FR3074908A1 - ACOUSTIC PROBE INTENDED FOR USE IN A SILICON SOLIDIFICATION FURNACE AND METHOD FOR MANUFACTURING THE SAME - Google Patents

Abstract

The invention relates to a method for manufacturing an acoustic probe (10) comprising a waveguide (20) whose free end (25) is partly delimited by a transmission face (22) intended to transmit a wave acoustic signal in a bath of molten silicon (5), comprising the following steps: - supply of the waveguide (20) whose free end (25) is made of a carbonaceous material (26) with zero open porosity, suitable for react with liquid silicon to form silicon carbide; contacting the free end (25) of the waveguide (20) in liquid silicon, so that a layer of silicon carbon (27) is formed by reaction between the carbonaceous material (26) ) and the liquid silicon and defines the transmission face (22).

Description

ACOUSTIC PROBE FOR USE IN A SILICON SOLIDIFICATION OVEN AND METHOD FOR MANUFACTURING SAME

TECHNICAL FIELD [ooi] The field of the invention is that of acoustic probes intended to be used in a solidification furnace containing a bath of molten silicon. Such acoustic probes can in particular be used in the context of monitoring the progress of solidification of silicon.

STATE OF THE PRIOR ART [002] Crystalline silicon for photovoltaic applications can be obtained in the form of ingots by directed solidification processes implemented using solidification ovens. Acoustic probes can be used to monitor the progress of the solidification of the silicon, or even to ensure the mixing of the molten silicon bath.

Figures 1A and 1B schematically illustrate an example of an acoustic probe îo used in a solidification furnace 1 for stirring a molten silicon bath 5. The solidification furnace 1 comprises an enclosure 2 delimited by a wall 3 thermally insulator, in which is disposed a crucible 4. The crucible 4 here receives a bath of molten silicon 5 during solidification. The silicon then takes the form of a crystalline solid phase and a liquid phase separated from each other by a solid / liquid interface 6.

The acoustic probe 10 includes a waveguide 20, a free end 25 of which is introduced into the liquid silicon. The waveguide 20 has an entry face of an acoustic wave, an exit face of the acoustic wave, and a side face which connects the entry and exit faces. It comprises a core 20.1 made of crystalline silica, a structure 20.3 of graphite or of silicon carbide SiC which surrounds the lateral face of the core 20.1, and a pulverulent intermediate layer 20.2 of silicon nitride SiN or of SiC disposed between the core 20.1 and the structure 20.3.

In operation, the acoustic probe îo is placed in the enclosure 2 of the solidification furnace 1 so that its free end 25 is introduced into the molten silicon bath 5. Ultrasonic waves are emitted by a transducer 31 to 1 inside the waveguide 20 then transmitted into the liquid silicon 5 so as to act on the convection regime and thus ensure mixing of the molten silicon bath 5.

However, the acoustic probe 10 presents a risk of contamination of the solidification furnace 1 insofar as the silica of the core 20.1 of the waveguide 20 is capable of reacting with the liquid silicon 5 and thus causing the release of silicon oxide SiO leading to physical and functional degradation of the waveguide, thus preventing the subsequent reuse of the latter (single use). Furthermore, the addition of oxygen to the molten silicon bath, by reducing the silica, is detrimental to the crystalline quality of the silicon.

PRESENTATION OF THE INVENTION The invention aims to at least partially remedy the drawbacks of the prior art, and more particularly to propose a method for manufacturing a reusable acoustic probe which in particular makes it possible to reduce the risks contamination of a silicon solidification furnace and which has good acoustic performance.

For this, the object of the invention is a method of manufacturing an acoustic probe intended for use in a silicon solidification furnace, the acoustic probe comprising a waveguide whose free end is in part delimited by a transmission face intended to transmit an acoustic wave in a bath of molten silicon. The process includes the following steps:

supply of the waveguide, the free end of which is made of a carbon material with zero open porosity, capable of reacting with liquid silicon to form silicon carbide, the surface of the free end being defined by the carbon material ;

- contacting the free end of the waveguide in liquid silicon, so that a layer of silicon carbon is formed by reaction between the carbonaceous material and the liquid silicon and defines the transmission face.

