Abstract:
The present invention concerns a method for manufacturing a resonator in a substrate characterized in that it includes the following steps:
       a) modifying the structure of at least one region of the substrate in order to make said at least one region more selective;   b) etching said at least one region in order to selectively manufacture said resonator.

Description:
[0001]    The invention concerns a resonator including a body used in deformation as a spiral resonator or a tuning fork. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is known from the prior art that some microtechnical parts made of single crystal or polycrystalline materials, such as resonators, are made by etching. This technique consists in taking the substrate to be etched and depositing a layer of photosensitive resin on top. A mask is placed on the resin and the whole assembly is exposed to light so that the photosensitive resin structure exposed to light is modified. This modified resin is eliminated by the action of a chemical element thus stripping the substrate bare at the places where the resin has been eliminated. 
         [0003]    Subsequently, these bare regions of the substrate are chemically etched in order to create hollow portions. The chemical agent is chosen to etch only the material forming the substrate and not the unmodified photosensitive resin. The duration of this chemical etching step determines the dimensions of the hollow portions. 
         [0004]    Similarly, it is possible to envisage making microtechnical parts by machining and/or polishing so that a drill or polisher may be used to form said parts. 
         [0005]    A first drawback of this chemical etching technique is that it cannot produce hollow portions with straight sides or walls. Indeed, the hollow portions obtained have inclined sides. This means that the surface of the hollow portion varies with depth, i.e. the surface becomes larger or smaller with the depth of the hollow. Generally the surface becomes smaller with depth. This observation means that theoretical calculations have to be adapted to obtain hollow portions with straight sides. Further, this variation in the profile of the hollow portions between theory and practice results in a variation in characteristics. 
       SUMMARY OF THE INVENTION 
       [0006]    It is an object of the invention to overcome the aforecited drawbacks by proposing a manufacturing method for making a component having hollow portions whose sides have an easily achievable inclination. 
         [0007]    The invention therefore relates to a method for manufacturing a resonator in a substrate characterized in that it includes the following steps: 
         [0008]    a) modifying the structure of at least one region of the substrate in order to make said at least one region more selective; 
         [0009]    b) etching said at least one region in order to selectively manufacture said resonator. 
         [0010]    An advantage of the present invention is that it makes it possible to create recesses located underneath the surface since the material used is selected to be laser-transparent. This enables the laser to aim at any point on the surface or underneath the surface of the resonator. The material is thus made selective so that the chemical etch that follows is only performed on the material that has undergone modification by the laser. A method is thus obtained which allows complex inner recesses to be created. 
         [0011]    Advantageous embodiments of the method of the present invention form the subject of the dependent claims. 
         [0012]    In a first advantageous embodiment, the method further includes the step of: 
         [0013]    c) releasing the component from said substrate. 
         [0014]    In a second advantageous embodiment, the structure of said at least one region is modified by a laser having a femtosecond pulse duration. 
         [0015]    In a third advantageous embodiment, the substrate is made of single crystal material. 
         [0016]    In another advantageous embodiment, the substrate is made of polycrystalline material. 
         [0017]    In another advantageous embodiment, the substrate is made of an amorphous material such as ceramic or glass. 
         [0018]    In another advantageous embodiment, the substrate is made of polymer. 
         [0019]    In another advantageous embodiment, the component is a resonator including a base from which extend at least two parallel arms each having an upper surface and a lower surface and further including at least one recess made on one of the surfaces of at least one of the at least two parallel arms. 
         [0020]    In another advantageous embodiment, said at least one recess takes the form of a groove having at least one vertical side or walls. 
         [0021]    In another advantageous embodiment, said resonator includes one groove per arm. 
         [0022]    In another advantageous embodiment, said resonator includes a groove on the upper surface of each arm and a groove on the lower surface of each arm. 
         [0023]    In another advantageous embodiment, said resonator has two grooves per arm. 
         [0024]    In another advantageous embodiment, said resonator includes two grooves on the upper surface of each arm and two grooves on the lower surface of each arm. 
         [0025]    In another advantageous embodiment, the method further includes a step consisting of modifying the resonator structure by laser so as to locally modify the refractive index of the material. 
         [0026]    In another advantageous embodiment, the component is a resonator including an inertia block coupled to a body formed of a bar coiled around itself to form a spiral, characterized in that said body has at least one hollow portion for locally modifying the rigidity thereof and thus adjusting the resonator frequency and/or regulating the isochronism defect. 
