EP3002638A2 - Method for manufacturing a thermocompensated hairspring - Google Patents

Abstract

A method of manufacturing a mechanical oscillator (1) for equipping a mechanical resonator with a watch movement, a MEMS sensor or other precision instrument; the oscillator (1) comprising a core (2) made of a material having a first thermoelastic coefficient (β1) and a peripheral coating (4) of an oxide having a second thermoelastic coefficient (β ₂ ); the method comprising depositing the coating (4) at a temperature which remains at or below 500 ° C, and heat treating annealing the coating (4) at a temperature of at least 550 ° C, to adjust the second thermoelastic coefficient (β ₂ ) of the coating (4) so that the latter compensates for the effect of the variation of the first thermoelastic coefficient (β ₁ ) of the core with the temperature.

Description

Technical area

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

State of the art

An oscillator or mechanical balance-spring resonator of a mechanical watch is conventionally composed of a flywheel, called a balance wheel and a spiral spring, called a spiral or spiral spring, fixed at one end to the axis of the balance wheel and by the other end on a bridge, called rooster, in which pivots the axis of the pendulum. More specifically, the spiral spring equipping, to date, the movements of mechanical watches is an elastic metal blade of rectangular section wound on itself spiral Archimedes and comprising from 12 to 15 turns.

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

The accuracy 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 spiral and the balance, as well as the variation of the Young's modulus of the spiral, modify the natural frequency of this oscillating assembly, disturbing the accuracy 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 frequency variations are based on the consideration that this natural frequency depends exclusively on the ratio between the constant of the return torque exerted by the balance spring on the balance and the moment of inertia of the latter, as indicated. in the following relation: $$\mathrm{F}=\mathrm{1}/\mathrm{2}\mathrm{\pi }{\left(\mathrm{VS}/\mathrm{l}\right)}^{\mathrm{0.5}}$$

where F is the natural frequency of the oscillator, C is the constant of the return torque exerted by the spiral of the oscillator, and I is the moment of inertia of the pendulum 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 Young's modulus (hereinafter CTE) thermal coefficient, the thermal compensation of the mechanical oscillator is obtained by adjusting the spiral CTE according to the coefficients of thermal expansion of the spiral and the balance. Indeed, by expressing the torque and the inertia from the characteristics of the balance spring and the balance, then by deriving the equation (1) with respect to the temperature, one obtains the thermal variation of the natural frequency: $$\mathrm{1}/\mathrm{F\; dF}/\mathrm{dT}=\mathrm{½}\left(\mathrm{1}/\mathrm{E\; dE}/\mathrm{dT}+\mathrm{3}{\mathrm{vs}}_{\mathrm{s}}-\mathrm{3}{\mathrm{vs}}_{\mathrm{b}}\right)$$

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

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

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

However, the spirals made of these alloys are difficult to manufacture. First of all, because of 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 setting of the regulating organ, which is the technique to ensure that the watch indicates at all times the most accurate time, 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 to have a constant return torque, to provide a very accurate electrical measuring device. However, this document is silent as to the thermal stability of the constant of the return torque of this spring. It can not 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 spiral shape with temperature variations. The thermal stability of the constant of the return torque of this spring is also not mentioned in this document.

The silicon CTE is strongly influenced by the temperature and a compensation of this effect is necessary for its use in horological applications. Indeed, the silicon CTE is of the order of -60 x 10 ^-6 / ° C and the thermal drift of a spiral spring silicon 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 discloses a spiral spring cut into a {001} monocrystalline silicon plate. The hairspring comprises a layer of SiO ₂ , having 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-hairspring assembly.

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

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

Brief summary of the invention

The present invention relates to a method for manufacturing a mechanical oscillator such as a spiral spring intended to equip a mechanical balance spring-balance resonator of a watch movement or other precision instrument. The spiral spring comprises a core made of a material chosen from metals and their alloys, metalloids of which silicon (amorphous, monocrystalline or polycrystalline), ceramics, carbon is 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 process comprising depositing the coating at a temperature of less than 500 °. C, and a thermal annealing treatment of the coating at a temperature of at least 550 ° C.

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

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

In yet another embodiment, the method may include deposition of an outer layer of Al ₂ O ₃ on the coating. The outer layer may 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 moisture.

