The present invention relates to a method for manufac¬ turing resonators of the kind defined in the preamble of Claim 1.

The invention also relates to plant for the manufacture resonators of the kind defined in the preamble of the main apparatus Claim.

A resonator may comprise, for instance, a piezoelectric crystal plate having mounted on its main surfaces a number of metallic electrodes which function to excite the crystal. The frequency of the resonator can be finely adjusted, by adjusting the mass of the electrodes for instance.

BACKGROUND ART

One problem with prior art technology is that it is not possible to series-manufacture resonators which will remain frequency stable over sufficiently long periods of time, and neither is it possible to produce resona¬ tors whose electrical properties can be reproduced sufficiently well, particularly when the resonators included in said series manufacture shall have mutually different frequencies and/or mutually different dynamic parameters.

In recent times, the demand for long-term resonator- frequency stability has become more strict and at pres¬ ent often lies beneath 1 ppm per year, which in the case of AT-crystals which oscillate in shear mode, for in¬ stance, corresponds to fractions of an atom layer,

calculated as mass on the crystal.

The technique hitherto used is described in "Hy-Q Hand¬ book of Quartz Crystal Devices, David Salt, Van Nostrand Reinhold (UK) Co. Ltd., 1987". This prior art technique comprises some twenty procedural steps, which can be roughly divided into six groups:

1. Mechanical working (grinding/polishing) in various stages to impart a suitable configuration to the crystal. Upon completion of this working process, the crystal has the form of a thin plate whose thickness (typically 0.05-0.5 mm) determines the resonance frequency. 2. Chemical wet etching and cleaning.

3. Application of electrodes, by vapour deposition in vacuum of metal on each side of the crystal.

4. Removal of the crystal in air, and fitting the crystal on a holder and establishing electrical contact with electrically-conductive glue, which is then hardened or cured.

5. Introducing the crystal into a vacuum, so as to further vapourize to a desired frequency while the crystal is connected to an oscillator circuit. 6. Removal of the crystal in air. 7. Encapsulation.

The technique described by Salt results in resonators which age at an unacceptably quick rate, i.e. resonators which have unsatisfactory long-term frequency stability. Furthermore, this known technique also results in crys¬ tals whose electrical properties cannot be reproduced sufficiently well, therewith causing a wide spread of the dynamic parameters C , L and R of the resonators (c.f. Salt, 1987, Figure 6.1).

In an article "Final Report LABCOM Resonator Phase III" from Pinellas Plant, U.S. Department of Energy, GEPP-FR-1046, UC-700, certain production conditions are recited for the manufacture of precision resonators of one single kind and frequency, wherein plates each carrying a plurality of crystals are transported in series through production equipment which includes a plurality of series-connected vacuum chambers, in which electrodes are applied to a coarse crystal in the elec- trode coating chamber, by vapourizing at a pressure of

—6 about 2 x 10 mbars. The coating chamber inlet and outlet are connected by means of closure valves to pretreatment chambers and after-treatment chambers which are maintained at a substantial vacuum, wherein the pressure difference from the electrode coating chamber to an adjacent pretreatment chamber or after-treatment chamber is considerably lower than the difference to atmosphere. The crystal frequency is finely adjusted during the electrode coating process.

An UV-treatment process and ozone treatment process is carried out in a pretreatment chamber maintained under vacuum conditions, while a "baking" process is carried out in another treatment chamber connected to said pretreatment chamber, i.e. the plate is subjected to temperature treatment at about 250°C and at a pressure of 2 x 10 mbars prior to being introduced into the electrode coating chamber. The crystal and its holder are sealed in a capsule in the after-treatment chamber.

According to "Final Report—", the known production equipment includes manually-operated manipulators for transferring the individual crystals in the coating chamber between a transport plate and electrode coating stations, which restricts manufacturing capacity.

DISCLOSURE OF THE PRESENT INVENTION

One object of the present invention is to provide a method and an arrangement of apparatus, hereinafter 5 referred to as plant, which will enable resonators to be produced in series more effectively and in a much more flexible manner than has previously been the case, and by means of which resonators which have high aging stability and which are highly reproducible with respect 0 to their electrical properties can be produced.

