FR2721471A1 - Ultrasonic transducer and method of manufacturing such a transducer - Google Patents

Abstract

An ultrasonic transducer (38) including a 0.1-0.5 mu m-thick silicon nitride membrane (4a) with an intrinsic mechanical stress of 100 MPa to 1.3 GPa, and a fixed electrode (15) made of a non-metallic rigid material and provided with a plurality of holes (25) with a small average diameter, said fixed electrode defining with said membrane an inner space 1-5 mu m thick. A method for making said ultrasonic transducer is also disclosed.

Description

The present invention relates to an ultrasonic transducer consisting of a membrane and a fixed electrode as well as to the processes for producing said membrane and fixed electrode and to the method of manufacturing said ultrasonic transducer from the membrane and the electrode. fixed.

Two major families of ultrasonic transducers are known, among which are the piezoelectric effect transducers which conventionally consist of a ceramic piezoelectric disk mounted on a substrate and provided with one or more adaptation layers. 'impedance.

The ceramic structure of such a transducer may resonate radially or according to its thickness or in combination.

These transducers are very resonant up to frequencies above 100kHz and a narrow frequency band but nevertheless offer poor performance in terms of radiated power.

The ultrasonic transducers of bimorph type constitute the second large family and these transducers generally consist of a vibrating membrane under which is disposed a piezoelectric disk.

These transducers, which are more sensitive than the aforementioned piezoelectric effect transducers, are very resonant but, on the other hand, do not make it possible to reach resonant frequencies greater than 100 kHz.

The Applicant has found that it would be interesting to be able to manufacture highly resonant ultrasound transducers with a defined resonance peak with an easily reproducible precision of a transducer to another and following an inexpensive manufacturing process. This resonance peak should be able to be obtained on the ultrasonic range and especially from 100 kHz.

The subject of the present invention is thus an ultrasonic transducer comprising a silicon nitride membrane with a thickness of between 0.1 and 0.5 cc with an intrinsic mechanical stress of between 100 MPa and 1.3 GPa, and a fixed electrode made of rigid non-metallic material and provided with a plurality of orifices of small average diameter, said fixed electrode defining with said membrane an inner space of thickness between 1 and 5 μm.

This ultrasound transducer possesses, due to its fabrication process, all the qualities required to have an adjustable ultrasonic resonance frequency, greater than 20 kHz, defined with great precision, which is moreover perfectly reproducible. This is due in particular to the low mass and high rigidity of the membrane that can be obtained by the manufacturing process.

 It is quite possible to confer on the membrane an intrinsic mechanical stress of between 100 MPa and 1.3 GPa and thus to adjust the resonant frequency of the ultrasonic transducer to a value between 20 kHz and 1 MHz.

According to other characteristics of the ultrasonic transducer: the silicon nitride membrane has a thickness preferably of between 0.3 and 0.5 μm, the fixed electrode is made of semiconductor material or ceramic, the electrode fixed at a thickness greater than 20 μm, the orifices of the fixed electrode have a mean diameter of between 25 and 100 μm.

The subject of the present invention is also a process for producing a membrane used in the manufacture of the above-mentioned ultrasonic transducer, characterized in that it consists in carrying out the following steps: a layer of electrical insulator is formed on two opposite faces called front and rear of a semiconductor material substrate, - a layer of silicon nitride is deposited on each of the two insulating layers, - is implanted ionically in the silicon nitride layer located on the front face of said substrate a material adapted to in order to reduce the intrinsic mechanical stress of said silicon nitride layer, - in order to form a silicon nitride membrane, the following steps a) to c) are carried out ::
a) selectively etching the silicon nitride and insulator layers located on the rear face of the substrate,
b) anisotropically etching said substrate from its rear face,
- c) and selectively etching the layer of insulation located on the front face of the substrate - and at least one electrical contact is made on said membrane on the rear face of the substrate.

Advantageously, this process includes a step of ion implantation of a layer of thin silicon nitride, for example of the order of 0.1 to 0.5 μm, which makes it possible to reduce the intrinsic mechanical stress of said layer .

Indeed, the ions thus implanted create defects in the silicon nitride layer, thereby inducing a relaxation of the intrinsic mechanical stress.

Ion implantation is performed with a dose of ions from a material belonging to columns III to V of the periodic table of the elements and, for example boron, between 5.1013 and 5.1015 ions / cm2 and the energy of Acceleration of these ions is between 35 and 150 keV.

The ion energy is calculated to obtain the maximum of ions at the interface between the silicon nitride layer and the underlying electrical insulator layer.

An annealing operation of the substrate coated with its ionically implanted silicon nitride layer is then carried out in order to increase the mechanical stress of said layer.

