EP3002638B1 - Method for manufacturing a thermocompensated hairspring - Google Patents

Description

Technical area

The present invention relates to a method of manufacturing a thermocompensated mechanical oscillator of a clockwork movement, of a MEMS sensor or of another precision instrument, such as in particular a thermocompensated spiral spring which is intended to equip a resonator. mechanical sprung balance. The present invention also relates to an oscillator obtained by the method.

State of the art

A mechanical balance-spring oscillator or resonator of a mechanical watch is conventionally composed of a flywheel, called a balance, and a spiral spring, called a spiral or spiral spring, fixed by one end to the axis of the balance. and by the other end on a bridge, called cock, in which the axis of the balance pivots. More precisely, the spiral spring equipping, to date, mechanical watch movements is an elastic metal blade of rectangular section wound on itself in an Archimedean spiral and comprising 12 to 15 turns.

The sprung balance oscillates around its equilibrium position (or neutral). When the balance leaves this position, it arms the hairspring. This creates a return torque which, when the balance is released, causes it to return to its equilibrium position. As it has acquired a certain speed, therefore kinetic energy, it goes beyond its neutral point until the opposite torque of the hairspring stops it and forces it to turn in the other direction. Thus, the hairspring regulates the period of oscillation of the balance.

The precision of mechanical watches depends on the stability of the natural frequency of the oscillator formed by the sprung balance. When the temperature varies, the thermal expansions of the hairspring and the balance, as well as the variation of the Young's modulus of the hairspring, modify the natural frequency of this oscillating assembly, disturbing the precision of the watch.

où F est la fréquence propre de l'oscillateur, C est la constante du couple de rappel exercé par le spiral de l'oscillateur, et I est le moment d'inertie du balancier de l'oscillateur.Most of the methods proposed to compensate for these variations in frequency are based on the consideration that this natural frequency depends exclusively on the ratio between the constant of the return torque exerted by the hairspring on the balance and the moment of inertia of the latter, as indicated. in the following relation: $$\mathrm{F}=1/2\mathrm{\pi }{\left(\mathrm{VS}/\mathrm{I}\right)}^{0.5}$$

where F is the natural frequency of the oscillator, C is the constant of the return torque exerted by the balance spring of the oscillator, and I is the moment of inertia of the balance of the oscillator.

où l'expression "1/E dE/dT" correspond au coefficient thermique du module de Young du spiral (CTE), c_s est le coefficient de dilatation thermique du spiral, et c_b est le coefficient de dilatation thermique du balancier.For example, since the discovery of Fe-Ni-based alloys having a positive thermal coefficient of Young's modulus (hereinafter CTE), the thermal compensation of the mechanical oscillator is obtained by adjusting the CTE of the hairspring as a function of the coefficients thermal expansion of the balance spring and balance. Indeed, by expressing the torque and inertia from the characteristics of the hairspring and the balance, then by deriving equation (1) with respect to the temperature, we obtain the thermal variation of the natural frequency: $$1/\mathrm{F}\mathrm{dF}/\mathrm{dT}=\mathrm{½}\left(1/\mathrm{E}\mathrm{of}/\mathrm{dT}+3{\mathrm{vs}}_{\mathrm{s}}-2{\mathrm{vs}}_{\mathrm{b}}\right)$$

where the expression "1 / E dE / dT" corresponds to the thermal coefficient of the Young's modulus of the hairspring (CTE), c _s is the coefficient of thermal expansion of the hairspring, and c _b is the coefficient of thermal expansion of the balance.

By adjusting the self-compensation term A = ½ (CTE + 3c _s ) to the value of the thermal expansion coefficient of the balance c _b , it is possible cancel equation (2). Thus, the thermal variation of the natural frequency of the mechanical oscillator can be eliminated.

Currently, complex alloys are used, both by the number of components and by the metallurgical processes used in order to obtain self-compensation for variations in the modulus of elasticity of the metal by combining two contrary influences: that of temperature and that of magnetoconstriction (contraction of magnetic bodies under the effect of magnetization).

However, balance springs made from these alloys are difficult to manufacture. First of all, due to the complexity of the processes used to make the alloys, the intrinsic mechanical properties of the metal are not constant from one production to another. Then, the adjustment of the regulator, which is the technique allowing the watch to indicate the most accurate time at all times, is tedious and long. This operation requires many manual interventions and many defective parts must be eliminated. For these reasons, production is expensive and maintaining consistent quality is an ongoing challenge.