Another object of the invention is to enable different resonators in the manufactured series to be given mutu¬ ally different frequencies and/or mutually different 5 dynamic parameters in a flexible manner. A further object is to enable the production stages to be effec¬ tively automated, or to enable the resonator elements to be passed manually through the different process sta¬ tions in a simple and efficient manner.

20

Still another object of the invention is to provide a method and plant which will enable the production equip¬ ment to be program controlled, and then particularly computer controlled.

25

A number of the aforesaid objects are achieved by means of the inventive method, having the characteristic features set forth in the independent method Claim. Further developments of the method are set forth in the 30 dependent method Claims.

Other objects of the invention are achieved with the plant defined in the accompanying independent apparatus Claim, while developments of the plant are set forth in 35. the dependent apparatus Claims.

The electrode treatment chamber is preferably an ultra- high vacuum (UVH)-chamber, in which there is established a base pressure which is lower than 10 mbars and

-9 preferably lower than 10 mbars.

Those chambers which connect with the electrode coating chamber will preferably be high vacuum (HV)-chambers in which there is established a base pressure which is

-5 -6 lower than 10 mbars, and preferably lower than 10 mbars or even lower.

By base pressure is meant the pressure that prevails subsequent to initial evacuation and in the absence of resonators. Subsequent to establishing a vacuum corre- sponding to these base pressures, process gas can be introduced into respective chambers in a controlled fashion, wherein synthetic air can be introduced into the first and the third chamber while argon can be introduced into the coating chamber. Switching of the carriers between the chambers is always effected at base pressure. The base pressures are established primarily in order to keep recontamination as low as possible.

The cleaning operation is preferably carried out in the coating chamber and includes a dry etching process effected by irradiation with atoms, ions, electrons, photons or plasma. This cleaning operation is carried out at an atomic level, and the piezoelectric plate can then be immediately coated with electrodes, which i - parts a very high aging stability to the crystals. The electrodes are formed in the second chamber by means of a PVD-process (Physical Vapor Deposition).

One advantage afforded by the invention is that the resonator element plates can be reached from both sides simultaneously, and according to one preferred

embodiment of the invention, electrodes are deposited simultaneously on the two mutually-opposing sides of the piezoelectric plate at the same time as the resultant resonator frequency is detected and the deposition or coating process is regulated so as to obtain a desired frequency with the resonator being electrode-coated. Electrodes are preferably applied to the resonator element in one single production stage.

The electrode coating chamber is connected to ambient atmosphere via vacuum chambers, and in one embodiment of the invention the carriers can be removed from the coating chamber via the pretreatment chamber through which said carriers are delivered to the treatment chamber. One pretreatment chamber may be connected to each end of the electrode coating chamber and while one array of carriers with resonator elements are pretreated in one chamber carriers with resonator elements which have earlier been pretreated in the other pretreatment chamber can be introduced into the electrode coating chamber for electrode deposition or coating and then returned to said pretreatment chamber, from which the carriers can be removed for encapsulation when the pretreated resonator elements in the first chamber are provided with electrodes in the electrode deposition or coating chamber, and vice versa.

The invention will now be described in more detail with reference to a preferred exemplifying embodiment thereof and with reference to the accompanying drawings, in which

Figure 1 is a schematic, top view of a piezoelectric resonator manufacturing plant constructed in accordance with the invention;

Figure 2 illustrates a carrier on which a resonator element is transported and treated in said plant;

Figure 3 illustrates schematically an arrangement for electrode coating a resonator element;

Figure 4 illustrates schematically a preferred embodi¬ ment of an electrode coating or deposition station in said plant;

Figure 5 is a schematic sectional view taken on the line V-V in Figure 1;

Figure 6 is a schematic sectional view taken on the line VI-VI in Figure 2;

Figure 7 illustrates a preferred embodiment of an elec¬ trical contact spring for connection to a resonator element during an electrode coating or deposition process;

Figure 8 illustrates schematically a carrier when car¬ ried on a storage rail in a first chamber of the plant; and Figure 9 is a schematic top view of a modified produc¬ tion plant.