The invention also relates to a process for monitoring the progress of solidification of silicon in a solidification furnace by means of an acoustic probe, comprising the following steps:

- manufacturing the acoustic probe by the method according to the preceding characteristic;

- emission of at least one acoustic wave in the molten silicon bath through the transmission face of the waveguide.

[ooio] Preferably, during the manufacturing step, the liquid silicon is that of the molten silicon bath.

[Ooii] The invention also relates to an acoustic probe intended to be used in a silicon solidification oven, comprising a waveguide comprising a free end intended to be introduced into a bath of molten silicon, which comprises a face of transmission of an acoustic wave in the molten silicon bath. The free end is made of a carbon material with zero open porosity, capable of reacting with liquid silicon, and a layer of silicon carbide which covers the carbon material and defines the transmission face.

[Ooi2] The carbonaceous material may have a carbon atom content greater than or equal to 95% by weight relative to its total weight.

[ooi3] The carbonaceous material can be vitreous carbon.

The invention also relates to a system for monitoring the progress of solidification of silicon in a solidification furnace, comprising:

- at least one acoustic probe according to any one of the preceding characteristics;

- a transmission / reception device, coupled to the acoustic probe, and adapted to emit a primary acoustic wave in the waveguide in the direction of the transmission face, and to detect in response a return acoustic wave;

- a processing unit, connected to the transmission / reception device, and adapted to determine the progress of the solidification of silicon from at least one signal transmitted by the transmission / reception device.

BRIEF DESCRIPTION OF THE DRAWINGS [ooi5] Other aspects, aims, advantages and characteristics of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of nonlimiting example, and made with reference to the accompanying drawings in which:

FIGS. 1A and 1B, already described, are schematic views in vertical section of a solidification furnace and of a system for stirring the molten silicon bath by means of an acoustic probe according to an example of the prior art, FIG. 1B being a detailed view of the free end of the acoustic probe;

FIG. 2 is a schematic view in vertical section of a solidification furnace and of a system for monitoring the progress of solidification of silicon by means of an acoustic probe according to one embodiment;

FIGS. 3A and 3B are partial views of the acoustic probe according to the embodiment, at the free end of the waveguide, during two stages of a process for manufacturing the acoustic probe, namely before (fig-3A) and after (fig.3B) bringing the free end of the waveguide into contact with liquid silicon.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS In the figures and in the following description, the same references represent the same or similar elements. In addition, the different elements are not shown to scale so as to favor the clarity of the figures. Furthermore, the different embodiments and variants are not mutually exclusive and can be combined with one another. Unless otherwise noted, the terms "substantially", "approximately", "in the order of" mean to the nearest 10%. Furthermore, the expression "comprising a" should be understood as "comprising at least one", unless otherwise indicated.

[ooi7] The invention relates to an acoustic probe and its manufacturing process, the acoustic probe being intended to be used in a solidification oven, for example for the purpose of monitoring the progress of the crystallization of silicon. By solidification is meant the phases of melting and crystallization of silicon during the production of a silicon ingot.

Figure 2 is a schematic view in vertical section of a solidification furnace 1 and a control system 30 of the progress of the solidification of silicon, this control system 30 comprising an acoustic probe 10 according to one embodiment.

[Ooi9] The solidification furnace 1 is suitable for carrying out the melting / crystallization of silicon. It thus comprises a thermally insulating wall 3 which delimits an enclosure 2. A crucible 4 receiving a bath of molten silicon 5 is placed in the enclosure 2. By way of illustration, the furnace 1 may be a directed solidification furnace, for example of Heat Exchanger Method type with resistive or inductive heating and argon scanning making it possible to produce silicon ingots for applications in particular photo voltaic. In this example, the molten silicon bath 5 corresponds to the liquid phase of the silicon which covers a solid phase of crystalline silicon. A solid / liquid interface 6 then separates the solid phase from the liquid phase. The position of the solid / liquid interface 6 of the silicon makes it possible to deduce the progress of solidification. The solidification furnace 1 can be used in the context of a directed solidification process of silicon in which a controlled vertical thermal gradient is imposed on the molten silicon bath 5.

The control system 30 includes an acoustic probe 10 with waveguide 20, an acoustic device 31 allowing the emission of acoustic waves in the waveguide 20 towards a transmission face 22 of that -this, as well as the reception of acoustic waves from the waveguide 20, and a processing unit 32 coupled to the acoustic device 31. It is suitable for determining the position of the solid / liquid interface 6 of the silicon from of the emission of a so-called primary acoustic wave S _P and of the reception of a response acoustic wave Sr, this signal coming from the backscattering or the reflection of part of the primary optical signal S _P at the solid / liquid interface 6 of silicon.