         [0027]    In another advantageous embodiment, said at least one hollow portion includes at least two parallel vertical sides or walls. 
         [0028]    In another advantageous embodiment, said body is made of quartz or ceramic or glass and includes sides that are locally parallel and vertical over the entire length of said body. 
         [0029]    In another advantageous embodiment, said body has a raised outer curve with respect to the other coils formed by the bar coiled on itself. 
         [0030]    In another advantageous embodiment, said body transmits and diffuses the light emitted by at least one luminous energy source. 
         [0031]    In another advantageous embodiment, the structure of said body is modified by laser so as to locally modify the refractive index of the material. 
         [0032]    In another advantageous embodiment, the structure of said body, the refractive index of whose material is modified by laser, is used to form a mirror. 
         [0033]    In another advantageous embodiment, the structure of said body, the refractive index of whose material is modified by laser, is used to form a light guide. 
         [0034]    The present invention concerns a resonator including a base from which extend at least two parallel arms each having an upper surface and a lower surface, and at least one group of electrodes located on one of the at least two parallel arms to electrically excite said at least two parallel arms, characterized in that it includes at least two straight parallel arms. 
         [0035]    In a first advantageous embodiment, one of the surfaces of one of the at least two parallel arms includes at least one recess taking the form of a groove having vertical sides or walls. 
         [0036]    In a second advantages embodiment, said resonator includes one groove per arm. 
         [0037]    In a third advantageous embodiment, said resonator includes one groove on the upper surface of each arm and one groove on the lower surface of each arm. 
         [0038]    In another advantageous embodiment, said resonator has two grooves per arm. 
         [0039]    In another advantageous embodiment, said resonator includes two grooves on the upper surface of each arm and two grooves on the lower surface of each arm. 
         [0040]    In another advantageous embodiment, the grooves include a projecting portion which optimises the electrical fields, while increasing alignment tolerances. 
         [0041]    In another advantageous embodiment, all of the sides of said resonator are straight and vertical. 
         [0042]    In another advantageous embodiment, all of the angles of said resonator are right angles. 
         [0043]    In another advantageous embodiment, all of the sides of said resonator are straight. 
         [0044]    The invention also concerns a resonator including an inertia block coupled to a body formed of a bar coiled on itself to form a spiral, characterized in that said body includes at least one hollow portion for locally modifying the rigidity thereof and thus adjusting the resonator frequency and/or regulating the isochronism defect. 
         [0045]    In another advantageous embodiment, said at least one hollow portion includes at least two parallel sides or walls. 
         [0046]    In another advantageous embodiment, said body is made of quartz or ceramic or glass and includes locally parallel sides or walls over the entire length of said body. 
         [0047]    In an advantageous embodiment, said body has a raised outer curve with respect to the other coils formed by the bar coiled on itself. 
         [0048]    In an advantageous embodiment, said body transmits and diffuses the light emitted by at least one luminous energy source. 
         [0049]    In an advantageous embodiment, the structure of said body is modified by laser so as to locally modify the refractive index of the material. 
         [0050]    In an advantageous embodiment, the structure of said body, the refractive index of whose material is modified by laser, is used to form a mirror. 
         [0051]    In an advantageous embodiment, the structure of said body, the refractive index of whose material is modified by laser, is used to form a light guide. 
         [0052]    In an advantageous embodiment, the resonator further includes a collet integral with said body, said collet has a hole arranged for mounting said resonator on an arbour, and the hole has at least one side on which at least one groove is arranged so that an intermediate element can be inserted between the collet and the arbour. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0053]    The objects, advantages and features of the method according to the present invention will appear more clearly in the following detailed description of embodiments of the invention, given solely by way of non-limiting example and illustrated in the annexed drawings, in which: 
           [0054]      FIGS. 1 and 2  show a schematic front view of a resonator according to the prior art. 
           [0055]      FIGS. 3   a  to  3   e  are schematic views of the steps of the method according to the present invention. 
           [0056]      FIGS. 4 and 5  show schematic views of a first embodiment of the resonator according to the present invention. 
           [0057]      FIG. 6  shows a schematic view of a variant of the first embodiment of the resonator according to the present invention. 