The process also relates to the treatment of resonator elements 9, as described below with reference to Figure 2.

Each element includes a piezoelectric plate 91, for instance a quartz crystal plate 91, which is mounted between two arms 92 on a holder 93, which may be the standard type HC-49/U or HC-50/U. In the illustrated case, the arms 92 are connected to respective conductive pins 94, which extend parallel with and perpendicularly to the holder-plate support surface, said plate 95 holding together the resilient arms 92 and isolating said arms electrically one from the other.

The resonator elements 9 are carefully inspected and adjusted, so that each plate 91 will have a well-defined position in relation to the holder plate 95 and its conductor pins 94, and also relative to the carrier 10 and its markings 104, as described herebelow.

A number of resonator elements 9, for instance 25 reso¬ nator elements, are mounted on the carrier 10, as illus¬ trated in Figures 2 and 6. The carrier 10 has the form of a rail which is provided with a pair of mutually-, parallel holes 101 for each element 9, said holes re¬ ceiving the pairs of pins 94 on respective elements. The rail 10 is also provided with a support surface 102 which extends perpendicularly to the axes of the holes 101 and which supports the plates 95 of said elements 9. The pins 94 lie in the plane of the plate or disc 91 and all of the holes 101 lie in a common plane, such that the discs 91 of all elements 9 will lie in a common plane with their respective centres located -on a straight line, when the elements 9 are mounted on a rail. The holes 101 are formed by ceramic tubes fitted in bores provided in the rail 10 and function to insu¬ late the conductor pins 91 from the rail 10.

Shown in Figure 1 are magazine chambers 1 and 4, each of which receives a large number of rails 10 for simulta¬ neous treatment of resonator elements carried by said rails. Also shown in Figure 1 are chambers 2 and 3 which are connected in series between the chambers 104 and through which respective rails 10 are advanced singly in their longitudinal directions. Figure 1 also shows the chamber 4 connected to a chamber 5 in which the treated resonator elements are encapsulated, e.g. by resistance welding in a dry and clean protective gas atmosphere or in vacuum.

The chambers 1-5 are arranged in series and are mutually separated by high vacuum feed-valves 52-55, wherein the first chamber 1 and the last chamber 5 have corresponding high vacuum feed-valves 51 and 56 respec¬ tively, through which rails 10 can be introduced and removed respectively.

Each of the chambers 1-5 can be connected to vacuum pumps and a gas supply system.

The vacuum pumps are intended to enable the chambers to be given a HV-status or a UHV-status, for instance so as to enable the chambers to be given the following base pressures, after initial down-pumping and in the absence of rails 10 with elements 9: Chambers 1, 2 and 4 better

-6 -9 than- ^j 10 mbars, and chamber 3 better than 10 mbars.

Subsequent to applying the aforesaid base pressures in the chambers, a suitable gas, which is controlled with respect to impurities, can be introduced into respective chambers, wherein the process can be carried out at a process pressure which is considerably higher than the base pressure.

As illustrated in Figure 1, the chambers 1 and 4 each include a rotational-symmetrical stand 70 which can be rotated about its symmetry axis and which carries a multiple of radially-extending guides 71 for respective rails 10. Also mounted in the magazine chambers 1 and 4 are linear drive mechanism 61, 62 which function to move the rails 10 onto and away from the guides 71. The mechanism 61, 62 may have the form of ball screws which are driven by separate stepping motors. The stand or rack 70 may have the form of a circular carousel plate or turntable which can be rotated to specific angular positions in which the guides 71 are directed towards

respective feed-valves 51, 52; 54, 55.

