The thickness of the layer 105 is for example between 50 and 500 nm, for example between 100 and 300 nm, for example of the order of 200 nm.

[0034] [Fig. 1B] illustrates the structure obtained at the end of a localized etching step of the stack formed by the layers 103 and 105. In this example, the layers 103 and 105 are kept only opposite the future cavities of the CMUT transducers. The layers 103 and 105 are for example etched by a dry etching process, for example by plasma etching. At the end of the etching, the upper face of the substrate 101 is exposed around the future cavities of the CMUT transducers.

[0035] [Fig.lC] illustrates the structure obtained at the end of a localized oxidation step of the silicon of the substrate 101, also called LOCOS (LOCal Oxidation of Silicon) process. During this step, thermal oxidation of the portions of the upper face of the substrate 101 exposed at the end of the etching step of [Fig.lB] is carried out. This leads to a growth of silicon oxide walls 107 opposite the exposed portions of the upper face of the substrate 101, i.e. around the future cavities of the CMUT transducers. The silicon nitride layer 105, on the other hand, blocks the oxidation at the locations of the future cavities of the substrate 101. The height of the silicon oxide walls 107 defines the thickness or depth of the future cavities of the CMUT transducers.More particularly, in this example, the depth of the future cavities of the CMUT transducers corresponds to the distance between the upper face plane of the silicon oxide walls 107 and the upper face plane of the silicon oxide layer 103, it being understood that the upper face plane of the oxide walls 107 is located above the upper face plane of the oxide layer 103. An advantage of the LOCOS method is that the height of the walls 107, and therefore the depth of the cavities, can be precisely controlled by controlling in particular the temperature and the oxidation time. This is particularly advantageous for the production of shallow cavities, for example less than 100 nm deep, for example of the order of 50 nm deep. The embodiments described are not, however, limited to shallow cavities. For example, the depth of the cavities of the CMUT transducers is between 10 nm and 1 μm.

[0036] As an illustrative, non-limiting example, the silicon oxide layer has a thickness of the order of 125 nm and the silicon oxide walls 107 protrude from the level of the upper face of the substrate 101 by a height of the order of 175 nm, so as to obtain a cavity depth of the order of 50 nm. It will be noted that part of the thickness of the substrate is consumed and transformed into silicon oxide during oxidation. For example, to obtain walls 107 protruding by approximately 175 nm relative to the upper face plane of the substrate 101, a thickness of the order of 137.5 nm of the substrate silicon is consumed during oxidation, leading to 107 walls with a total height of around 312.5 nm (137.5 + 175 nm).

[0037] [Fig. 1D] illustrates the structure obtained at the end of a step of removing the silicon nitride layer 105. The layer 105 is removed by a process of selective etching of the silicon nitride relative to the silicon oxide. The layer 105 is preferably removed by wet etching, which makes it possible to preserve the quality of the underlying silicon oxide layer 103. In particular, this makes it possible, compared to a dry etching process, to avoid the formation of microcracks on the upper face of the silicon oxide layer 103. At the end of this step, only the silicon oxide walls 107 laterally surrounding the cavities 109 of the CMUT transducers, the silicon oxide layer 103 whose upper face defines the bottom of the cavities 109, and the underlying substrate 101 are retained.

[0038] [Fig. 1E] illustrates the structure obtained at the end of a transfer and fixing step, on the side of the upper face of the structure of [Fig. 1D], of a stack comprising, in order from the upper face of the structure of [Fig. 1D], a silicon oxide layer 111, a silicon layer 113, a silicon oxide layer 115, and a support layer 117, for example made of silicon. By way of example, each of the layers 111, 113, 115 and 117 extends continuously and with a substantially uniform thickness over the entire surface of the structure of [Fig. 1D]. The layers 111, 113, 115 and 117 are for example substantially planar. In this example, the substrate 117 is in contact, by its lower face, with the upper face of the layer 115, the layer 115 is in contact, by its lower face, with the upper face of the layer 113, and the layer 113 is in contact, by its lower face, with the upper face of the layer 111.Layer 117 corresponds, for example, to a silicon wafer or a portion of a silicon wafer.

[0039] The stack of layers 111, 113, 115 and 117 is formed separately, then transferred and fixed to the upper face of the structure of [Fig.1D].