The acoustic probe 10 includes a waveguide 20 which extends longitudinally between a first face 21 called connection and a second face 22 called transmission, opposite the first face 21 along the longitudinal axis of the guide d wave 20. A lateral face 23 extends parallel to the longitudinal axis and connects between them the connection 21 and transmission 22 faces.

The connection face 21 is acoustically coupled to the acoustic device 31 and the transmission face 22 is intended to transmit an acoustic signal in the molten silicon bath 5, here in the direction of the solid / liquid interface 6 of the silicon. The transmission face 22 is therefore placed in the molten silicon bath 5. Here it has a planar shape, orthogonal to the longitudinal axis of the waveguide 20, but can have any other shape, in particular a conical tip.

The waveguide 20 has a first end 24 removably assembled to the wall 3 of the enclosure 2 of the solidification furnace 1, and a second end 25, called free, opposite the first end along the longitudinal axis of the waveguide 20. The first end 24 has the connection face 21. It can be coated by a protective sleeve at its attachment to the wall 3 of the enclosure 2. Furthermore, the end free 25 comprises the transmission face 22. It is intended to be introduced into the liquid silicon so that the transmission face 22 is disposed there. This end 25 is said to be free in the sense that it is not assembled to the wall 3 of the solidification furnace 1.

The waveguide 20 is made of at least one material whose acoustic impedance Z is greater than that of the surrounding medium, and can take the form of a bar or a rigid rod with a length of a few centimeters to hundreds of centimeters, advantageously between 10cm and 150cm and for example equal to about 50cm, and with a diameter of the order of a few millimeters to tens of millimeters, for example equal to 10mm. In this example, the waveguide 20 is made of the same material but can be made of several distinct materials assembled together, in particular by brazing. The different materials preferably have acoustic impedances substantially equal to each other.

The waveguide 20 is made at least in part from a dense carbon material 26, that is to say from a material formed essentially based on carbon atoms whose carbon content is preferably greater than or equal to 95%, for example greater than or equal to 99% by weight relative to its total weight. More specifically, at least the free end 25 of the waveguide 20 is made of dense carbon material 26. The dense carbon material 26 is capable of reacting with liquid silicon to form a layer 27 of silicon carbide SiC according to the chemical reaction: Si + C -> SiC.

The carbonaceous material 26 is said to be dense, insofar as it has zero open porosity. Porosity is said to be open (or free) when pores communicate with each other so as to create an internal space which is accessible from the outside and therefore which can be filled. The open porosity is said to be zero when the pores are not connected to each other, which corresponds to an open porosity value of less than or equal to 1%, preferably equal to 0%. This value of open porosity can be measured by the mercury porosimetry method or by pycnometer, or even by missing mass methods (see for example the article by Panneton and Gros entitled A Missing Mass Method To Measure The Open Porosity Of Porous Solids, Acta-Acustica 91 (2), 342-348 (2005)), or even by optical techniques such as image analysis by X-ray tomography and optical microscopy.

The dense carbon material 26 is preferably vitreous carbon (glassy carbon or vitreous carbon, in English). Glassy carbon is a material which is sometimes used for the production of a solidification furnace crucible. It is a carbonaceous material obtained by thermal decomposition of a three-dimensional crosslinked polymer with zero open porosity. It has physical properties which remain stable at high temperatures, and in particular at temperatures above the melting temperature of silicon (approx. 1414 ° C.). Preferably, the carbonaceous material is vitreous carbon marketed under the reference V25 by the company Mersen, insofar as it has a heat treatment at 25OO ° C. making it possible to further improve the stability of its physical and therefore acoustic properties at susceptible temperatures. to be applied inside the solidification furnace 1.

The waveguide 20 further comprises a layer 27 of silicon carbide SiC, which at least partially covers the dense carbon material 26 and defines the transmission face 22. The layer 27 of silicon carbide SiC is located at level of the free end 25 of the waveguide 20, and here also defines a part of the lateral face 23 of the waveguide 20 which extends from the transmission face 22. The layer 27 of carbide silicon SiC is obtained naturally when the dense carbon material 26 of the waveguide 20 is brought into contact with liquid silicon, the latter preferably being that of the molten silicon bath 5.