In the document JP6117470 , a spiral-shaped spring is made of monocrystalline silicon. It is dimensioned so as to have a constant return torque, to provide a high precision electrical measuring device. However, this document is silent as to the thermal stability of the constant of the return torque of this spring. It cannot therefore be used directly as a spiral spring in a timepiece.

In the document DE10127733 , a spiral spring is made of monocrystalline silicon coated with silicon dioxide so as to obtain good stability of the shape of the balance spring with temperature variations. The thermal stability of the constant of the return torque of this spring is also not mentioned in this document.

The CTE of silicon is strongly influenced by temperature and compensation for this effect is necessary for its use in horological applications. Indeed, the CTE of silicon is of the order of -60 x 10 ^-6 / ° C and the thermal drift of a silicon spiral spring is thus about 155 seconds / day, for a temperature variation of 23 ° C +/- 15 ° C. This makes it incompatible with watchmaking requirements which are of the order of 8 seconds / day.

The document EP1422436 describes a spiral spring cut from a {001} monocrystalline silicon wafer. The hairspring comprises a layer of SiO ₂ , exhibiting a CTE opposite to that of silicon and formed around the outer surface of the hairspring, in order to minimize the thermal drift of the balance-spring assembly.

The document WO2013064351 describes a thermocompensated resonator comprising a body formed by ceramic where at least part of the body comprises at least one coating whose variations in Young's modulus as a function of temperature (CTE) are of opposite sign to those (CTE) of material used for the core in order to allow said resonator to have a frequency variation as a function of the temperature at least at the first order substantially zero. According to this document, ceramics can include both positive and negative first order and second order thermoelastic coefficients, and the coating (s) used can incidentally include both negative and positive first order and second order thermoelastic coefficients. Germanium oxide (GeO ₂ ) or tantalum oxide (Ta ₂ O ₅ ) and / or oxides of zirconium or hafnium can be used as coatings. Preferably, the coatings also form a moisture barrier, and a primer layer may be deposited between the core and the coating.

The document EP2395662B1 describes a thermocompensated resonator comprising a body formed from a cutting plate in a quartz crystal and comprising a coating, for example of oxide of germanium or tantalum oxide, deposited at least partially against the core.

The document CH699780 discloses research towards optimal coating materials and dopants to effect better compensation by adjusting the value of the thermoelastic coefficient of the coating to compensate for the change in the thermoelastic coefficient of the core with temperature.

Brief summary of the invention

The present invention relates to a method of manufacturing a mechanical oscillator as claimed in claim 1, such as a spiral spring, as claimed in claim 15, for equipping a mechanical sprung balance resonator of a timepiece movement. or other precision instrument. The spiral spring comprises a core made of a material chosen from metals and their alloys, metalloids including silicon (amorphous, monocrystalline or polycrystalline), ceramics, carbon and its various allotropic forms, or composite materials having a first coefficient thermoelastic of a first sign. The spring also comprises a peripheral coating of an oxide, preferably SiO ₂ , having a second thermoelastic coefficient of a second sign opposite to the first sign of the first thermoelastic coefficient, the method comprising depositing the coating at a temperature below 500 ° C, and an annealing heat treatment of the coating at a temperature of at least 550 ° C.

In one embodiment, the heat treatment for annealing the coating may be preceded by a gradual increase in temperature.

In another embodiment, the method can comprise the deposition of a tie layer of Al ₂ O ₃ between the core and the coating. The tie layer can be deposited with a thickness of about 5 nm.

In yet another embodiment, the method may include depositing an outer layer of Al ₂ O ₃ on the coating. The outer layer can be deposited with a thickness of about 300 nm. The outer layer can then control the frequency of the oscillator and / or protect the assembly formed by the core and the coating against humidity.

The present invention relates to a spiral spring obtained by the method as well as a mechanical sprung balance resonator comprising such a spiral spring. The present invention also relates to other types of mechanical oscillators such as tuning forks and MEMS sensors.