[0040] For example, the stack of layers 117, 115 and 113 is a SOI (Silicon On Insulator) type structure, layer 117 constituting the support substrate of the SOI structure, layer 115 being the buried silicon oxide layer of the SOI structure, and layer 113 being the active monocrystalline silicon layer of the SOL structure. The thickness of substrate 117 is for example between 10 μm and 1 mm, for example between 400 and 800 μm. The thickness of silicon oxide layer 115 is for example between 50 nm and 2 μm. The thickness of silicon layer 113 is for example between 0.5 and 5 μm. Silicon layer 113 is preferably relatively heavily doped. For example, the doping level of the 113 layer is between 1013 and 1018 atoms / cm3. Alternatively, the 113 layer may be non-intention- heavily doped.

[0041] The silicon oxide layer 111 is for example formed on and in contact with the lower face (in the orientation of [Fig.lE]) of the silicon layer 113, before transferring the stack 111-113-115-117 onto the upper face of the structure of [Fig.lD]. The layer 111 is for example formed by thermal oxidation in dry growth, for example under the same conditions as the layer 103 ([Fig.lA]). The thickness of the layer 111 is for example between 20 and 300 nm, for example between 100 and 150 nm, for example of the order of 125 nm. Preferably, the thickness of the layer 111 is substantially equal to the thickness of the layer 103.

[0042] The stack of layers 111, 113, 115 and 117 is then transferred and fixed to the upper face of the structure of [Fig. 1D]. The fixing of the stack is preferably carried out by direct bonding or molecular bonding of the lower face of the silicon oxide layer 111 on the upper face of the silicon oxide walls 107, without the addition of intermediate material.

[0043] To improve the quality of the bonding, annealing of the structure at a relatively high temperature can be provided, for example at a temperature between 700 and 1100°C. This is called fusion bonding.

[0044] At the end of this step, the cavities 109 of the CMUT transducers are hermetically closed. The lower face of the silicon oxide layer 111 defines the upper face of the cavities 109. The bonding can be carried out under vacuum so as to obtain cavities 109 having a pressure lower than atmospheric pressure.

[0045] At the end of these steps, the substrate 117 and the buried silicon oxide layer 115 can be removed. The silicon layer 113 is retained and forms the membrane of the CMUT transducers. The layer 113 can furthermore, when doped, form, in part, the upper electrode of the CMUT transducers. A conductive layer, for example metallic (not visible in [Fig.lG], corresponding to the layer 129 in the example of [Fig.lH]), can furthermore be deposited on and in contact with the upper face of the semiconductor layer 113, and form in part the upper electrode of the transducers.

[0046] In this example, the substrate 101 forms the lower electrode of the CMUT transducers. Various steps of reconnecting the electrodes of the CMUT transducers and of electrically isolating the electrodes of the CMUT transducers can also be implemented. FIGS. 1F to II illustrate a non-limiting example of implementing such steps.

[0047] [Fig. 1F] illustrates the structure obtained at the end of a step of thinning the substrate 101, by its lower face, and of forming contact recovery elements on the side of the lower face of the thinned substrate 101.

[0048] For example, the substrate 101 is thinned by grinding using the layer 117 as a handle. At the end of the thinning step, the thickness of the substrate 101 is for example between 10 and 150 μm, for example between 20 and 100 μm.

[0049] After thinning, isolation trenches 121 are formed from the lower face of the substrate 101, opposite the silicon oxide walls 107 of the CMUT transducers. The trenches 121 extend vertically through the substrate 101, over the entire thickness of the substrate 101, and open onto the lower face of the silicon oxide walls 107.

[0050] More particularly, in this example, for each elementary cell of CMUT transducer, or for each element of CMUT transducer, an isolation trench 121, for example of annular shape, is formed on the periphery of the cell or of the element, for example on the periphery of the cavity 109 of the cell (on the right side of the cavity in the example shown), laterally delimiting a zone 123 of the substrate 101 intended to be electrically connected to the upper electrode of the transducer. In this example, the zone 123 is entirely surrounded and electrically isolated from the rest of the substrate 101 by the isolation trench 121.

[0051] The trench 121 is for example filled with an electrically insulating material, for example silicon oxide. As a variant, the side walls of the trench 121 are coated with an electrically insulating material, for example silicon oxide, then the trench is filled with an electrically insulating material, for example undoped polycrystalline silicon or silicon oxide.