The annealing is carried out in a temperature range of 500 to 800 ° C for a period of between 15 minutes and a few hours.

Thanks to annealing, it is therefore possible to precisely adjust the ultrasound resonant frequency to the desired value.

Moreover, the annealing makes it possible to obtain a better reproducibility of the mechanical stress values in the membrane from one substrate to the other with respect to a simple ion implantation.

In addition, during the series production of several membranes on several substrates, it is possible to settle for an excessive ion implantation identical for all the substrates which will therefore greatly reduce the mechanical stresses in the silicon nitride layers and then to adjust the final mechanical stress sought by suitable annealing.

The annealing also makes it possible to improve the long-term stability of the mechanical stress in the silicon nitride layer and thus to improve the stability of the future ultrasonic transducer.

The Applicant has found that it is more advantageous to use a layer of silicon nitride (membrane) with a thickness of between 0.3 and 0.5 μm in order to reduce its fragility and thus to improve its mechanical behavior. during the following steps of formation of the membrane by etching as well as the other steps.

In addition, this range of thickness is the best possible compromise between the strength of the membrane and its sensitivity, in terms of mechanical energy conversion into acoustic energy and also conversion of acoustic energy into mechanical energy.

For example, the electrical insulating layer is formed by thermal oxidation of the two faces of the substrate and the insulating layer, for example, has a thickness of 1 μm, and this thickness may even be less than 1 μm. The realization of the electrical contact is obtained by metallization of the rear face of the substrate.

In addition, prior to the making of the electrical contact and after anisotropic etching of the bulk of the substrate, is removed by selective etching the silicon nitride and insulation layers remaining on the rear face of the substrate.

According to another embodiment of the invention, it is also possible to envisage, before depositing the silicon nitride layer on the front face of the substrate coated with a layer of electrical insulator, to etch said layer of insulation throughout its thickness so as to leave a layer of peripheral insulation.

After this operation, the deposition of the silicon nitride layer, its ionic implantation, and optionally its annealing is carried out, and then, said layer of ionic silicon nitride implanted ionically so as to leave exposed the layer of peripheral insulation which will be used to define the internal space and which will receive for example a sealing agent such as "PYREX" for the final assembly of the ultrasonic transducer.

Thus, this embodiment makes it possible to better control the formation of the internal space between the membrane and a fixed electrode.

However, in this embodiment which will be described later in more detail, the thickness of the electrical insulation layer which is formed for example by thermal oxidation of the two faces of the substrate has a thickness which must be greater than 1 bed thus allowing to predetermine the thickness of the internal space.

The present invention also relates to a method for producing a fixed electrode used in the manufacture of the aforementioned ultrasonic transducer, characterized in that it consists in performing the following steps from a semiconductor material substrate having two opposite faces. said front and rear :: - at least one protective layer is formed on the rear face of said substrate - the fixed electrode is formed, on the one hand, by selectively etching said protective layer located on the rear face of the substrate and, d on the other hand, by anisotropically etching said substrate from its rear face, after having previously protected the front face with respect to the anisotropic etching, a principal cavity located opposite said fixed electrode is etched in said substrate; at least one secondary cavity for the electrical contacts, at least one layer of electrical insulation is formed in the central cavity and in at least one a secondary cavity is formed - a metal protective layer is formed on the front face of the substrate - the said metal, insulating and fixed electrode layers are selectively etched in the said main cavity so as to form a plurality of orifices in said fixed electrode. The main cavity has a depth of between 1.5 and 2.5 μm.

The protective layer on the rear face of the substrate is made of an electrical insulator resistant to etching agents and may consist of silicon nitride but it is then preferable to perform an ion implantation of said layer with a material belonging to the columns III to V of the periodic table of the elements in order to reduce the intrinsic mechanical stress of the latter and therefore to adapt this constraint to that of the substrate.

 It is also possible to form two protective layers on the rear face of the substrate, a first layer in contact with said substrate being formed of an electrical insulator such as an oxide and a second silicon nitride layer on which it It is not necessary to carry out an ion implantation since it is not in direct contact with the substrate.

To avoid that the front face is attacked during the anisotropic etching of the substrate by the rear face, it must be protected and one can, for example, be content with a protection identical to that formed in the rear face of said substrate.

 It is possible to etch in the substrate at least two secondary cavities, one for electrical contacts of the fixed electrode and the other for making electrical contact with the substrate.

The electrical insulating layer is then formed in the main cavity and in at least one secondary cavity by deposition of an electrical insulating layer and then etching of said layer.

In particular, the layer of electrical insulation is etched in the secondary cavity intended to establish electrical contact with the substrate.