The present invention relates to a spiral spring obtained by the method as well as a mechanical spring 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 evolution of the value of the Young's modulus, is not or can not 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 non-silicon core. However, the solution of the present invention is also applicable, for example, to an oscillator comprising a silicon core and an SiO ₂ coating where the coating is formed by a deposition process itself and not by the growth of a thermal oxide. . Obtaining an oscillator coated by such a method may be advantageous where it is desired to better control the size and 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 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, 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 of 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 of silicon dioxide (SiO ₂ ), deposited and at least partially covering the outer surface of the core 2. An attachment 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. However, it will be understood that the geometry of the soul may be other than that illustrated in this example, for example, the soul may have a straight or circular section.

The core 2 may be made of monocrystalline silicon, with an orientation such as {001}, {111} or the like, or it may be made of a polycrystalline or amorphous material (polycrystalline silicon, amorphous silicon, quartz glass, silica glass). The core material may also be a metallic material whose melting point remains compatible with the heat treatment step (described below) of the present invention. Moreover, the material of the core may 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 may be a composite of carbon fibers, the list of materials mentioned here being absolutely not 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:

• supplying 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.

In one embodiment, the coating 4 is deposited using a low temperature deposition process, that is to say, at a temperature substantially lower than that used for post-treatment, especially below 500 ° C. Preferably, the deposition of the coating 1 is performed at a temperature below 300 ° C, even more preferred at a temperature below 200 ° C and alternatively at a temperature of about 100 ° C. The deposition temperature may vary, and in particular it may go down during this deposition step.

In particular, the coating 4 may be deposited using a thin film deposition process which may include, but is not limited to, methods such as physical vapor deposition (PVD) or a chemical vapor deposition (CVD). Other thin layer deposition methods are also conceivable for the deposition of the coating 4 provided that the deposition temperature remains at or below 500 ° C. For example, the coating 4 can be deposited using a plasma enhanced chemical vapor deposition (PECVD), high density plasma (HDPCVD), molecular vapor deposition (MVD) method. Atomic Layer Deposition (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 low temperature physical or chemical deposition process, as described above, does not necessarily make it possible to obtain, as such, sufficient compensation for the effect of the variation of the CTE of the soul with the 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 not comprising the coating (column 0), a spiral spring comprising a fastening layer 3 Al ₂ O ₃ deposited by ALD (column 1) and with coating 4 of SiO ₂ deposited by PECVD (column 2). In all cases, the variation of the CTE with the temperature remains important. 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 annealing 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 variation in humidity [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 an SiO ₂ coating obtained by thermal oxidation of silicon at substantially higher temperatures, for example the order of 1000 ° C. The coating 4 of SiO ₂ can also include a small percentage of hydrogen or other impurities. Under these conditions, the thermoelastic coefficient of SiO ₂ is not necessarily (or 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 annealing coating 4, to make the thermoelastic coefficient sufficiently compensatory (or enough abnormal in the context of SiO ₂ ) and stabilize it. In particular, the annealing heat treatment can be carried out with an annealing temperature of at least 550 ° C, and preferably a temperature of between about 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 deposition operation of the coating 4 or indifferently spaced apart by a time interval of several days.

According to the materials used for the core 2 and the coating 4, the annealing heat treatment of the coating 4 can make 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 it compensates 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 charging temperature, for example 200 ° C. After annealing, the temperature can be decreased 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 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 about 800 ° C and 1050 ° C. This effect is not as important when moving 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 modification of the value of the coefficient of thermal expansion of the coating [ Cao Z. et al., J. Appl. Phys. 96 (8), 2004, p 4273-4280 ]. Hiller and. al. [ Hiller D. et al. J. Appl. Phys, 107, 064314, 2010, p 1-10 ], discusses the effect of an RTA (Rapid Thermal Annealing) type heat treatment on the hydrogen concentration in an SiO ₂ deposit. 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 .

Alternatively, the oxide coating 4 may 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 streams, so as to reduce the melting temperature of SiO ₂ , which modifies the post-treatment temperature accordingly. It should be noted that the use of fluxes such as Al ₂ O ₃ or B ₂ O ₃ can increase the value of the Young's modulus. Using the feed CaO can likewise increase the tensile strength of the coating 4 and act as a stabilizer against the absorption of water by SiO ₂ .

The thickness of the coating 4 can be adjusted 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 core material 2 and 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 about 2 microns.