The proposed solution makes it possible to manufacture a spiral spring whose core consists of a material whose control of the thermoelastic behavior, in particular the abnormal change in the value of the Young's modulus, is not or cannot be obtained for various reasons by the growth of a thermal oxide on its surface, but instead involves the deposition of an oxide on the surface of the core by a deposition process. This is the case, for example, where an SiO ₂ coating is formed on a core not comprising silicon. However, the solution of the present invention is also applicable, for example, to an oscillator comprising a silicon core and a SiO ₂ coating where the coating is formed by a deposition process proper and not by the growth of a thermal oxide. . Obtaining a coated oscillator by such a process can be advantageous where it is desired to better control the dimensions and the frequency of the oscillator, because with a growth of a thermal oxide on the surface of a silicon core a portion of the core becomes oxidized during the oxidation step. This solution is applicable to a set of materials compatible with the heat treatment processes used to stabilize the compensation, and is intended, inter alia, for functions exploiting the stability of mechanical properties, and in particular elastic properties, such as oscillators and resonators.

Brief description of the figures

• la figure 1 montre une vue du dessus d'un ressort spiral selon l'invention; et
• les figures 2a et 2b montrent une vue en coupe transversale droite (figure 2a) et longitudinale (figure 2b) du ressort spiral comprenant une âme et un revêtement, selon un mode de réalisation.

Examples of implementation of the invention are indicated in the description illustrated by the appended figures in which:

• the figure 1 shows a top view of a spiral spring according to the invention; and
• the figures 2a and 2b show a right cross-sectional view ( figure 2a ) and longitudinal ( figure 2b ) of the spiral spring comprising a core and a coating, according to one embodiment.

Example (s) of embodiment of the invention

The figure 1 shows a top view of a spiral spring 1 and the figures 2a and 2b show a longitudinal and transverse sectional view of the spiral spring 1 according to the invention. The spiral spring 1 comprises a core 2 formed from a material having a first thermoelastic coefficient β ₁ of a first sign (typically negative or normal). The spiral spring 1 also comprises a peripheral coating 4 of an oxide, preferably silicon dioxide (SiO ₂ ), deposited and at least partially covering the outer surface of the core 2. A tie layer 3 can also be deposited. between the core 2 and the coating 4. In the example of figures 1 and 2 , the core has a helical shape and comprises at least one turn of rectangular section of thickness w and height h. It will be understood, however, that the geometry of the core may be other than that illustrated in this example, for example, the core may have a straight or circular section.

The core 2 can be made in monocrystalline silicon, with an orientation such as {001}, {111} or other, or it can be made in a polycrystalline or amorphous material (polycrystalline silicon; amorphous silicon, quartz glass, etc. silica glass). The material of the core can also be a metallic material whose melting point remains compatible with the heat treatment step (described below) of the present invention. Furthermore, the material of the core can comprise a ceramic material, in particular a silicon nitride, a silicon carbide, or a silicon oxynitride. The core material may further comprise a composite material or a polymer or carbon material. For example, the core 2 can be a composite of carbon fibers, the list of materials mentioned here not being by any means exhaustive. Preferably, the core is made of a material whose coefficient thermoelastic is normal and the melting point is compatible with the heat treatments applied as indicated below.

• fournir l'âme 2 dans le matériau ayant le premier coefficient thermoélastique β₁; et
• déposer le revêtement oxyde 4 d'un second coefficient thermoélastique β₂ au moins partiellement sur la surface extérieure de l'âme 2.

In one embodiment, a method of manufacturing the spiral spring 1 comprises the steps of:

• providing the core 2 in the material having the first thermoelastic coefficient β ₁ ; and
• depositing the oxide coating 4 with a second thermoelastic coefficient β ₂ at least partially on the outer surface of the core 2.

The coating 4 is deposited using a low temperature deposition process, that is to say, at a temperature substantially lower than that used for the post treatment, in particular below 500 ° C. Preferably, the deposition of the coating 1 is carried out at a temperature below 300 ° C, even more preferred at a temperature below 200 ° C and according to a variant at a temperature of approximately 100 ° C. The deposition temperature can vary, and in particular it can drop, during this deposition step.