After forming a metal protective layer on the front face of the substrate on the aforementioned insulation layers, said metal protective layer is etched so as to form electrical contacts vis-à-vis the main and secondary cavities.

 It is also possible subsequently to make the electrical contact with the substrate and, on the other hand, to form the orifices of the fixed electrode before making this contact, but this further complicates the process.

The invention also relates to a method of manufacturing the above-mentioned ultrasonic transducer from the membrane and the fixed electrode respectively obtained by the above-mentioned methods. According to this method of manufacture, a sealing agent is deposited on a peripheral part of the front face of the membrane or of the fixed electrode, the membrane is positioned opposite the fixed electrode and assembled with so that they form between them an internal space of small thickness, for example between 1 and 5 lm.

The sealant used may be an adhesive, "PYREX" (registered trademark) or other suitable sealant.

When the sealing agent is "PYREX", the assembly is done by anodic sealing and it is therefore expected, before depositing the "PYREX", to remove from the front and rear faces of the fixed electrode the remaining parts the various layers having served as protection, remove from the front face of the membrane the optional protective layer and form on the unetched areas of said front face of the fixed electrode a final protective layer.

Other features and advantages of the invention will become apparent from the following description given by way of illustrative and non-limiting example and with reference to the accompanying drawings, in which: - Figures 1 to 12 are views in cross section each representing an important step in the process of producing a membrane used for the manufacture of an ultrasonic transducer according to one embodiment of the invention, - Figures 13 to 23 are cross-sectional views each representing a step of the method of producing a fixed electrode used for the manufacture of an ultrasonic transducer according to an embodiment of the invention, - Figure 24 is a cross-sectional view showing the ultrasonic transducer assembled according to the invention, - FIG. 25 represents a frequency response curve of an ultrasonic transducer according to the invention with a resonant frequency at 100 kHz.

Under usual manufacturing conditions, the production methods according to the invention obviously make it possible to simultaneously produce several membranes and several fixed electrodes from one and the same substrate for each process and thus to subsequently simultaneously manufacture several ultrasonic transducers in the form of a network of transducers.

However, in order to simplify the following description, only a membrane and a fixed electrode will be represented.

We will focus first on the realization of the membrane which will be described with reference to Figures 1 to 12.

As represented in FIG. 1, a substrate 1 of p-doped average resistivity of between 1 and 20 Ω / cm is used, and for example a monocrystalline silicon wafer whose crystalline orientation is of the <100> type of equal thickness. at 520 μm and which has two large opposite faces 1 a, lb said front and rear. This wafer 1 is cleaned in the usual manner and then is formed on each of its large opposite faces an electric insulation layer 2, 3.

For example, a thermal oxidation of the silicon is carried out for example 111m as shown in FIG.

The next step, the result of which is illustrated in FIG. 2, is to delimit the internal space which will form with the membrane and the fixed electrode a variable capacity.

To do this, a layer of photosensitive resin (not shown) is deposited, for example by spinning, on the oxide layer 2 on the front face of the wafer 1, an annealing is carried out at 600.degree. photogravure (not shown), with said wafer, this resin layer is insulated through said mask, the insolated resin layer is developed with the mask, another annealing is carried out at 120 ° C., etching of the film layer is carried out oxide 2 in the central portion using an etchant such as hydrofluoric acid in dilute solution to form the inner space.

Then, all that remains is to remove the remaining photosensitive resin layer and then clean the surfaces in a conventional manner.

A peripheral zone 2a of the oxide layer defining at its center the internal space is then obtained.

The next step (FIG. 3) of the process consists in depositing a layer of silicon nitride 4, between 0.1 and 0.5 μm and for example approximately 0.3 μm on each of the two oxidized faces. partially oxidized platelet 1.

This deposition is carried out for example by a known technique of low pressure chemical vapor deposition (LPCVD).

An ion implantation, in the silicon nitride layer 4 situated on the front face of the wafer, of a material belonging to columns III to V of the periodic table of elements, such as, for example, boron, in order to reduce the intrinsic mechanical stress of said silicon nitride layer (FIG. 4).

It is also possible to use another suitable material for the ion implantation of oxygen.

For this implantation, a dose of ions of between 5 × 10 13 and 5 × 10 15 ions / cm 2 and for example equal to 2 × 10 15 ions / cm 2 is used.

The ion acceleration energy is between 35 and 150 kev and is for example equal to 100 kev. The choice of the ion dose and the acceleration energy is made according to their size and mass.

