Piezoelectric oscillator
BACKGROUND OF THE INVENTION 
     This invention relates to piezoelectric oscillators comprising a piezoelectric element and a casing, and is more particularly concerned with an oscillator, for use in e.g. a timepiece, in which the casing includes a pair of glass plates which are sealed to one another and whose insides are formed with recesses defining a housing for the piezoelectric element and defining a pedestal for the latter. 
     The manufacture of such oscillators has not so far been trouble-free. Attempts to seal the two glass plates with adhesive have been unsatisfactory because the adhesives available are too permeable to maintain for long enough the vacuum required by these oscillators. Attempts have also been made to seal the two plates with a metal solder but the solder crosses a pair of electrically conductive tracks acting as terminals for the piezoelectric element, hence the need to provide intermediate insulating material of sufficient width to avoid a short-circuit through the solder, thereby substantially increasing the volume of the casing. 
     SUMMARY OF THE INVENTION 
     A main object of the invention is to reduce the cost and size of such oscillators. This object is achieved by forming in the plate with the pedestal, opposite the latter, a hole that opens onto and is sealed by one of the conductive tracks, which hole contains an electrically conductive material forming one of the terminals of the piezoelectric element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings given by way of example: 
     FIG. 1 is a cross-sectional view of an oscillator according to the invention; 
     FIG. 2 is a plan view of the FIG. 1 oscillator; and 
     FIG. 3 illustrates diagrammatically various stages involved in the manufacture of the oscillator shown in FIGS. 1 and 2. 
    
    
     DETAILED DESCRIPTION 
     The oscillator shown in FIGS. 1 and 2 comprises a piezoelectric element 10, shaped like a tuning fork, whose arms 12a and 12b and whose base 14 bear control electrodes 16 and 18. It further comprises a parallelipipedic casing 20 formed by a pair of glass plates 22 and 24. The insides of these plate have recesses 26 and 27 which together define a housing 28. Portion 22a of the inside of plate 22 provides a pedestal for the tuning fork. It bears electrically conductive tracks 30 and 32 which are electrically connected to electrodes 16 and 18 by means of two spots of electrically conductive adhesive or solder not shown. These spots further serve to fix the tuning fork, by its base 14, to pedestal 22a. 
     The peripheral portions of the inner surfaces of plates 22 and 24 are wholly coated by annular electrically conductive tracks 38 and 40 which lie opposite one another and are separated by a sealing frame 42, e.g. of tin alloy, which serves to connect the two plates. Track 32 is in effect an extension of annular track 38. 
     Lower plate 22 is formed with crater shaped holes 44 and 46, one under pedestal 22a, near one end of the plate, the other in the opposite end portion of the plate. Holes 44 and 46 are only shown in FIG. 1 so as not to clutter up FIG. 2. Holes 44 and 46 are sealed at their inner end by conductive tracks 30 and 38 and contain electrically conductive layers 48 and 50 which extend over the outer surface of plate 22 to provide supply terminals 45 and 47 for piezoelectric element 10. 
     Hole 46 may be formed anywhere in the area occupied by annular track 38 but is best located as far away as possible from hole 44 to reduce the risk of a short-circuit, in particular when the casing is being mounted on a printed circuit. 
     FIG. 3 diagrammatically illustrates various stages in one mode of manufacture of the oscillator shown in FIGS. 1 and 2. A pair of glass plates 52 and 54 (FIG. 3a), several centimetres long and wide and having the same thickness as plates 22 and 24, are metallized on both sides (FIG. 3b), first with a layer of chrome, then with a layer of gold. Plates 52 and 54 are then chemically etched (FIG. 3c) to produce alveolate structures 56 and 57, the alveoli respectively corresponding to the recesses 27 and 26 of the casing in FIG. 1. During that same operation, holes 44 and 46 (not shown in FIG. 3) are formed in plate 54. The metal layers initially deposited on the two plates are then chemically etched so as only to leave electrically conductive tracks 30, 32,38,40,45 and 47. A tuning fork 10 is then fixed, by its base 14, with adhesive to an edge portion of each alveolus 57 of plate 54 (FIG. 3d). After depositing a frame of solder (corresponding to frame 42 in FIG. 1) around each alveolus 56, the two plates are then placed on top of one another (FIG. 3e) such that each recess 47 overlies a tuning fork 10. The assembly is then heated in vacuo to soften frame 42 and sealingly to bond the plates to one another. 
     Once the plates have been bonded, metallic layers 48 and 50 (FIG. 1) are deposited in recesses 44 and 46. Instead, these recesses may be filled with a tin based alloy or with electrically conductive adhesive. 
     Both plates are then scribed (FIG. 3e) and broken along the scratched lines to separate the oscillators (FIG. 3f). 
     With this above described and illustrated arrangement, the risk of short-circuiting as a result of electrically conductive tracks being crossed by the sealing frame is eliminated, the size of the oscillator is reduced since the terminals are located on one of the major surfaces of the casing instead of being provided on its ends. The manufacture of the oscillator is as a result simplified firstly because no insulating material need be provided between the conductive tracks and the sealing frame, and secondly, because the two plates of the oscillator casing may be of equal length, unlike before when one plate was made longer than the other to accommodate the terminals provided at the ends of the oscillator casing, thereby enabling the steps described with reference to FIGS. 3e and 3f.