The rails 10 and the elements 9 carried thereby are subjected to a heat treatment process in chamber 1 (at a temperature of 200 ^D C or higher) in vacuum for a rela¬ tively long period of time, so as to expel water vapour and other volatile substances that may have settled on the surfaces of the rails and elements. The pressure in the chamber 1 is measured by way of a process check, this pressure being dependent on the evacuation of gas from the crystals 91. The heat treatment process is continued in the chamber 1 until the correct pressure prevails therein. The rails 10 and the element 9 car¬ ried thereby are then allowed to cool, whereafter one rail 10 at a time is introduced into the chamber 2, where the elements are subjected to an oxidizing treat¬ ment process with ozone and UV ight. Those organic impurities, such as hydrocarbons, which remain on the elements 9 subsequent to their treatment in the chamber 1 will be decomposed and oxidized to carbon dioxide and water vapour, which is pumped away. A positive assess¬ ment of the degree of purity of the crystals can be obtained by measuring the partial pressure of, e.g., carbon dioxide in the chamber 2, so as to achieve uni- form purity of the crystals from rail to rail. This residual gas analysis is carried out in a separate system, which is normally held closed from the chamber 2, except when samples are taken.

In an alternative embodiment of the inventive equipment or plant, the chamber 2 can either be omitted or used as a feed-valve between the chamber 1 and the treatment chamber 3, while the UV-ozone-treatment process can be carried out in the chamber 1 simultaneously with the vacuum-heat-treatment process carried out therein.

As illustrated in Figure 5, the rail 10 is moved through the chambers 2 and 3 in the direction of its longitudi¬ nal axis, by means of a transport arrangement 80. Provided in the upper surface of the rail 10 is a groove 106 which extends in the longitudinal direction of the rail 10 and which is preferably V-shaped in cross-sec¬ tion (see Figure 6). The undersurface of the rail 10 is provided with a corresponding groove 107, which extends parallel with the groove 106. The transport arrangement 80 includes a number of wheel pairs 81, 82, the wheel peripheries of which engage in the grooves 106, 107 on the rail 10. As indicated in Figure 5, one wheel 82 may be spring biased in a direction towards the other wheel 81, and one of the wheels, e.g. 81, may be driven for axial movement of the rail 10.

The guides/rails 71 (see Figure 1) can be aligned on the transport arrangement 80 so as to enable rails 10 to be transferred linearly to and from said arrangement. As will be seen from Figure 2, at least the forward end of respective rails 10, as seen in the withdrawal direc¬ tion, as a magnetic screw 108 which enables the rails to be coupled to the linear drive mechanism 61, 62 concerned.

The chamber 3 is constructed so as to be ultra-high-

-9 vacuum compatible (UHV) , i.e. to permit a vacuum of 10 mbars to be established therein. One condition which must be satisfied in order to be able to maintain a pressure of such low magnitude in the chamber 3, despite the large passage of material therethrough, is primarily that the rails 10 and the elements 9 introduced into said chamber are sufficiently clean to ensure that no gas will be released therefrom and into the chamber 3. It is also necessary to surround the chamber 3 with chambers (the chambers 2 and 4 respectively) which are

maintained at a pronounced subpressure when the feed- valve 53, 54 is opened, thereby minimizing the amount of contaminating material that can possibly be transferred to the chamber 3.

The illustrated chamber 3 has two treatment positions 3a, 3b which are mutually separated in the longitudinal direction of the chamber.

As illustrated in Figure 2, the rail 10 has markings 104, which in the illustrated case have the form of milled notches which correspond to the positions of each respective resonator element along the rail 10. When the rail 10 is moved in the direction of its longitudi- nal axis, the position of each marking is detected, so as to enable the rail to be locked in a position which unambiguously defines the position of the associated crystal element 9. It will be understood that the notches 104 can be replaced with corresponding holes, pins or some other identification means, and that the position of respective markings can be detected mechani¬ cally, electromechanically inductively, capacitively or optically. Shown in Figure 2 is a position setting device 11 which includes a pivotally mounted arm 110 which has mounted on its free end a rotatably journalled roller, or alternatively a pointed latching device 111. A spring 112 biases the arm 110 in a direction towards the undersurface of the rail 10, such as to drive the device 111 into a recess 104, so as to place an element 9 in a given position. A detecting device 114 may be provided for detecting engagement of the pointed device 110 in a recess 104, for the purpose of controlling the drive arrangement 80 or for indicating that a crystal 9 has taken a given position.