For example, using the Blister method described in the article by SD Senturia, MG Allen and M. Mehregany, "Microfabricated structures for the in-situ measurements of residual stress, Young's modulus, and ultimate strain of the films". , Appl. Phys. Lett. 51, (4), (1992), 241-243, the intrinsic mechanical stress of the silicon nitride layer which provides a value less than 100 MPa.

As previously explained, it may be desired to adjust the ultrasonic resonance frequency accurately and this is accomplished by annealing in an oven at a temperature of 600 ° C for 30 minutes.

 It should be noted that the annealing thus makes it possible to increase the intrinsic mechanical stress of the silicon nitride and to obtain with great precision this stress and hence the resonant frequency of the future ultrasonic transducer, which is very useful if the The stress has been too strongly reduced during the ion implantation or if the stress approaches by a value lower than the desired value but with insufficient precision.

Thanks to the annealing, it is therefore not necessary to master the ion implantation step.

When the intrinsic mechanical stress of the silicon nitride has been adjusted to the desired value which will make it possible to obtain precisely the desired resonance ultrasound frequency, for example 400 MPa, the membrane is then structured from the face back.

For this purpose, a photoresist layer (not shown) is deposited by spinning on the rear face of the wafer, ie on the outer face of the layer 5, an annealing is carried out at 600.degree. photogravure (not shown) with the wafer, this resin layer is insulated through said mask, develops and performs another annealing at 12000.

Next, the silicon nitride layer 5 is etched selectively by reactive ion etching (RIE) in the presence of sulfur hexafluoride and oxygen ions (dry etching), and then the oxide layer 3 by wet etching. in 1: 7 HF / NH4F solution.

The remaining photosensitive resin layer is then removed and the surfaces are conventionally cleaned.

As shown in FIG. 5, a peripheral zone formed of a portion 5a of the silicon nitride layer and a part 3a of the oxide layer leaving on a central zone the rear face 1b of the plate 1.

A photoetching is then carried out on the front face of the wafer so as to etch the layer 4 of silicon nitride to reveal the peripheral zone 2a of the oxide layer 2.

This photogravure is identical to that described above and comprises the following steps: - deposition of a layer of photoresist on the front, - annealing at 60 "C, - exposure through a mask, - development, - annealing at 1200C.

The silicon nitride layer 4 is etched by reactive ion etching in the presence of sulfur hexafluoride ions and oxygen, then the remaining photosensitive resin layer is removed and the surfaces are conventionally cleaned (FIG. 6). .

As shown in FIG. 7, a layer 6 of * PYREX 7740® (registered trademark) of between 1 and 3 cm and for example 2 μm is deposited by cathodic deposition or evaporation under vacuum.

A photoetching is then carried out on the front face of the wafer 1 so as to etch the layer 6 of "PYREX" so as to reveal the remaining central zone 4a of the layer 4 of ionically implanted silicon nitride and thus form a peripheral zone 6a of the layer of "PYREX *" (Figure 8).

This photogravure is carried out according to the steps explained above: - deposition of a layer of photoresist on the front face, - annealing at 60 ° C, - exposure through a mask, - development, - annealing at 1200C.

The layer 6 of "PYREX" is then etched (wet etching) in a dilute HF solution.

 It should be noted, however, that if the "PYREX" is not used as a sealing agent, the step illustrated in FIG. 2 which consists of preforming the internal space by structuring a peripheral zone 2a of the oxide layer 2 on the front face of the wafer 1.

In this case, the sealing agent which may be an adhesive and for example a polyimide or an epoxy resin, is deposited after having made the membrane and the fixed electrode, just before assembly.

Another variant could consist in preforming the internal space by structuring a peripheral zone 2a of the oxide layer 2 in the front face of the wafer 1 as already described, but not in depositing the PYREX layer until after having made the membrane and the fixed electrode.

For this variant, it is however necessary to engrave this layer of "PYREX" before considering the sealing of the membrane and the fixed electrode.

Now, we will focus on the structuring of the silicon nitride membrane which is made from the rear face lb of the wafer as shown in Figures 9 to 11.

According to a first step (FIG. 9), the silicon wafer 1 is etched anisotropically from its back face 1b, using a solution containing the KOH etching agent diluted to 30% and brought to a temperature of the order of 70 to 90 ° C.

As shown in FIG. 9, the wafer 1 is etched to obtain a thickness of silicon, for example 5 μm, before reaching the central zone 4a of the silicon nitride layer 4.

During this step, it is possible to protect, for example mechanically with a glass plate, the front face of the wafer.

Rinsing is then carried out with deionized water and the possible protection on the front face is removed and the surfaces are cleaned in a conventional manner.