[0052] [Fig. 1F] further illustrates the formation of contact metallizations on the lower face of the substrate 101. More particularly, in this example, for each CMUT transducer or for each CMUT transducer element, two separate contact metallizations 125a and 125b are formed on and in contact with the lower face of the substrate 101. The metallization 125a is located at the periphery of the cavity and is in contact only with the lower face of the zone 123 of the substrate 101 electrically insulated from the rest of the substrate by the trench 121. The metallization 125b is located opposite the cavity 109 and extends for example over the majority of the surface of the cavity 109, for example over substantially the entire surface of the cavity 109. The metallization 125b is not in contact with the zone 123 of the substrate 101.

[0053] It will be noted that in the example shown, each CMUT transducer element comprises two cavities 109 (corresponding for example to two elementary transduction cells) excited simultaneously and separated laterally by a silicon oxide wall 107. The embodiments described are not limited to this particular example. As a variant, each CMUT transducer may comprise a single cavity 109 or a number of cavities 109 greater than 2.

[0054] [Fig. IG] illustrates the structure obtained at the end of a step of removing the support layer 117 and the silicon oxide layer 115 from the structure of [Fig. 1F], so as to expose the upper face of the silicon layer 113, forming the membrane of the CMUT transducers.

[0055] [Fig. 1H] illustrates the structure obtained at the end of a step of resuming an electrical contact on the membrane (and upper electrode) 113 of each CMUT transducer.

[0056] More particularly, in this example, an opening 127 extending vertically through the membrane 113, the silicon oxide layer 111 and the silicon oxide wall 107, and opening onto the upper face of the zone 123 of the substrate 101 delimited laterally by the trench 121, is formed in each CMUT transducer from the upper face of the structure of [Fig. 1G], in line with the zone 123 of the substrate 101 delimited laterally by the trench 121.

[0057] A metallization 129 is then formed extending over and in contact with the upper face of the membrane 113, opposite the cavity 109 of the transducer, for example over the majority of the surface of the cavity 109 or over substantially the entire surface of the cavity 109. The metallization 129 also extends over the sides and at the bottom of the opening 127. In particular, the metallization 129 comes into contact with the upper face of the zone 123 of the substrate 101 delimited laterally by the trench 121. Thus, the metallization 129 is electrically connected to the lower metallization 125a of the transducer via the zone 123 of the substrate 101. The metallization 129 is, on the other hand, electrically insulated from the rest of the substrate 101. Thus, the metallizations 125a and 125b are electrically connected to the upper electrode and the lower electrode of the CMUT transducer respectively.For example, to form the metallizations 129, a metal layer is first deposited full plate, over the entire upper surface of the structure, this layer then being etched locally to electrically isolate the electrodes of the different transducers from each other. In the case where the semiconductor layer 113 is doped, the etching can be extended through the layer 113, so as to electrically isolate the electrodes of the different transducers from each other.

[0058] [Fig.II] illustrates the structure obtained at the end of a step of transferring and fixing the structure of [Fig.1H] on an electronic control circuit 150. The electronic circuit 150 is for example an integrated circuit previously formed in and on a semiconductor substrate, for example a silicon substrate. The electronic circuit 150 is for example produced in CMOS technology (from the English “Complementary Metal Oxide Semiconductor”). The electronic circuit 150 is for example configured to, during an emission phase, apply between the electrodes of each transducer an alternating excitation voltage superimposed on a direct bias voltage, so as to cause the emission of an ultrasonic acoustic wave by the transducer, and, during a phase of reception, applying a DC bias voltage between the electrodes of each transducer and reading between said electrodes an AC voltage generated under the effect of a received ultrasonic acoustic wave. The substrate in and on which the circuit 150 is integrated corresponds, for example, to a silicon wafer or a portion of a silicon wafer. Thus, in this example, the transfer implemented during this step is a wafer-to-wafer transfer. We also speak of wafer-level packaging or assembly, or WLP.

[0059] In the example shown, the electronic circuit 150 comprises, on its upper face side, for each CMUT transducer of the structure of [Fig. 1H], two connection metallizations 151a and 151b intended to be connected respectively to the connection metallizations 125a and 125b of the transducer.

[0060] In this example, during the transfer, each metallization 125a of the structure of [Fig. 1H] is brought into contact, by its lower face, with the upper face of a corresponding metallization 151a of the electronic circuit 150, and each metallization 125b of the structure of [Fig. 1H] is brought into contact, by its lower face, with the upper face of a corresponding metallization 151b of the electronic circuit 150. The fixing of the metallizations 125a, 125b to the metallizations 150a, 150b can be a fixing by direct bonding, by thermocompression, by eutectic bonding, by means of a layer of solder, solder pads, solder balls, or by any other known means of fixing and electrical connection of contact metallizations.