           [0058]      FIG. 7  shows a schematic view of a second embodiment of the resonator according to the present invention. 
           [0059]      FIGS. 8 and 9  show schematic views of a first variant of the second embodiment of the resonator according to the present invention. 
           [0060]      FIG. 10  shows a schematic view of a second variant of the second embodiment of the resonator according to the present invention. 
           [0061]      FIG. 11  shows a schematic view of a third embodiment of the resonator according to the present invention. 
           [0062]      FIG. 12  shows a schematic view of a variant of the first embodiment of the resonator according to the present invention. 
           [0063]      FIG. 13  shows a schematic view of the spiral part of the sprung balance resonator according to the present invention. 
           [0064]      FIGS. 14 and 15  shows schematic views of variants of the spiral part of the sprung balance resonator according to the present invention. 
           [0065]      FIG. 16  shows a schematic view of the fastening part of the sprung balance resonator according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0066]      FIGS. 4 and 5  show schematic views of a component according to the present invention. 
         [0067]    The component  101 , manufactured in accordance with the method of the present invention, is a resonator  300 . A conventional resonator, shown in  FIG. 1 , includes a body  100  used in deformation. This body  100  takes the form of a base  302  placed on a support base  306  and from which extend at least two parallel arms  304 . These two parallel arms  304  carry metallisations which form, on the arms, two groups of electrodes  308  for subjecting them to electrical fields and making them vibrate. Resonator  300  further includes, on base  302 , connection pads  309 , respectively connected to groups of electrodes  308  as seen in  FIG. 5 . Each arm  304  includes an upper surface  314  and a lower surface  316 . The typical dimensions of this type of resonator  300  are: length 1 mm, width 0.1 mm, thickness 0.1 mm. The resonator also has sides or walls  311 . 
         [0068]    In order to improve the characteristics of resonator  300 , it is known to form recesses  200  taking the form of grooves  310  or channels located on arms  304 . These grooves  310  increase the piezoelectric coupling, which decreases the electric power consumption. This decrease in power consumption is a result of the decrease in electrical resistance representing losses in the equivalent circuit of resonator  300 . 
         [0069]    The present invention proposes to provide a manufacturing method for forming grooves  310  of component  101 . In the present case, the emphasis will be placed on the creation of grooves  310  or recesses  200 . 
         [0070]    In a first step shown in  FIG. 3   a , component  101 , which is resonator  300  here, is provided with no grooves  310  formed therein. It is thus understood that resonator  300  is pre-formed. Resonator  300  is made by the most suitable method according to its constituent material, referred to as the “first material” here, such as for example machining or chemical etching. 
         [0071]    A second step consists in taking a laser (L) having a femtosecond pulse duration here, namely 10 −15  of a second (typically 100 fs). The pulse duration may range from a femtosecond to a picosecond (10 −12  of a second). This laser (L) is then used to modify the structure of said arm  304  as seen in  FIG. 3   b . To this end, the first material is chosen to be laser-transparent. This means that the focal point (P) of the laser (L) can aim at a point (P) which may be on or underneath the surface of arm  304 . For the laser (L) having a femtosecond pulse duration here, the first material may be a single crystal material such as quartz, sapphire or synthetic ruby, or a polycrystalline material, such as polycrystalline ruby, or an amorphous material like glass, such as silica or a ceramic. The focal point (P) is then aimed at resonator  300  on regions located on or underneath the surface of resonator  300 . This is all carried out in a predetermined or desired sequence so as to cause a local modification of the structure by multiphoton absorption. Indeed, modification of the structure of the material by multiphoton absorption requires a very high energy density. It is currently only possible to obtain such a high energy density with lasers whose pulse duration is very low, i.e. around a femtosecond or picosecond. These lasers are in fact capable of supplying this energy density at the focal point, i.e. where the energy density is highest. The structure of a first arm  304  is thus modified as seen in  FIG. 3   c  and attention is turned to the other arm  304 . Resonator  300  is thus obtained, as shown in  FIG. 3   d.    