In particular, the coating 4 can be deposited using a thin-film deposition process which may include, in a non-exhaustive manner, processes such as physical vapor deposition (PVD) or even a process of coating. chemical vapor deposition (CVD). Other thin film deposition methods can also be envisaged for the deposition of the coating 4 provided that the deposition temperature remains equal to or less than 500 ° C. For example, coating 4 can be deposited using a plasma assisted chemical vapor deposition (PECVD), high density plasma (HDPCVD), molecular vapor deposition (MVD) process. , deposition of atomic layers (Atomic Layer Deposition, or ALD), or deposits obtained using sol-gels.

Our experimental results, as indicated in Table A below, however demonstrate that the coating 4 deposited by a physical or chemical deposition process at low temperature, as described above, does not necessarily make it possible to obtain, as is, sufficient compensation for the effect of varying the core CTE with temperature. In Table A, the variation of the CTE for an amorphous carbon core on a daily basis as a function of temperature is compared for a spiral spring without the coating (column 0), a spiral spring comprising a tie layer 3 of Al ₂ O ₃ deposited by ALD (column 1) and with the coating 4 of SiO ₂ deposited by PECVD (column 2). In all cases, the variation of CTE with temperature remains significant. The variation of CTE on a daily basis as a function of relative humidity (RH%) is also shown in Table A. Table A Testing Process step 0 1 2 3 4 5 Step 1 - ALD Al ₂ O ₃ ALD Al ₂ O ₃ ALD Al ₂ O ₃ ALD Al ₂ O ₃ ALD Al ₂ O ₃ 2nd step - - PECVD SiO ₂ PECVD SiO ₂ PECVD SiO ₂ PECVD SiO ₂ step 3 - - - annealing 800 ° C annealed 1050 ° C ALD Al ₂ O ₃ step 4 - - - ALD Al ₂ O ₃ ALD Al ₂ O ₃ annealing 800 ° C variation in temp. [s / d / ° C] -3.7 -4.2 -5.2 -1.8 -1.8 -3.9 humidity variation [s / d / RH%] -0.2 -0.05 -0.2 -0.02 -0.03 -1.5

Indeed, the SiO ₂ coating obtained from such a low-temperature deposition process is in principle structurally or chemically different from a SiO ₂ coating obtained by thermal oxidation of silicon at substantially higher temperatures, for example, of around 1000 ° C. _{SiO 2} coating 4 can also include a small percentage of hydrogen or other impurities. Under these conditions, the thermoelastic coefficient of SiO ₂ is not necessarily (or is not sufficiently) abnormal to obtain the desired thermocompensation.

According to the present invention, the method of manufacturing the spiral spring 1 comprises a heat treatment for annealing the coating 4, making it possible to make the thermoelastic coefficient sufficiently compensatory (or even sufficiently abnormal in the context of SiO ₂ ) and to stabilize it. In particular, the annealing heat treatment is carried out with an annealing temperature of at least 550 ° C, and preferably a temperature of between approximately 800 ° C and 1050 ° C. According to one embodiment, the annealing temperature is either about 800 ° C or about 1050 ° C. Preferably, the annealing time is between 2 to 6 hours, and in one embodiment it is around four hours. Moreover, this annealing operation can take place in continuity with the operation of depositing the coating 4 or equally spaced at a time interval of several days.

Depending on the materials used for the core 2 and the coating 4, the annealing heat treatment of the coating 4 makes it possible to modify the coating 4 so that the second thermoelastic coefficient β ₂ of the coating 4 has a second sign opposite to the first sign of the first thermoelastic coefficient β ₁ of the core 2 in order to adjust the second thermoelastic coefficient β ₂ so that the latter compensates for the effect of the variation of the first thermoelastic coefficient β ₁ of the core with temperature.

The annealing heat treatment can be carried out under an inert atmosphere, for example under a nitrogen atmosphere. The annealing temperature can be reached by a gradual rise in temperature of the order of 10 ° C / min from a loading temperature, for example 200 ° C. After annealing, the temperature can be lowered to a speed of the order of 3 ° C / min, up to an unloading temperature, for example of about 200 ° C.

The annealing heat treatment makes it possible to modify the structure, in particular to densify the coating and to reduce the internal stresses of the coating 4. The annealing heat treatment of the coating 4 deposited by the low-temperature deposition process can therefore modify the thermoelastic behavior of the coating. coating 4 and make it possible to obtain a compensation for the effect of the variation of the CTE of the core with the temperature. In particular, according to the experiments of the inventors, the desired compensation effect is preferably obtained when the annealing temperature is between approximately 800 ° C and 1050 ° C. This effect is not as great as one moves away from these annealing temperatures and is generally not obtained below a temperature of 550 ° C. The annealing heat treatment also makes it possible to stabilize the properties of the SiO ₂ coating 4 obtained by the low temperature deposition process.