In position 3a (Figure 1), which corresponds to one station in which the individual elements 9 are temporarily held, there is positioned on each side of the position of the crystal disc 91 an irradiating device which dry-etches off any remaining contaminants with the aid of energy-rich beams of atoms, ions, pho¬ tons or electrons. This etching process is continued until the major surfaces of the disc 91 are atomically clean, or may be continued until the thickness ^' of the disc 91 has been reduced, whereby this process step is able to serve to establish a quartz disc or plate of given thickness. Alternatively, the etching process in the station 3a may be used to subsequently remove (sput¬ ter off) electrode material so as to increase the fre- quency of the element 9. This etching method may also be used to adjust the frequency of "lateral field reso¬ nators" which lack electrodes on their major surfaces. The energy-rich atoms, ions or electrons are generated in "atom guns", in ion beam sources or by high frequency discharge on an appropriate process gas, such as argon for instance.

The rail 10 is advanced in the direction of its longitu¬ dinal axis such that each cleaned element 9 is held temporarily in the position 3b (Figure 1), where the element 9 is coated with an appropriate electrode metal, e.g. gold, by means of a PVD-process (Physical Vapor Deposition), e.g. by vapourizing or preferably sputter¬ ing (cathode atomization) from two coating devices placed on a respective side of the crystal in position 3b, wherein preferably two sputter magnetrons are sta- tionarily mounted on a respective side of the movement path of the crystal discs 91.

The beams of sputtered or vapourized metal are defined by masks 191 which are provided with apertures 192 which

determine the desired electrode form on the crystal, Figure 3.

Illustrated to the left of Figure 2 is an element 9 on which electrodes 98 are applied, each electrode being connected to its respective holder arm 92.

As illustrated in Figure 4, the sputtering magnetrons 86 are directed towards one another equidistant from the path along which the discs 91 move, and the masks 191 are disposed on each side of said path, the centre plane of which is indicated by the chain line 117.

In accordance with one preferred embodiment, the masks 191 have the form of two mutually parallel discs which are provided with pairs of different mask-apertures or openings around their periphery and which can be rotated so as to index the mask-apertures into positions oppo¬ site a crystal disc which is held temporarily stationary in the position 3b. The different mask-apertures may be adapted to different types of resonator elements and, as illustrated in Figure 4, are rotated forwards to working positions by means of an externally driven mechanism.

Also illustrated schematically in Figure 4 is the pair of contact springs 170 which establish contacts with the pins 94 on an element 9 located in the treatment posi¬ tion 3b. A preferred embodiment, 171, of the springs is shown in Figure 7.

As shown in Figure 3, the sputtering magnetron 86 may be screened by a housing 87 having an opening 88 which functions to direct metal vapour essentially solely onto the element 9 in position 3b.

It is important to have accurate control over the crys¬ tal temperature during the coating process, since the resonator frequency is also dependent on temperature. The disc or plate 91 is unavoidably heated, as it is bombarded with the metal atoms which build-up the metal film (mean energy about 1 eV) . Furthermore, thermal electrons (mean energy about 2-5 eV, i.e. electron temperatures 20,000-50,000 K) are generated from the gas discharge plasma in the magnetrons. If these thermal electrons should strike the disc 91, the disc would be heated so powerfully as to render accurate adjustment of the frequency impossible. Consequently, the magnetron 86 is provided with an electron trap 89, e.g. in the form of means which will generate a transversal or axial magnetic field (mirror field) at the orifice 88 of the housing 87, or in the form of a retarding electric field. The chamber 3 is also provided with a heat exchanger, for instance in the form of a conduit, which is in contact with the outside of the chamber 3 and through which cold water flows, so as to regulate the temperature of the chamber walls. This enables a suit¬ able temperature (e.g. +25 or +85°C) to be maintained in respect of the element 9 during the coating process.

As indicated in connection with Figure 4, the pins 94 on elements 9 are connected via electric connector springs 170 to an appropriate high frequency circuit (active oscillator or passive transmission circuit or reflection circuit) which through the piezoelectric effect engen- dered excites mechanical oscillations in the crystal (in the illustrated embodiment shear oscillations), where¬ with the resonance frequency falls (decreases) during the electrode coating process, the frequency being monitored so as to enable the element 9 to be given a desired frequency.