A second step (FIG. 10) then consists in eliminating from the rear face 1b of the wafer the peripheral zones 3a and 5a of the respective layers of oxide and silicon nitride. This step can be carried out by selective etching, on the one hand, by reactive ionic etching in the presence of hexafluoride ions of zone 5a of the silicon nitride layer and, on the other hand, by wet etching in a solution.
HF / NH4F1: 7 of the zone 3a of the oxide layer 3.

 It should be noted that it is preferable to carry out this step when the entire thickness of silicon has not been etched as just described so as not to damage the membrane and more particularly the central zone 4a of the layer. 4.

However, this step can be performed after total silicon etching but it is more delicate for the reasons explained above.

FIG. 11 shows the third step of structuring the membrane, which consists in performing another anisotropic etching in a solution containing the 30% diluted KOH etching agent and brought to a temperature of 80.degree. C. from the rear face lb already partially etched to remove the last remaining thickness of silicon and thus clear the central zone 4a of the layer 4.

Then the surfaces are rinsed with deionized water and cleaned in a conventional manner.

We can of course protect as already specified the front face of the wafer 1 during this third step.

 It then remains only to make at least one electrical contact on the structured rear face of the wafer 1.

To do this, it metallizes, for example, the entire rear face of the wafer as shown in Figure 12 with a layer 7 formed of an alloy of nickel, chromium and gold about 0.05 lm, chromium being disposed in contact with the rear face of the wafer about 0.005 Rm.

This metallization layer 7 may be carried out by cathodic deposition or by evaporation under vacuum.

The method of producing the fixed electrode will now be described with reference to FIGS. 13 to 23.

As shown in FIG. 13, a p-doped average resistivity substrate 10, between 1 and 20 Ω / cm, and for example a monocrystalline silicon wafer whose crystalline orientation is of the <100> type, of thickness equal to 5201um and which has two large opposite faces 10a and 10b said front and rear.

This wafer 10 is cleaned in the usual manner and then at least one protective layer consisting of an electrical insulator resistant to etching agents is formed on the rear face 10b of said wafer. For example, two protective layers are formed on each face 10a, 10b of the wafer.

A first layer 11, (remainder), is formed by thermal oxidation of the silicon on the front face 10a (or on the rear face 10b) on a thickness of between 0.1 and 0.3 μm and for example equal to 0.2 um.

A second layer 13, (resp.14) of thickness between 0.1 and 0.3 clam, for example equal to 0.2 μm, is formed on the layer 11 (resp.12) by chemical vapor deposition at low pressure (Fig. 13).

As shown in FIG. 14, the first step of producing the fixed electrode is carried out by selective etching of the silicon nitride and oxide protection layers 12 located on the rear face 10b of the wafer 10.

For this purpose, a photoetching of the rear face 10b is carried out by carrying out the steps already described in detail during the production of the membrane: depositing a layer of photosensitive resin on the front face, annealing at 60.degree. sunstroke through a mask, - development, - annealing at 1200C.

The silicon nitride layer 14 is etched by reactive ion etching (RIE) in the presence of sulfur hexafluoride and oxygen ions (dry etching), and then the oxide layer 12 is etched by wet etching in an HF / NH4F solution. 1: 7.

The remaining photosensitive resin layer is then removed and the surfaces are conventionally cleaned.

FIG. 14 shows that the protective layers have been etched in their central part in order to disengage the central part of the rear face 10b of the wafer which will undergo anisotropic etching.

The respective remaining portions 12a and 14a of the protective layers 12 and 14 form a peripheral zone.

The next step is to anisotropically etch the silicon wafer 10 from its back side 10b with a solution containing an etching agent such as KOH diluted to 30% and brought to a temperature of 75 ° C.

The silicon is etched to a silicon thickness of 50 m thus forming the fixed electrode (FIG 15).

Rinsing with deionized water is then carried out and then a conventional cleaning of the surfaces.

Obviously, it is possible to protect the surface located on the front face of the wafer with a glass plate during the etching KOH.

Subsequently, a main cavity 10 having a depth of between 1.5 and 2.5 μm and for example equal to 1 μm located opposite the fixed electrode 15 is formed in the manner to be described in detail below, and at least one secondary cavity 17 for electrical contacts.

A second secondary cavity 18 is likewise formed in order to make electrical contact with the front face 10a of the wafer 10. Thus, a photogravure is carried out on the front face of the wafer 10 according to the conventional steps: of a layer of photosensitive resin on the front face, - annealing at 60 ° C, - exposure through a mask, - development, - annealing at 1200C.

The silicon nitride layer 13 is etched by reactive ion etching (RIE) in the presence of sulfur hexafluoride ions and oxygen (dry etching), then the oxide layer 11 is etched by wet etching in a solution. HF / NH4F1: 7.