[0061] As a variant, not shown, to improve the mechanical strength of the assembly during the various steps of the method, the fixing of the integrated control circuit 150 on the CMUT transducer structure can be carried out after the steps of [Fig. 1F] and before the step of removing the support layer 117 ([Fig. 1G]).

[0062] The structure of [Fig. II] can then be cut into a plurality of individual chips each comprising one or more CMUT transducers, for example a matrix of CMUT transducers, and an electronic circuit for controlling the CMUT transducer(s) of the chip. The cutting is for example carried out by sawing, for example in a manner similar to that illustrated in the example described below of [Fig. 2D],

[0063] Figures 2A to 2E illustrate another example of implementation of contact recovery steps on the CMUT transducers produced by the method of Figures 1A to 1E.

[0064] The method of Figures 2A to 2E differs from the method of Figures 1F to H mainly in that, in the method of Figures 2A to 2E, the assembly of the transducers and the electrical circuit for controlling the transducers is carried out at the scale of the individual chip, and not at the scale of the substrate or wafer (before cutting into individual chips) as in the method of Figures 1F to H.

[0065] The method of FIGS. 2A to 2F comprises steps identical or similar to those described above in relation to [Fig. 1F] of thinning the substrate 101, of forming insulation trenches 121 delimiting connection zones 123 of the substrate, and of forming contact metallizations 125a, 125b on the lower face of the substrate 101.

[0066] [Fig.2A] illustrates more particularly a step of transferring and fixing the structure of [Fig.1F] on an interconnection structure 210.

[0067] The interconnection structure 210 is for example formed in and on a semiconductor substrate 211, for example a silicon substrate. The substrate 211 is preferably relatively heavily doped. For example, the substrate 211 is a silicon substrate with a doping level of between 1013 and 1018 atoms / cm3. The thickness of the substrate 211 is for example between 30 μm and 1 mm.

[0068] The interconnection structure 210 comprises, for each CMUT transducer of the structure of [Fig. 1F], two contact metallizations 213a and 213b arranged on and in contact with the upper face of the substrate 211 and intended to be connected respectively to the contact metallizations 125a and 125b of the transducer. The interconnection structure 210 further comprises, for each CMUT transducer of the structure of [Fig. 1F] two contact metallizations 215a and 215b arranged on and in contact with the lower face of the substrate 211, for example vertically above the contact metallizations 213a and 213b respectively. The interconnection structure 210 further comprises vertical isolation trenches 217, for example similar to the trenches 121 of the structure of [Fig. 1F], extending over the entire thickness of the substrate 211.The trenches 217 are arranged so that, for each CMUT transducer, the connection metallizations 213a and 215a associated with the transducer are electrically connected to each other by an area of ​​the substrate 211 and are electrically isolated from the contact metallizations 213b and 215b of the transducer as well as from the contact metallizations 213a, 215a, 213b, 215b of the other transducers.

[0069] The interconnection structure 210 can be produced separately, then transferred and fixed on the lower face of the structure of [Fig. 1F]. The transfer is for example carried out at the scale of the substrate or wafer (WLP).

[0070] In this example, during the transfer, each metallization 125a of the structure of [Fig.lF] is brought into contact, by its lower face, with the upper face of a corresponding metallization 213a of the interconnection structure 210, and each metallization 125b of the structure of [Fig.lH] is brought into contact, by its lower face, with the upper face of a corresponding metallization 213b of the interconnection structure 210. The fixing of the metallizations 125a, 125b to the metallizations 213a, 213b can be a fixing by direct bonding, by thermocompression, by means of a layer of solder, solder pads, solder balls, or by any other known means of fixing and electrical connection of contact metallizations.

[0071] [Fig.2B] illustrates the structure obtained at the end of a step of removing the support layer 117 and the silicon oxide layer 115 from the structure of [Fig.2A], so as to expose the upper face of the silicon layer 113, forming the membrane of the CMUT transducers.

[0072] [Fig.2C] illustrates the structure obtained at the end of a step of resuming an electrical contact on the membrane (and upper electrode) 113 of each CMUT transducer. This step is similar to what was described above in relation to [Fig.1H],

[0073] [Fig.2D] illustrates a step of cutting the structure of [Fig.2C] into a plurality of individual chips each comprising one or more CMUT transducers, for example a matrix of CMUT transducers, and a corresponding portion of the interconnection structure 210. The cutting is for example carried out by sawing.