The acoustic device 31 comprises a transmitter and a receiver of acoustic waves, in particular of ultrasound. It is acoustically coupled to the connection face 21 of the waveguide 20 on the one hand, and is connected to the processing unit 32 on the other hand.

The ultrasonic transmitter / receiver 31 can be formed by the same transducer 31, for example a PZT ceramic transducer (lead and zirconium titanate), placed outside the enclosure 2 and coupled to the connection face of the waveguide 20. It is thus adapted to emit an ultrasonic acoustic wave called primary S _P in the waveguide 20 from the connection face 21, so that the primary acoustic wave S _P propagates towards the transmission face of the free end 25. It is also suitable for detecting a return ultrasonic acoustic wave Sr propagating in the waveguide 20 from the transmission face 22, l ' return acoustic wave Sr having been backscattered or reflected at the solid / liquid interface 6 of the silicon. The transducer 31 is connected to the processing unit 32 and is adapted to emit signals, for example electrical signals such as a voltage, representative of the emission of the primary acoustic wave S _P and of the detection of the acoustic wave return Sr.

The processing unit 32 is connected to the transducer 31, and is adapted to determine, from the electrical signals received from the transducer 31, the position of the solid / liquid interface 6 of the silicon and therefore to deduce therefrom the progress of solidification of silicon. The processing unit 32 comprises the usual technical means, for example a processor and read and / or read-only memories, a data transmission bus, etc.

The method of manufacturing the acoustic probe 10 is now described with reference to Figures 3A and 3B which illustrate a longitudinal sectional view of part of the waveguide 20, before (fig.3A) and after (fig .3B) bringing the dense carbon material 26 of the free end 25 of the waveguide 20 into contact with liquid silicon. In this example, the liquid silicon is the molten silicon bath 5 for which it is desired to check the progress. Furthermore, the carbon material of the waveguide 20 is advantageously glassy carbon.

As illustrated in fig-3A, in a first step, an acoustic probe 10 is provided comprising a waveguide 20 whose free end 25 is made of glassy carbon 26. Thus, the glassy carbon 26 defines the surface of the free end 25, namely the downstream face 22.1 opposite the connection face 21, and the lateral face 23. The layer 27 of silicon carbide SiC is not yet present at the free end 25.

In a second step, the downstream face 22.1 of the free end 25 of the waveguide 20 is brought into contact with liquid silicon, so that a layer 27 of silicon carbide SiC is formed naturally by reaction chemical between the glassy carbon 26 and the liquid silicon 5. The layer 27 of silicon carbide SiC then covers the glassy carbon 26 at its downstream face 22.1, and then defines the transmission face 22 through which an acoustic wave is intended to be transmitted in a bath of molten silicon 5. Furthermore, the vitreous carbon 26 has good wetting properties with liquid silicon, so that the latter wets the side face 23 (contact angle less than 90 °) , which results in the formation of the SiC layer 27 on the face 23.

As illustrated in fig-3B, preferably, the liquid silicon is the molten silicon bath 5 located in the crucible 4 of the solidification furnace 1 for which it is desired in particular to monitor the progress of solidification . The layer 27 of silicon carbide SiC then forms by chemical reaction until it forms a tight barrier between the vitreous carbon 26 and the liquid silicon 5. When this tight barrier is formed, that is to say after a few minutes at a few tens of minutes, for example 20 min, the chemical reaction ceases and the layer 27 of silicon carbide SiC has a constant and substantially homogeneous thickness along the surface of the free end 25. It can be of the order of a few microns to a few tens of microns, for example equal to approximately πm.

As explained below, an adaptation of acoustic impedance is then naturally carried out at the level of the transmission face 22 of the waveguide 20, thereby making it possible to improve the acoustic transmission coefficient of the waveguide 20. Furthermore, it is particularly advantageous for the liquid silicon making it possible to form the layer 27 of silicon carbide SiC to be the molten silicon bath 5, the state of progress of solidification of which is particularly desired. Indeed, the step of bringing the dense carbon material 26 into contact with the liquid silicon of the manufacturing process corresponds to the step of introducing the free end 25 of the waveguide 20 into the bath of molten silicon 5 the process for checking the progress of solidification.