         [0072]    The third step consists in taking a chemical agent. This chemical agent is chosen to allow regions Z 1  whose structure has been modified, to be dissolved more quickly than regions Z 2  whose structure has not been modified. This means that the etching speed of regions Z 1  whose structure has been modified by focal point (P) of the laser (L) is higher than the etching speed of regions Z 2  whose structure has not been modified by the laser (L). In fact, the local modification of the structure by the focal point (P) of the femtosecond laser (L) means that it is possible to select a chemical agent which is more reactive to modified regions Z 1  than to unmodified regions Z 2 . Consequently, dipping said resonator for a determined time period in a bath formed of the chemical agent, dissolves all of the regions Z 1  whose structure has been modified by focal point (P) of the laser (L). Of course, the dimensions of resonator  300  are calculated to take account of the etch of the chemical agent on the unmodified regions and thus not to excessively dissolve the regions whose structure has not been modified. Resonator  300  is thus obtained, as shown in  FIG. 3   e.    
         [0073]    Further, it will be noted that in order for the dissolution of regions Z 1  whose structure has been modified to occur, regions Z 1  must be accessible to the chemical agent. It is thus understood that at least one surface or at least one region Z 1  close to the surface is modified. Indeed, this method makes it possible to create internal structures, but requires that the chemical agent has access to regions Z 1  whose structure is modified. If a region Z 1  whose structure is modified is at the surface, said chemical agent is able to dissolve modified regions Z 1  immediately. However, it is possible to envisage having regions Z 1  whose structure has been modified which are not located on the surface but just underneath the surface. The chemical agent then dissolves the few unmodified regions Z 2  separating said chemical agent from modified regions Z 1  and then dissolves said modified regions. Of course, this means that each recess  200  of resonator  300  must be configured accordingly or be linked. Once all of modified regions Z 1  have been dissolved, said resonator is removed from the bath. 
         [0074]    A fourth step consist in cleaning said resonator to remove all of the residues of the chemical agent. This stops the chemical reaction for good. 
         [0075]    The method of the present invention has a twofold advantage. Indeed, the method makes it possible to locally modify the structure of the material of the component so that each place on the surface or underneath the surface of the component can be structurally modified. It is thus possible to structurally modify resonator  300  in complex shapes which will be chemically etched in a precise manner. For example, it is possible to envisage having trapezoidal arms as seen in  FIG. 12 . 
         [0076]    This possibility of complex shapes is then used to make sides or walls  311  and hollow portions  310  having straight vertical sides or walls  312 . Indeed, modified regions Z 2  are arranged to provide straight sides or walls  312  and because the chemical etching step only acts on the modified regions, the verticality of sides  312  is maintained. It is possible to generalise by stating that the advantage of the method is in retaining the original shapes of recesses  200 . The presence of straight vertical sides  312  improves the piezoelectric coupling. Indeed, these sides  312  lead to an increase in the motion capacity of resonator  300 . This motion capacity represents the piezoelectric coupling efficiency. For resonator  300  shown in  FIG. 4 , the increase in piezoelectric coupling gain is around 30%. 
         [0077]    In a first implementation seen in  FIGS. 4 and 5 , the resonator adopts the model of prior art resonator  300 , i.e. it has two grooves  310 . These two grooves  310  are each located on upper surface  314  of one of arms  304 . Each arm  304  thus has one groove  310 . Preferably, groove  310  of each arm  304  is placed centrally as shown in  FIG. 4 . These grooves  310  may thus have the same shape as that of the arms, namely trapezoidal in the case of  FIG. 12 . 
         [0078]    In a variant of this first implementation of the invention, each surface  314 ,  316  of each arm  304  includes one groove  310 . It is understood that for a resonator  300  having two arms  304 , said resonator  300  has four grooves  310 . There is thus one groove  310  on upper surface  314  of each arm  304  and one groove  310  on the lower surface  316  of each arm  304 . In this variant, grooves  310  of each arm  304  are opposite with respect to the plane A-A′ seen in  FIG. 6 . 
         [0079]    In a second implementation, there are at least two grooves  310  on one of surfaces  314 ,  316  of each arm  304 , as seen in  FIG. 7 . Preferably, the at least two grooves  310  of an arm  304  are placed on the same surface  314 ,  316  as the at least two grooves  310  of the other arm  304 . This arrangement optimises the electrical fields in resonator  300 . 
         [0080]    This second implementation may have a variant wherein each surface  314 ,  316  of each arm  304  includes at least two grooves  310  as seen in  FIG. 8 . 