[0074] Each individual chip can then be attached and electrically connected to an electronic control circuit, for example an integrated circuit chip having lateral dimensions smaller than those of the transducer.

[0075] [Fig.2E] illustrates the structure obtained at the end of a step of transferring and fixing a transducer chip obtained at the end of the step of [Fig.2D], on an electronic control circuit chip 250 previously formed from a semiconductor substrate, for example a silicon substrate. The electronic circuit 250 is for example produced in CMOS technology. In the example shown, the electronic circuit 250 comprises, on its upper face side, for each transducer CMUT of the transducer chip, two connection metallizations 251a and 251b intended to be connected respectively to the connection metallizations 215a and 215b of the transducer chip.

[0076] The metallizations 215a, 215b of the transducer chip are connected respectively to the metallizations 251a, 251b of the control chip by any suitable connection means, for example by direct bonding, by thermocompression, by means of a solder layer, solder pads, solder balls, etc.

[0077] An advantage of the embodiment of Figures 2A to 2E is that the interconnection structure 210 can allow, to a certain extent, to adapt the arrangement of the contacts of the CMUT (125a and 125b) to different arrangements of the contacts of the CMOS (opposite the contacts 215a and 215b). Thus, for the same design of CMUT transducer, it is possible to adapt to different CMOS circuits by changing only the design of the interconnection structure 210, generally simpler to manufacture. Thus, although in the figures there are metallizations 215a, 215b and isolation trenches 217 aligned vertically with the contacts 215a and 215b, respectively, metallizations 125a, 125b and with the isolation trenches 121, in practice, the metallizations 215a, 215b and the isolation trenches 217 may not be vertically aligned with the metallizations 125a, 125b and the isolation trenches 121 respectively, as illustrated for example in [Fig.2F].

[0078] Another advantage of the interconnection structure 210 is that it makes it possible to stiffen the CMUT + interconnection structure assembly after removal of the support layer 117 and before transferring the control circuits.

[0079] An advantage of the embodiments described in relation to figures 1A to 111, 2A to 2E and 2F is that the silicon oxide layers 103 and 111 located respectively on the lower face side and on the upper face side of the cavity 109 have a high quality on the one hand because they are produced by a full-plate dry growth thermal oxidation process making it possible to obtain a high-quality silicon oxide, and on the other hand because they do not undergo any partial thickness etching or etching step during the process. This makes it possible to significantly limit the parasitic phenomena of injection and trapping of electrical charges in the layers 103 and 111, which are likely to degrade the operation of the transducers.

[0080] Preferably, the layers 103 and 111 have the same thickness or substantially the same thickness, which makes it possible to make the structure symmetrical and to promote the balancing of the distribution of the charges injected into the layers 103 and 111.

[0081] [Fig. 3] is a sectional view illustrating an alternative embodiment of the device of [Fig.2E],

[0082] In this variant, the dielectric layer 103 has been removed in one of the two cavities 109 of the CMUT transducer and retained in the other cavity. This makes it possible to obtain two different cavity heights, for example for two cavities of different shapes and / or lateral dimensions, on the same substrate. In particular, the absence of the layer 103 in one of the cavities makes it possible to benefit from a greater deflection of the membrane and thus to use a membrane with a larger surface area. This makes it possible, for example, to obtain a transducer adapted to emit simultaneously at two distinct acoustic frequencies. The localized etching of the layer 103 is for example implemented after the step of removing the silicon nitride layer 105 ([Fig. 1D]) and before closing the cavities 109 by bonding the silicon oxide layer 111 and the membrane 113 ([Fig. 1E]).

[0083] The variant of [Fig. 3] can of course be combined with the method of Figures 1F to IL

[0084] Various embodiments and variations have been described. Those skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will occur to those skilled in the art. profession. In particular, the described embodiments are not limited to the dimension examples mentioned in this description.

[0085] Furthermore, it will be noted that in the examples described above, the silicon oxide walls 107 laterally delimiting the cavities 109 of the transducers are formed by a LOCOS process on the upper face of the substrate 101, forming the lower electrode of the transducers. As a variant, the walls 107 can be formed by a LOCOS process on the lower face of the silicon layer 113, forming the membrane and the upper electrode of the transducers. In this case, the lower silicon oxide layer 103 of the structure is not etched. The silicon nitride layer 105 is then formed on the lower face (in the orientation of [Fig. 1E]) of the oxide layer 111, and the stack of the layers 111 and 105 is etched in a localized manner to be kept only opposite the future cavities 109 of the transducers.