The process for monitoring the progress of solidification can then be carried out. For this, the acoustic transducer 31 emits a primary acoustic wave S _P in the waveguide 20 in the direction of the transmission face 22. A part St of the primary acoustic wave S _P is transmitted outside the waveguide 20 , through the transmission face 22, in the molten silicon bath 5. The acoustic power of the transmitted wave St is all the more important as, on the one hand, the layer 27 of silicon carbide SiC achieves a adaptation of acoustic impedance making it possible to maximize the transmission coefficient, and that, on the other hand, the layer 27 of silicon carbide SiC is obtained by chemical reaction between the vitreous carbon 26 and the liquid silicon 5, and not by deposition in CVD thin layer.

A part Srd of the transmitted wave St is then backscattered at the solid / liquid interface 6 of the silicon in the direction of the waveguide 20, then is transmitted inside of it and propagates in the direction of the connection face 21. The transducer 31 receives the return acoustic wave Sr. The processing unit 32, from electrical signals, emitted by the transducer 31, and representative of the emission of the primary acoustic wave S _P and the detection of the return acoustic wave Sr, determines the position of the solid / liquid interface 6 of the silicon and deduces therefrom the state of progress of the solidification of the silicon.

The inventors have found that the waveguide 20 has high acoustic performance due to the formation by chemical reaction of the layer 27 of silicon carbide SiC covering the dense carbon material 26 of the free end 25 of the guide wave 20, insofar as the presence of this layer 27 of silicon carbide SiC results in an adaptation of acoustic impedance making it possible to significantly improve the coefficient of acoustic transmission in liquid silicon through the transmission face 22. The acoustic transmission between the waveguide 20 and the molten silicon 5 is also improved by the wetting of the free end 25 by the molten silicon.

In fact, the transmission coefficient of an acoustic wave at normal incidence through the transmission face 22 separating a medium 1 from a medium 2 can be expressed, at the first order, like the ratio 4.Zi .Z2 / (Zi + Z ₂ ) ² . In the case where the transmission face 22 separates the vitreous carbon of acoustic impedance Zcv vis-à-vis the liquid silicon of acoustic impedance Zsi, the transmission coefficient Tcv / sî at the level of the transmission face 22 is equal to 45.3% approximately, with Zcv = 6.00.107 kg / m ² / s and Zsî = 8.97.107 kg / m ² / s.

However, with the presence of the layer 27 of silicon carbide SiC defining the transmission face 22, the overall transmission coefficient Tcv / sî between the carbonaceous material and the liquid silicon (in one direction as in the other) can be written, as a first approximation, as the product of the transmission coefficient Tcv / sîc between the carbonaceous material and the silicon carbide, and of the transmission coefficient Tsic / si between the silicon carbide and the liquid silicon. The transmission coefficient Tcv / sîc is at least equal to approximately 78.2% (with Zsîc = 2.18.107 kg / m ² / s) and the transmission coefficient Tsîc / sî is equal to approximately 82.6%. Thus, the overall transmission coefficient Tcv / sî is at least equal to approximately 64.6%, which corresponds to an increase in the acoustic transmission of more than 40% compared to the case where the layer 27 of silicon carbide SiC would be absent. The transmission of the acoustic signal of the liquid silicon in the waveguide is also particularly high (symmetrical relationship).

In addition, the inventors have found that the overall transmission coefficient Tcv / sî between the glassy carbon and the liquid silicon has a value greater than 64.6%, insofar as there is high acoustic transmission between the carbon vitreous and silicon carbide (effective Tcv / sîc greater than 78.2%) due to the gradual transition between vitreous carbon and silicon carbide. In fact, this gradual transition between these two materials is due to the fact that the silicon carbide was obtained by chemical reaction and not by thin layer deposition, of the CVD type (for Chemical Vapor Deposition, in English) or other, directly on the carbonaceous material.

It therefore appears that the waveguide 20 has high acoustic performance due to the presence of the layer 27 of silicon carbide SiC which provides impedance matching between the carbonaceous material and the liquid silicon, and the fact that the layer 27 of silicon carbide SiC is formed naturally by chemical reaction between the carbonaceous material and the liquid silicon and not by CVD type deposition on the carbonaceous material.