A small thickness of about 2 μm of the silicon wafer is then etched by reactive ion etching (RIE) in the presence of sulfur hexafluoride ions and oxygen. The remaining photosensitive resin layer is removed and the surfaces are conventionally cleaned.

As shown in FIG. 16, a central cavity 16 and two secondary cavities 17 and 18 are thus obtained.

The following step makes it possible to obtain the result illustrated in FIG. 17 and consists in forming at least one layer of electrical insulator 20 with a thickness of between 0.5 and 2.5 μm in the central cavity 16 and in at least one secondary cavity and for example equal to 1 .mu.m.

For example, an insulation layer 21, 22 is formed in each of the two secondary cavities 17 and 18.

The insulating layer having a thickness of 1 cm is, for example, obtained by thermal oxidation at 1100 ° C. of the silicon located at the bottom of the main and secondary cavities 17, 18 at 1000 ° C. in a dry atmosphere for 1 hour, then in an atmosphere. wet for 1 hour 30 and finally in a dry atmosphere for 1 hour.

As shown in FIG. 18, the oxide layer 20 in the central cavity 16 is etched to form a plurality of holes 23 with a small average diameter of between 25 and 100 KLm and for example equal to 50 μm. etches the oxide layer 22 located in the second secondary cavity 18 in order to make electrical contact with the wafer 10.

For this, the following steps are carried out: deposition of a layer of photosensitive resin in front face 10a, annealing at 60 ° C., exposure through a mask, development, annealing at 1200 ° C.

etching the oxide layers 20, 22 with a solution
HF / NH4F1: 7 - removal of the remaining photoresist layer - conventional surface cleaning.

FIG. 19 illustrates the following step of forming a metal protective layer 24 on the front face 10a of the wafer.

This layer 24, for example aluminum with a thickness equal to 1 μm, is deposited on the front face by cathodic deposition or evaporation under vacuum.

The metal layers 24 and oxide 20 are then photogravure in the conventional manner: - depositing a layer of photosensitive resin on the front face, - annealing at 60 ° C, - exposure through a mask, - development, annealing at 1200 ° C., etching in phosphoric acid of the aluminum and of the oxide located in line with the holes 23 formed in the step corresponding to FIG. 18 so as to reform these holes (FIG. 20); rinsing with deionized water, - removal of the remaining photosensitive resin layer and conventional cleaning.

In the next step, the result of which is illustrated in FIG. 21, a plurality of orifices 25 are formed in the fixed electrode by performing a dry etching of the silicon of the fixed electrode throughout its thickness. (about 50 clam) by reactive ionic etching in the presence of sulfur hexafluoride ions.

The penultimate step (Fig. 22) consists in carrying out a photoetching of the metal protective layer 24 so as to form electrical contacts facing the main 16 and secondary cavities 17 and 18.

This step is carried out as follows: depositing a layer of photosensitive resin on the front face, annealing at 60 ° C., exposure through a mask, development, annealing at 1200 ° C., etching of aluminum in phosphoric acid, - rinsing with deionized water, - removal of the remaining photosensitive resin layer and conventional cleaning.

Electrical contacts 30 are thus obtained in the secondary cavity 17, electrical contacts 31 in the main cavity 16 between the holes 23 in direct contact with the face 10a of the wafer 10.

The remaining parts 12a and 14a of the protective layers 12 and 14 on the rear face and the remaining parts of the protective layers 11 and 13 on the front face of the wafer 10 are then removed in the last step (FIG.

Both sides of the wafer 10 are etched selectively with silicon nitride 14 by reactive ion etching in the presence of sulfur hexafluoride and the oxide layer by wet etching in 1: 7 HF / NH 4 F solution.

In the embodiment which has just been described and where the layer 6 of "PYREX" has been deposited during the formation of the membrane, the ultrasonic transducer 38 is then manufactured (FIG. previously conventionally the two respective front faces of the membrane and the fixed electrode, by aligning said membrane and said fixed electrode, ie by positioning the membrane 4a vis-à-vis the fixed electrode 15 and putting them in contact with each other so that they form between them an internal space 40 of small thickness.

Anodic sealing of the membrane and the fixed electrode is then carried out as described in the article "Silicon-to-silicon anodic with a borosilicate glass layer", Anders Hanneborg,
Martin Nese and Per Ohlckers, J. Micromech. Microeng.1 (1991) 139144.

The internal space, after sealing, has a thickness of between 1 and 5 μm and for example equal to 3 μm.

The fact of precisely obtaining an internal space of small thickness makes it possible to obtain a good efficiency in terms of conversion of electrical energy into mechanical energy and vice versa.