The effect of the annealing heat treatment on the oxide coating 4 can be explained by the change in the value of the thermal expansion coefficient of the coating [ Cao Z. et al., J. Appl. Phys. 96 (8), 2004, p 4273-4280 ]. Hiller et. al. [ Hiller D. et al. J. Appl. Phys, 107, 064314, 2010, p 1-10 ], discusses the effect of a heat treatment of the RTA (Rapid Thermal Annealing) type on the hydrogen concentration in a deposit of SiO ₂ . Jansen and. al. [ Jansen F. et al. Appl. Phys. Lett, 50 (16), 1987, p 1059-1061 ] reports the effect of the residual hydrogen concentration on the thermoelastic properties of SiO ₂ .

In a variant, the oxide coating 4 can be deposited in the presence of a flux, including in particular one of Na ₂ O, K ₂ O, Li ₂ O, CaO, MgO, Al ₂ O ₃ , B ₂ O ₃ or a combination of these flows, so as to reduce the melting temperature of SiO ₂ , which accordingly modifies the post-treatment temperature. It will be noted that the use of fluxes such as Al ₂ O ₃ or B ₂ O ₃ can increase the value of Young's modulus. The use of flow CaO can likewise increase the tensile strength of coating 4 and act as a stabilizer against absorption of water by SiO ₂ .

The thickness of the coating 4 can be adjusted so as to obtain a desired value of the thermoelastic coefficient of the spiral spring. Indeed, the thermoelastic coefficient of the spiral spring depends on the combination of the first thermoelastic coefficient β ₁ of the material of the core 2 and of the second thermoelastic coefficient β ₂ of the coating 4. In practice, the thickness of the coating 4 is between 0.1 µm and 10 µm, and preferably between 1 µm and 5 µm. In one embodiment, the coating 4 is deposited with a thickness of approximately 2 μm.

As the core 2 of the spiral spring 1 is a flexible structure subjected to mechanical stresses, a separation of the coating 4 deposited on the core 2 is possible. Detachment can be caused, for example, by delamination. Such separation is all the more possible when the mechanical properties, such as the coefficient of thermal expansion, the Young's modulus, of the material making up the core 2 from that of the coating 4 differ. In addition, the internal stresses in the deposited coating 4 can be high.

In one embodiment, the method of manufacturing the spiral spring 1 comprising the deposition of a tie layer 3 between the core 2 and the coating 4. Preferably, the tie layer 3 consists of a aluminum oxide (Al ₂ O ₃ ). The tie layer 3 can be deposited using an ALD deposition process. The ALD deposition process has very good microscopic distributing power. In other words, the ALD deposition has the advantage of being able to deposit the aluminum oxide in a conforming manner to the surface of the core 2, including in the pores present on the surface of the core 2. The quality of the l The anchoring of the tie layer 3 is all the better.

The deposition of the _{bonding layer 3 in Al 2} O ₃ makes it possible to form strong bonds between the aluminum oxide and the functional groups active agents available at the surface of the core 2, in particular of the various forms of carboxyls and hydroxyls present on the surface of the core 2 (which depends on the material constituting the core).

Preferably, the tie layer 3 is deposited with a thickness of about 5 nm. A small thickness of the bonding layer 3 has the advantage of not significantly modifying the mechanical properties of the core 2, and in particular the frequency of the spiral balance oscillator. A small thickness of the tie layer 3 also makes it possible to make the method of manufacturing the spiral spring 1 economically more interesting.

The coating 4, especially when it comprises SiO ₂ , can have hydrophilic properties, even if the annealing heat treatment decreases the hydrophilic character of the coating 4. Still in one embodiment, the method of manufacturing the spiral spring 1 comprises the deposition of an outer layer 5 of an aluminum oxide (Al ₂ O ₃ ) at least partially covering the outer surface of the coating 4. The outer layer 5 is preferably deposited with a thickness of about 300 nm. The step of depositing the outer layer 5 makes it possible to make the core 2 and coating 4 assembly less sensitive to the effects of humidity. In particular, the mechanical properties of the SiO ₂ coating 4 can be adversely affected in the presence of humidity. The Applicant has discovered in the course of elaborate experiments that in order to maintain both the effects of thermoelastic compensation and of protection against the effects of humidity, the heat treatment step should preferably be carried out before the deposition of the outer layer 5. A heat treatment at the end of the deposition of the layer 5 does not produce the desired effects.