Furthermore, due to the zero open porosity of the dense carbon material 26, the process of infiltration of the liquid silicon inside the dense carbon material 26 is almost nonexistent. Also, only the layer 27 of silicon carbide SiC is formed, the thickness of which becomes constant when a tight barrier is formed between the dense carbon material 26 and the liquid silicon in the bath 5. The acoustic properties of the waveguide 20 thus remain stable over time, which allows the reuse of the acoustic probe 10 in other molten silicon baths without degradation of the acoustic properties of the waveguide 20. In addition, the zero open porosity results in good transmission acoustic within the dense carbon material 26, which improves the acoustic properties of the waveguide 20. In fact, it has been noted that a material with a non-zero open porosity can result in a low acoustic transmission.

Finally, the acoustic probe 10 is not likely to induce contamination of the solidification furnace 1 and, therefore, a degradation of the crystalline quality of the silicon obtained, as is the case for example in the example of the prior art mentioned above where there is a continuous emission of oxygen and silicon oxide. Indeed, according to the invention, only the layer 27 of silicon carbide SiC is formed on the surface of the dense carbon material 26, without there being any release of pollutants in the molten silicon bath 5. It It therefore follows that the acoustic probe 10 according to the invention makes it possible to limit the risks of contamination of the solidification furnace 1, while exhibiting good acoustic performance.

As mentioned above, the acoustic probe 10 also has great ease of production and use, insofar as the layer 27 of silicon carbide SiC can be formed naturally during the introduction of the free end 25 in the molten silicon bath 5 which it is desired in particular to monitor the progress of solidification.

Particular embodiments have just been described. Different variants and modifications will appear to those skilled in the art. Thus, the progress control system can comprise a single acoustic probe 10 or a plurality of acoustic probes. The processing unit 32 then deduces the progress of the solidification of the signals transmitted by the transducers relating to the various acoustic probes.

Claims (7)

1. Method for manufacturing an acoustic probe (îo) intended to be used in a solidification furnace (1) of silicon, the acoustic probe (îo) comprising a waveguide (20) including a free end (25) is partly delimited by a transmission face (22) intended to transmit an acoustic wave in a molten silicon bath (5), characterized in that it comprises the following stages: - Supply of the waveguide (20), the free end (25) of which is made of a carbonaceous material (26) with zero open porosity, capable of reacting with liquid silicon to form silicon carbide, the surface (22.1 ) of the free end (25) being defined by the carbonaceous material (26); - contacting the free end (25) of the waveguide (20) in liquid silicon, so that a layer of silicon carbon (27) is formed by reaction between the carbonaceous material (26 ) and liquid silicon and define the transmission face (22). 2. Method for monitoring the progress of solidification of silicon in a solidification furnace (1) by means of an acoustic probe (10), comprising the following steps: - manufacture of the acoustic probe (10) by the method according to claim 1; - emission of at least one acoustic wave in the molten silicon bath (5) through the transmission face (22) of the waveguide (20). 3. Control method according to claim 2, wherein, during the manufacturing step, the liquid silicon is that of the molten silicon bath (5). 4. Acoustic probe (10) intended to be used in a solidification furnace (1) for silicon, comprising a waveguide (20), comprising a free end (25) intended to be introduced into a bath of molten silicon ( 5), which comprises a transmission face (22) of an acoustic wave in the molten silicon bath (5), characterized in that the free end (25) is produced by: o a carbon material (26) with zero open porosity, capable of reacting with liquid silicon, and in o a layer of silicon carbide (27) which covers the carbon material (26) and defines the transmission face (22). 5. Acoustic probe (10) according to claim 4, wherein the carbonaceous material (26) has a carbon atom content greater than or equal to 95% by weight relative to its total weight. 6. Acoustic probe (10) according to claim 4 or 5, wherein the carbonaceous material (26) is vitreous carbon. 7. Control system (30) for the progress of solidification of silicon in a solidification furnace (1), comprising: o at least one acoustic probe (10) according to any one of claims 4 to 6; o a transmission / reception device (31), coupled to the acoustic probe (10), and adapted to emit a primary acoustic wave (S _P ) in the waveguide (20) in the direction of the transmission face ( 22), and to detect in response a return acoustic wave (Sr); o a processing unit (32), connected to the transmission / reception device (31), and adapted to determine the state of progress of the solidification of silicon from at least one signal transmitted by the device transmission / reception (31) ·