Advantageously, because of this small thickness and the small membrane thickness, an ultrasonic transducer is obtained which has a very good efficiency and therefore consumes relatively little electrical energy.

As already mentioned, it is also possible to assemble the membrane and the fixed electrode by means of another sealing agent such as a polyimide which is then deposited for example manually on the membrane or the fixed electrode. .

The membrane and the fixed electrode are then aligned as described above and secured by heating at a temperature of 300 ° C.

FIG. 25 represents a frequency response curve of an ultrasonic transducer according to the invention revealing a perfectly defined resonance peak at a frequency value of 100 kHz.

This curve is typically obtained for an ultrasonic transducer having a membrane of thickness equal to 0.3 m of width equal to 1 mm, the intrinsic mechanical stress (after annealing) is 200
MPa and which has an internal space of 3 μm thick.

 It should be noted that the method according to the invention makes it possible not only to adjust the resonance frequency of the ultrasonic transducer but also to adjust the bandwidth of said transducer according to the intended application.

Claims (37)

Ultrasonic transducer (38), characterized in that it comprises  a membrane (4a) made of silicon nitride of thickness  between 0.1 and 0.5um with intrinsic mechanical stress  between 100 MPa and 1.3 GPa,  a fixed electrode (15) made of non-metallic rigid material  and provided with a plurality of orifices (25) of small average diameter,  said fixed electrode defining with said membrane a space  internal thickness between 1 and 5 Am. Ultrasonic transducer according to claim 1, characterized in that  the silicon nitride membrane (4a) has a thickness of  preferably between 0.3 and 0.5 lim. Ultrasonic transducer according to claim 1 or 2, characterized  in that the intrinsic mechanical stress of the membrane in  silicon nitride is preferably between 100 and 600 MPa. Ultrasonic transducer according to one of claims 1 to 3,  characterized in that the fixed electrode is made of material  semiconductor. Ultrasonic transducer according to one of claims 1 to 3,  characterized in that the fixed electrode (15) is made of material  ceramic. Ultrasonic transducer according to one of claims 1 to 6,  characterized in that the fixed electrode (15) has a thickness  greater than 20 lim. Ultrasonic transducer according to one of claims 1 to 6,  characterized in that the orifices (25) of the fixed electrode (15) have a  average diameter between 25 and 100 μm. 8. Process for producing a membrane used in the manufacture  an ultrasonic transducer (38), characterized in that it consists of  to perform the following steps:  a layer of electrical insulation (2, 3) is formed on two sides  opposing so-called front (la) and rear (lob) of a substrate (1) in  semiconductor material,  a layer of silicon nitride (4, 5) is deposited on each  two layers of insulation (2, 3),  - Is implanted ionically in the silicon nitride layer  located on the front face (4) of said substrate a suitable material for  to reduce the intrinsic mechanical stress of said layer  silicon nitride,  in order to form a silicon nitride membrane (4a),  perform the following steps a) to c) ::  a) the layers of silicon nitride (5) are selectively etched  of insulation (3) located on the rear face of the substrate,  b) anisotropically etching said substrate (1) from its  back side (1b),  c) and selectively etching the insulating layer (2) located in  front face of the substrate  and at least one electrical contact (7) is made on said  membrane on the back of the substrate. 9. Method according to claim 8, characterized in that one forms  the layer of electrical insulation (2, 3) by thermal oxidation of  two faces of the substrate (1). Method according to claim 8 or 9, characterized in that  each layer of insulation (2, 3) has a thickness greater than 111m. 11. The method of claim 8, characterized in that each  silicon nitride layer (4, 5) has a thickness of between  0.1 and 0.5 μm. Method according to claim 11, characterized in that each  silicon nitride layer (4, 5) has a thickness preferably  between 0.3 and 0.5 m. 13. Method according to one of claims 8 to 12, characterized in that  that the suitable material belongs to columns III to V of the  periodic classification of the elements and is, for example, the Boron. 14. Process according to claims 8 and 13, characterized in that  the ion implantation is performed with a dose of ions included  between 5.1 o13 and 5.1015ionsicm2. 15. Method according to claim 13 or 14, characterized in that  ion implantation is performed with acceleration energy  ions between 35 and 150 keV. 16. Method according to one of claims 8 to 15, characterized in that  that annealing of the substrate (1) is carried out consecutively to  ion implantation to adjust the intrinsic stress of the  silicon nitride layer (4) at a predetermined value  desired between 100 MPa and 1.3 GPa and preferably  between 100 and 600 MPa. 17. Process according to claim 16, characterized in that the annealing  is carried out at a temperature between 500 and 8000C  for a period ranging from 15 minutes to several hours. 18. Method according to one of claims 8 to 17, characterized in that  that the realization of the electrical contact (7) is obtained by  metallization of the rear face (lob) of the substrate. 