By way of illustration, Table A shows the variation values of the CTE with temperature and relative humidity, for a spiral spring comprising the bonding layer 3 of Al ₂ O ₃ deposited by ALD the coating 4 of SiO ₂ deposited by PECVD and the outer layer 5 of Al ₂ O ₃ filed by ALD. (columns 3 to 5). In the case where annealing at 200 ° C and 1050 ° C is performed after deposition of the coating, the variation of CTE with temperature and relative humidity is minimal. A notable increase in the variation of the CTE is however observed when the annealing is carried out after the step of depositing the outer layer 5.

Reference numbers used in figures

1

spiral spring

2

soul

3

tack coat

4

peripheral coating

5

outer layer

β1

first thermoelastic coefficient

β2

second thermoelastic coefficient

h

height

w

thickness

Claims (16)

1. Process for the production of a mechanical oscillator (1) intended to equip a mechanical resonator of a clockwork movement, a MEM sensor or another precision instrument; the oscillator (1) comprising a core (2) made of a material having a first thermoelastic coefficient (β1) of a first sign and a peripheral coating (4) in an oxide presenting a second thermoelastic coefficient (β2); characterized in that the process comprises:

   - the deposition of the coating (4) at a temperature remaining equal or below 500°C; and

   - a thermal annealing treatment of the coating (4) at a temperature of at least 550°C, so that the second thermoelastic coefficient (β2) of the coating (4) has a second sign, opposite the first sign of the first thermoelastic coefficient (β1) and provides sufficient compensation after said thermal treatment wherein the later compensates the effect of the variation of the first thermoelastic coefficient (β1) of the core with the temperature, wherein said first and second thermoelastic coefficients define the variations of the Young modulus as a function of the temperature of the core and of the coating, respectively.
2. Process according to claim 1, wherein the coating (4) is deposited by means of a process of thin layer deposition.
3. Process according to claim 2, wherein said process of thin layer deposition comprises physical vapour deposition (PVD) or a chemical vapour deposition (CVD) or their derived processes such as a plasma enhanced chemical vapour deposition (PECVD) or high-density plasma chemical vapour deposition (HDPCVD), molecular type (MVD) or atomic type (ALD) of deposition or deposition obtained by means of sol-gels.
4. Process according to one of claims 1 to 3, wherein the deposition of the coating (4) is realised at a temperature remaining equal or below 300°C, or remaining equal or below 200°C or at a temperature of about 100°C.
5. Process according to one of claims 1 to 4, wherein the thermal annealing treatment of the coating (4) occurs at a temperature of at least about 600°C, preferably above 800°C.
6. Process according to one of claims 1 to 5, wherein the thermal annealing treatment of the coating (4) occurs at a temperature below about 1050°C.
7. Process according to one of claims 1 to 6, wherein the thermal annealing treatment of the coating (4) is applied for a duration of 2 to 6 hours.
8. Process according to one of claims 1 to 7, wherein the coating (4) is deposited with a thickness in the order of a micrometre, preferably 2 µm.
9. Process according to one of claims 1 to 8, further comprising the deposition of an adhesion layer (3) of Al₂O₃ between the core (2) and the coating (4).
10. Process according to claim 9, wherein the adhesion layer (3) is deposited by means of a process of atomic layer deposition (ALD).
11. Process according to one of claims 1 to 10, further comprising the deposition of an external layer (5) of Al₂O₃ on the coating (4).
12. Process according to claim 11, wherein the external layer (5) is deposited after the thermal annealing treatment of the coating (4).
13. Process according to one of claims 1 to 12, wherein the material making the core comprises the carbon, the silicon or the ceramic.
14. Process according to on of claims 1 to 13, wherein the coating comprises silicon dioxide.
15. Oscillator in chape of a spiral spring (1) obtained according to the process according to one of claims 1 to 14.
16. Mechanical balance hairspring resonator comprising the spiral spring according to claim 15.

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