         [0081]    This variant may consist in having two channels each including a straight side or wall  312   b  and an inclined side or wall  312   a , the two grooves  310  being separated by a central element  313  as seen in  FIG. 9 . These channels are arranged so that central element  313  is formed by the inclined sides  312   a  of the two grooves  310 . These sides  312   a  are inclined so that the surface of groove  310  decreases as the depth increases. This arrangement takes advantage of the straight sides  312   b  of grooves  310  to increase coupling. Further, the fact of having two small grooves  310  instead of one very wide groove  310  means that not too much material is removed from resonator  300  and it is therefore weakened less. 
         [0082]    This second implementation and the variant thereof also have the advantage of making it possible to shorten the duration of the method since there is less material to be removed and thus less material to be modified with the laser (L). 
         [0083]    However, to achieve this second implementation, it is possible to use the method of the invention in a different manner. Indeed, the invention was described as using a laser (L) having a femtosecond pulse duration for modifying regions of resonator  300  and dissolving them by chemical action. It is also possible to use the laser (L) to cut resonator  300 . Laser (L) is used to modify the contours of a region to be eliminated. The chemical etching step is then performed and dissolves the contours of the region to be eliminated. Since the contours of the region are dissolved by the action of the chemical agent, the region to be eliminated is separated from resonator  300 . The surface of resonator  300  can then be sculpted and a part thereof removed, for example to make grooves  310 , without modifying the structure of the entire part. The time saving is consequently significant. 
         [0084]    In a second variant seen in  FIG. 10 , each groove  310  includes a relatively large surface portion  314 . These surface portions  314  take the form of a projecting part  315  located at the surface of resonator  300 . This projecting part  315  has a rectangular profile and extends towards the centre of the arms. In addition to the advantages of providing an optimised electrical field, this arrangement improves the alignment of the masks. Indeed, in the case of this second implementation, surface portions  314  are used to increase the distance between the lateral electrodes  308  and those placed at the centre of the tuning fork arms. Greater alignment precision or tolerance of the various masks is thus obtained and therefore an increase in the reliability and efficiency of the method. It is thus clear that the resonator, grooves  310  and surface portions  314  are made prior to the step of creating the electrodes. The method according to the present invention makes it possible to use a shading or photosensitive spray masking method. 
         [0085]    In a third implementation seen in  FIG. 11 , it is possible for resonator  300  to include a third arm  304  so that resonator  300  has a similar shape to that of a trident. This third arm  304  is used to secure resonator  300  and more specifically to modify its centre of gravity. Indeed, this third arm  304  is used as a securing point so that it is in contact with support base  306  supporting resonator  300 . This arrangement of the third arm  304  which acts as the central arm  304  makes it possible to position the centre of gravity and obtain the best equilibrium of resonator  300 . Of course, resonator  300  with three arms  304  may be arranged to have a shape which substantially resembles that of the letter W or the letter M. 
         [0086]    Advantageously, the method according to the present invention may be used simply to form the grooves  310  of arms  304  of resonator  300 . Indeed, since the method of the present invention is precise and can achieve complex shapes, it is also possible to envisage using the invention to create all or part of resonator  300  from a substrate. This possibility of forming all of resonator  300  using the method of the present invention is advantageous since it shortens the duration of the method. The use of this method makes it possible to perform, in a single step, the step of forming the contours and the step of forming grooves  310  of resonator  300 . Of course, it will be understood that this procedure can also be used for the case of a standard resonator  300  with two arms  304 . Moreover, it is possible to envisage only using the method of the present invention to make a resonator  300  with two or three arms  304  having no grooves  310  on one and/or the other of arms  304 . Another advantage of this technique lies in the possibility of eliminating the residues of the chemical etch between the two arms and, when fins are arranged at the end of arms  304 , at the base of the fins. 
         [0087]    Indeed, an etch by photolithography and chemical etching has the drawback of being imprecise since the chemical etching is not perfectly controlled. Consequently, it is observed that etch residues remain on some portions of a resonator part to be created. For example, it is noted in  FIG. 2  showing a resonator made by the photolithography and chemical etching method, that etch residues  502  remain in region  500  located between the two arms  304  or at the base of the fins of the fin region  501 . These residues may cause an unbalance in the part since the distribution of material is not necessarily perfect. 