19. Method according to one of claims 8 to 17, characterized in that  than prior to the deposition of the silicon nitride layer (4)  on the front face (la) of the substrate coated with an insulation layer (2)  said insulation layer is etched all the way through  leaving a layer of peripheral insulation (2a). 20. Process according to claims 8 and 19, characterized in that one  structure the ionically implanted silicon nitride layer (4)  so as to leave clear the layer of peripheral insulation (2a). 21. Method according to one of claims 8 to 20, characterized in that  than prior to making an electrical contact (7) on the  rear face (1b) of the substrate and after anisotropic etching of the  most of the substrate, selective etching is removed.  layers of silicon nitride (5) and insulation (3) which remain on  said rear face. 22. A method of producing a fixed electrode used in the  manufacturing an ultrasonic transducer (38), characterized in that  that it consists of performing the following steps from a substrate  (10) semiconductor material having two opposite faces said  front (10a) and rear (10b) ::  at least one protective layer is formed on the rear face  (10b) of said substrate (10)  the fixed electrode (15) is formed, on the one hand, by selectively etching  said protective layer located on the rear face of the substrate (10)  and, on the other hand, by anisotropically etching said substrate (10) to  from its back, after having previously protected the face  before (10a) with respect to anisotropic etching,  - a main cavity (16) located in  view of said fixed electrode (15) and at least one cavity  secondary for electrical contacts,  at least one layer of electrical insulator (20) is formed in the  main cavity and in at least one secondary cavity,  a metal protective layer (24) is formed on the front face  (10a) of the substrate (10),  - selectively gravitates to the right of said main cavity said  metal protection layers (24), insulation and fixed electrode  (15) so as to form a plurality of orifices (25) in said  fixed electrode. 23. The method of claim 22 characterized in that the layer  protection consists of an electrical insulator resistant to  engraving agents. 24. The method of claim 22 or 23 characterized in that the  protective layer is made of silicon nitride. 25. Process according to claims 22 and 24, characterized in that  implanted ionically in the nitride protective layer  of silicon a material belonging to columns III to V of the  periodic classification of the elements in order to reduce the  intrinsic mechanical stress of said nitride layer  silicon. 26. Process according to claim 22, characterized in that one forms  two protective layers (12, 14) on the back side of the substrate  (10), a first layer (12) of an electrical insulator in contact  with said substrate and a second silicon nitride layer (14). 27. Method according to one of claims 22 to 26, characterized in that  that the front face (10a) of the substrate (10) is protected from the  anisotropic etching by forming at least one protective layer  on said front face. 28. Method according to one of claims 22 to 27, characterized in that  that the main cavity (16) has a depth of between 1.5 and  2.5 tm. 29. Method according to one of claims 22 to 28, is engraved in the  substrate (10) at least two secondary cavities (17, 18), one  intended for electrical contacts (31) of the fixed electrode (15) and  The other (18) provided for making an electrical contact (32) with the  substrate (10). 30. Method according to one of claims 22 to 29, characterized in that  that the layer of electrical insulation (20) is formed in the cavity  in at least one secondary cavity by depositing a  layer of electrical insulation and then etching said layer. 31. Process according to claims 29 and 30, characterized in that one  etches the layer of electrical insulation (22) into the secondary cavity  (18) for establishing an electrical contact (32) with the substrate  (10). 32. Method according to one of claims 22 to 31, characterized in that  that the metal protective layer (24) is etched so as to  forming electrical contacts (30, 31, 32) opposite the cavities  principal (16) and secondary (17, 18). 33. A method of manufacturing an ultrasonic transducer (38) from  of the membrane (4a) and the fixed electrode (15) respectively  made according to one of claims 8 to 21 and according to one of  22 to 32, according to which an agent of  sealing on a peripheral portion of the front face of said  membrane or of said fixed electrode, the membrane is positioned  towards the fixed electrode and assemble them so that they  that they form between them an internal space (40) of weak  thickness. 34. The method of claim 33, characterized in that the space  internal (40) thus formed has a thickness of between 1 and 5 clam. 35. Process according to claim 33 or 34, characterized in that  the sealant is a glue. 36. The method of claim 33 or 34, characterized in that  the sealant is "PYREX". 37. Process according to claims 26 and 36, characterized in that  prior to the deposit of the sealant,  front and rear faces of the fixed electrode (15) the remaining parts  layers used as protection, we remove from the front face of the  membrane (4a) the possible protective layer and form a  other protective layer on the unetched areas of the said  front face of the fixed electrode.