The high frequency measuring circuit may be constructed, for instance, in accordance with standard IRV-444. However, the connector elements will preferably be resilient in the case of the illustrated embodiment, and to this end each of the connector springs is preferably constructed in the manner illustrated in Figure 7, namely as a folded spring strip with a small gap between the legs and with a large width, wherein the free end- part 171 of the spring element is intended to be re- ceived in the recess 103 beneath the holder plate 95 and to establish electrical contact with a respective pin 94 when the element 9 is in the position 3b. The other end of the connector spring is connected first to a pi- network and thereafter to the measuring equipment of the circuit, by means of a coaxial cable, the configuration of the spring connector provide a minimum inductance.

Subsequent to coating each element 9 on a rail with electrodes and adjusting the frequency of said each element, the rail 10 is moved into the magazine chamber 4. When the chamber 4 has been filled with a batch of rails 10, the elements 9 are subjected to an annealing process in vacuum over a suitable period of time, there¬ with relaxing any mechanical stresses that may have built up in the electrofilm 98 and removing any residual noble gas from the magnetron discharges.

The rails 10 are then moved into the chamber 5, where the elements 9 are encapsulated.

Figure 9 illustrates plant which includes chambers 1 and 4 of the earlier described construction, having feed- valves 51 for the introduction of carriers 10 with resonator elements to be treated in accordance with the invention.

The chambers 1 and 4 are also connected to a common electrode coating or deposition chamber 3, via valves 52 and 54. Furthermore, each of the chambers 1 and 4 is connected to an associated encapsulating chamber 5, via valves 55, from which carriers with encapsulated resona¬ tor elements can be taken through a feed-valve 56.

The initial process step in which the resonator elements are cleaned before being introduced into the electrode coating chamber is normally a time-consuming operation which may take as long as six hours, for instance. The working capacity of the chamber 3 can be extended two¬ fold, by dimensioning the chambers 1 and 4 in a manner which will enable the chambers to accommodate a large number of carriers and resonator elements. For instan¬ ce, during the time in which resonator elements are cleaned in the chamber 1, elements which have already been cleaned in the chamber 4 can be introduced into the chamber 3 carried by respective carriers 10.for elec- trode coating and frequency adjustment, one carrier being introduced into the chamber 3 at a time and then returned to its rail in the chamber 4. Treated carriers with resonator elements can then be removed via the valve 55 and encapsulated in the chamber 5, whereafter other carriers with elements to be cleaned are intro¬ duced into the chamber 4, while elements that have been cleaned in the chamber 1 are electrode coated in the chamber 3.

If required, the chamber 3 can be equipped with several working positions or may be divided into a sequence of separate chambers. This affords the possibility of new or alternative process steps, such as:

a. Coating the resonator elements with an alter¬ native metal, for instance aluminium or sil¬ ver; b. Pre-coating the resonator elements with ate- rial which will improve the adhesion of the electrode film; c. Pre-coating the resonator elements with mate¬ rial which will minimize mechanical stresses; d. Coating the resonator elements with a protec- tive layer which will passivate the electrode surface against gas adsorption, etc. ; e. Oxidizing the electrosurface (particularly in the case of aluminium electrodes) ; f. Separate coating positions for the applica- tion of a primary plating and a complementary coating for frequency adjustment purposes; g. Frequency adjustment by sputtering-off elec¬ trode material applied with a primary coat¬ ing; and h. Frequency adjustment by etching the quartz surface in the manufacture of "lateral field resonators".

The sputter magnetrons can either be operated with solely a negative d.c. voltage which is applied to the cathode (the target electrode), or with a d.c. voltage together with a superposed high frequency voltage (e.g.

13.56 MHz). The HF-discharge gives rise to a population of energy-rich argon ions, which bombard the substrate during the ongoing plating process and knock loose or move atoms which are weakly bound to the electrode film.

This results in a denser metallic film having the same electrical conductivity as solid metal.

Alternatively, ion beam sputtering can be used. In this case, the target electrode has the form of a rod or a

strip whose one end is sputtered with the aid of a high energy ion beam from an ion source. The rod is advanced successively as it is consumed.