[0086] In another variant, the LOCOS method can be implemented on the side of the lower face of the membrane and on the side of the upper face of the substrate 101, before transferring the membrane onto the substrate 101. In this case, it will be necessary to align the lower portions of the walls 107 with the upper portions of the walls 107 during the transfer.

[0087] Finally, the practical implementation of the embodiments and variants described is within the reach of those skilled in the art from the functional indications given above.

Claims

Claims

1. A method of manufacturing a CMUT transducer, comprising the following steps: a) forming a first silicon oxide layer (103) on one face of a first silicon layer (101) defining a first electrode of the transducer; b) forming a second silicon oxide layer (111) on one face of a second silicon layer (113); c) after step a), forming, on the side of said face of the first silicon layer (101), by localized oxidation of the silicon of the first silicon layer (101), silicon oxide walls (107) having a height greater than the thickness of the first silicon oxide layer (103), said walls (107) laterally delimiting a cavity (109) of the transducer;and d) after steps b) and c), transferring and fixing the assembly comprising the second silicon layer (113) and the second silicon oxide layer (111) onto the assembly comprising the first silicon layer (101), the first silicon oxide layer (103) and the silicon oxide walls (107), so as to close the cavity (109) of the transducer, said cavity (109) extending vertically from the face of the first silicon oxide layer (103) opposite the first silicon layer (101) to the face of the second silicon oxide layer (111) opposite the second silicon layer (113).;

2. A method according to claim 1, wherein, in step a), the first silicon oxide layer (103) is formed by dry growth thermal oxidation of said face of the first silicon layer (101) and, in step b), the second silicon oxide layer (111) is formed by dry growth thermal oxidation of said face of the second silicon layer (113).

3. The method of claim 1 or 2, wherein step c) comprises a step of depositing a silicon nitride layer (105) on the face of the first silicon oxide layer (103) opposite the first silicon layer (101), followed by a step of localized etching of the silicon nitride layer (105) and the first silicon oxide layer (103) at the desired locations of the silicon oxide walls (107), followed by a step of thermal oxidation to form the silicon oxide walls (107), followed by a step of removing the ... silicon nitride (105).

4. The method of claim 3, wherein the removal of the silicon nitride layer (105) is performed by wet etching.

5. A method according to any one of claims 1 to 4, wherein, in step d), the assembly comprising the second silicon layer (113) and the second silicon oxide layer (111) is fixed to the assembly comprising the first silicon layer (101), the first silicon oxide layer (103) and the silicon oxide walls (107) by direct bonding.

6. Method according to claim 5, in which the direct bonding carried out in step d) comprises annealing at a temperature between 700 and 1100°C.

7. Method according to claim 5 or 6, in which the direct bonding carried out in step d) is a bonding of the face of the second silicon oxide layer (111) opposite the second silicon layer (113) on the face of the silicon oxide walls (107) opposite the first silicon oxide layer (101).

8. A method according to any one of claims 1 to 7, wherein the first silicon layer (101) is a fixed substrate and the second silicon layer (113) is a flexible membrane of the transducer.

9. A method according to any one of claims 1 to 8, wherein the thickness of the first silicon oxide layer (103) is substantially equal to the thickness of the second silicon oxide layer (111).

10. Method according to any one of claims 1 to 9, further comprising steps of forming, on the face of the first silicon layer (101) opposite the first silicon oxide layer (103), contact metallizations (125a, 125b) of the transducer, and a step of connecting said contact metallizations to a control integrated circuit (150; 250) of the transducer.

11. A method according to any one of claims 1 to 10, wherein the first silicon layer (101) is doped.

12. Method according to any one of claims 1 to 11, comprising a step of forming, on a face of the second silicon layer (113) opposite the second silicon oxide layer (111), a metal layer (129) defining a second electrode of the transducer.

13. CMUT transducer comprising: - a first silicon layer (101) defining a first electrode of the transducer; - a first layer of silicon oxide (101) arranged on and in contact with the upper face of the first layer of silicon (101); - localized walls of silicon oxide (107) extending vertically higher than the upper face of the first layer of silicon oxide (103) and partially penetrating into the first layer of silicon, said walls (107) laterally delimiting a cavity (109) of the transducer; - a second layer of silicon oxide (111) closing the cavity by its upper face, the cavity extending vertically from the upper face of the first layer of silicon oxide (103) to the lower face of the second layer of silicon oxide (111); and - a second layer of silicon (113) arranged on and in contact with the upper face of the second layer of silicon oxide (111).

Images