         [0088]    Conversely, with the method of the invention, the femtosecond laser illumination of the contours is precise which has the advantage of providing a precise and more easily controllable etch and thus of balancing the arms. It is therefore possible to obtain a resonator  300  wherein all the sides  311 ,  312  are straight and vertical and wherein all the angles are acute regardless of whether or not they are right angles as seen in  FIG. 4 . 
         [0089]    It is thus possible to imagine the resonator of the invention taking the form of a sprung balance resonator. This sprung balance resonator includes an inertia block called a balance wheel on which a balance spring is coaxially mounted. The balance wheel includes an annular mass called a felloe held by at least two arms and the balance spring includes a body  100  which takes the form of a bar  5  coiled on itself to form a balance spring  400  as seen in  FIG. 13 . This type of balance spring  400  may be manufactured using the method of the invention. To achieve this, the contours of balance spring  400  are drawn on a substrate using a laser (L) having a femtosecond or picosecond pulse duration. The chemical etching step then separates balance spring  400  from the rest of the substrate. 
         [0090]    The advantage of the method of the present invention is that it makes it possible to precisely create the shape required for balance spring  400 . This method also makes it possible to obtain a balance spring having straight sides or walls over the entire length thereof, i.e. to have parallel sides locally. This therefore avoids the overhanging portions in some crystallographic orientations which are present after a conventional etching technique, and thus provides improved balance of the spiral resonator. 
         [0091]    Further, owing to the precision of the shapes obtained, the method used makes it possible to create the most complex shapes such as for example a Breguet overcoil. This type of coil differs by having a Phillips terminal curve, i.e. a raised outer curve. A three dimensional balance spring is thus obtained. Thus, with the method of the present invention, it is easy to draw the contours of the Breguet overcoil with the laser (L) having a femtosecond pulse duration. 
         [0092]    The method of the present invention also makes it possible to facilitate the adjustment of resonator  300 . Indeed, the two great characteristics of a sprung balance resonator are frequency and isochronism, i.e. the capacity of a resonator  300  to have an oscillation duration independent of the amplitude of oscillation and the position of the watch. To adjust the frequency and isochronism of a sprung balance resonator, local modification of the rigidity of the spiral is one solution. To achieve this, the precision of the method of the invention is useful since it makes it possible to form recesses  310  or grooves or channels along the coils of the spiral resonator. These recesses or grooves or channels are made locally so as to locally decrease the rigidity of the resonator and take advantage of the precision of the method. These recesses  310  have at least two straight parallel sides, but it is possible for the sides to be in parallel pairs. The sides of the recesses may also be locally parallel. The method therefore makes it possible to create the contours and recesses  310  of the spiral resonator in the same single step, thereby simplifying the manufacture of the spiral resonator. 
         [0093]    In a variant, it is possible for the spiral, made of a laser-transparent material, to be illuminated. To achieve this, a light device is placed in proximity to a location on balance spring  400  preferably at one end thereof. The light emitted by the device is then diffused in the balance spring. Since the method of the invention allows the inner structure of body  100  of the spiral resonator to be modified by the laser, this thus makes it possible to locally modify the refractive index of the material. This feature makes it possible to produce inner structures  600 . These inner structures  600  may be “conventional” mirrors  601 , or use two or three-dimensional crystal photonic systems as seen in  FIG. 14 . 
         [0094]    This modification of the refractive index of the material may be achieved so as to form a light guide  602  seen in  FIG. 15 . To achieve this, the material is modified so as to form a channel of square or circular cross-section which may cover or all part of the coil. 
         [0095]    Likewise, it is possible to envisage using the laser illumination and etching method to improve the securing of said spiral resonator as seen in  FIG. 16 . Indeed, a spiral resonator  400  is secured to an arbour  700  via a collet  401  which may be integral with the body  100  coiled to form a coil. This collet is located at the inner end of body  100  so as to be at the centre of spiral resonator  400 . Collet  401  includes a through hole  402  which may be circular or rectangular and into which arbour  700  is inserted. To improve securing, at least one recess or one groove  701  may be made on sides  403  of hole  402  so that an intermediate element  702  can be inserted between the collet and the arbour when said spiral resonator is secured. This intermediate element may be metallic or take the form of an O-ring joint. 
         [0096]    It will be clear that various alterations and/or improvements and/or combinations evident to those skilled in the art may be made to the various embodiments of the invention set out above without departing from the scope of the invention defined by the annexed claims.