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. The uncoupling can be caused, for example, following a delamination. Such uncoupling is all the more possible as the mechanical properties, such as the coefficient of thermal expansion, the Young's modulus, of the material making up the core 2 of 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 fastening layer 3 between the core 2 and the coating 4. Preferably, the attachment layer 3 consists of a aluminum oxide (Al ₂ O ₃ ). The attachment layer 3 can be deposited using an ALD deposition process. The ALD deposition process has a very good microscopic distribution power. In other words, the ALD deposition has the advantage of being able to deposit the aluminum oxide in accordance with the surface of the core 2, including in the pores present on the surface of the core 2. The quality of the anchoring of the attachment layer 3 is all the better.

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

Preferably, the attachment layer 3 is deposited with a thickness of about 5 nm. A small thickness of the attachment 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 attachment layer 3 also makes it possible to make the spiral spring production process 1 economically more interesting.

The coating 4, especially when it comprises SiO ₂ , may have hydrophilic properties, even if the annealing heat treatment decreases the hydrophilic character of the coating 4. In another embodiment, the method of manufacturing the spiral spring 1 comprises 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 deposition step of the outer layer 5 makes the entire core 2 and coating 4 less sensitive to the effects of moisture. In particular, the mechanical properties of the SiO ₂ coating 4 may be adversely affected in the presence of moisture. The applicant has discovered in elaborate experiments that in order to maintain both the effects of thermoelastic compensation and of protection against the effects of moisture, the heat treatment stage must preferably be carried out before the deposition of the outer layer. A heat treatment at the end of 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 the temperature and the relative humidity, for a spiral spring comprising the attachment 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 coating deposition, the variation of the CTE with temperature and relative humidity is minimal. However, a significant increase in the variation of the CTE is observed when the annealing is performed after the deposition step of the outer layer 5.

Reference numbers used in the figures

1

spiral spring

2

soul

3

tie layer

4

peripheral coating

5

outer layer

β ₁

first thermoelastic coefficient

β ₂

second thermoelastic coefficient

h

height

w

thickness

Claims (16)

A method of manufacturing a mechanical oscillator (1) for equipping a mechanical resonator with a watch movement, a MEMS sensor or other precision instrument; the oscillator (1) comprising a core (2) made of a material having a first thermoelastic coefficient (β ₁ ) and a peripheral coating (4) of an oxide having a second thermoelastic coefficient (β ₂ );
characterized in that the method comprises: deposition of the coating (4) at a temperature which remains less than or equal to 500 ° C; and a heat treatment for annealing the coating (4) at a temperature of at least 550 ° C, in order to adjust the second thermoelastic coefficient (β ₂ ) of the coating (4) so that the latter compensates for the effect of the variation of the first thermoelastic coefficient (β ₁ ) of the core with temperature. Process according to claim 1,
wherein the coating (4) is deposited using a thin film deposition process. Process according to claim 2,
wherein said thin film deposition process comprises a physical vapor deposition (PVD) or a chemical vapor deposition (CVD) or derivative processes such as plasma enhanced chemical vapor deposition (PECVD) or plasma assisted deposition high density (HDPCVD), molecular-type (MVD) or atomic (ALD) deposits, or deposits obtained using sol-gels. Method according to one of claims 1 to 3,
wherein the deposition of the coating (4) is carried out at a temperature which remains less than or equal to 300 ° C, or which remains less than or equal to 200 ° C or a temperature of about 100 ° C. Method according to one of claims 1 to 4,
wherein the annealing heat treatment of the coating (4) takes place at a temperature of at least about 600 ° C, preferably greater than 800 ° C. Method according to one of claims 1 to 5,
wherein the annealing heat treatment of the coating (4) takes place at a temperature below about 1050 ° C. Method according to one of claims 1 to 6,
wherein the annealing heat treatment of the coating (4) is applied for a period of 2 to 6 hours. Method according to one of claims 1 to 7,
wherein the coating (4) is deposited with a thickness of about one micron, preferably 2 microns. Method according to one of claims 1 to 8,
further comprising depositing an Al ₂ O ₃ bonding layer (3) between the core (2) and the coating (4). Process according to claim 9,
wherein the bonding layer (3) is deposited using an atomic layer deposition (ALD) method. Process according to one of Claims 1 to 10,
further comprising depositing an outer layer (5) Al ₂ O ₃ on the coating (4). Process according to claim 11,
wherein the outer layer (5) is deposited after the annealing heat treatment of the coating (4). Method according to one of Claims 1 to 12,
wherein the core material comprises carbon, silicon or a ceramic. Method according to one of claims 1 to 22,
wherein the coating comprises silicon dioxide. Spiral spring oscillator (1) obtained by the method according to one of claims 1 to 14. Spiral balance mechanical resonator comprising the spiral spring according to claim 15.

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