DE2516307A1 - Quartz crystal oscillator element - has high frequency stability over whole of life span - Google Patents

Abstract

A simple quartz crystal oscillator element is designed so that high resonance frequency stability over its life-span is achieved. The final stable frequency of the element is achieved sooner than for traditional elements, and that frequency is maintained accurately for the remainder of its life. In its simplest form the element comprises a small thin disc (1) of quartz with a slightly thicker section (2) at the centre of the disc. Two electrodes (3), one above and one below the disc, and arranged diametrically opposite to each other, run across the element over extended radii. The configuration of energy when the element oscillates is described. A number of similar, alternative forms of element construction are also described.

Description

"Electromechanical resonator arrangement that can be used as a frequency standard" The invention relates to an electromechanical resonator arrangement that can be used as a frequency standard with a flat or convex disk made of piezoelectric material, preferably Made of quartz, which contains at least one resonator area, the one in the middle has a lower resonance frequency than its surroundings, for generating normal frequencies Oscillator arrangements are preferred for medium frequency stability requirements used, which is located in a housing as a frequency-determining element Have a quartz resonator with high mechanical oscillations, which is controlled by a thermostat is maintained at a substantially constant temperature. The natural frequency Such resonators, however, experience even with the most favorable construction of the arrangement ene as Aging known constant change over time, so that There are limits to the use of such resonators with the highest stability requirements are. For example, with the help of complex and costly manufacturing and Cleaning method ideally maximum relative frequency changes of 10 10 / day for quartz resonators. These are all lens crystals in the frequency range between approx. 1 and 5 MHz, which is in the fundamental or harmonic of the Thickness shear vibration are operated.

Because of the lens contouring it is achieved that the vibration energy remains largely concentrated under the excitation electrodes and a reduction vibration quality and aging due to the interaction with the disc edge and the Bracket are largely excluded.

Especially in connection with the required very precise compliance the crystallographic orientation of the center plane of the quartz lens (usual tolerances In the arc minute range) the production of such lenses is very expensive.

With the planar quartz disks, which are easier to manufacture, however, must through correspondingly thick electrode coatings for a significant reduction in frequency the usually used thickness shear oscillation of the excited quartz region the Electrodes are provided opposite the surrounding quartz disk, the concentration of the vibrational energy on the known as "energy capture" to reach the stimulated area. However, since the acoustic damping processes in the Metal are orders of magnitude higher than in the quartz material and therefore the metal coating the quality of oscillation of the quartz is drastically reduced with increasing thickness, are higher Layer thickness problematic, the compromise between As thick as possible netall occupancy on the one hand to achieve extensive energy capture and the thinnest possible metal coating to ensure a high vibration quality in addition, that with such crystals only medium SchwingCite and stability values are achieved will.

In addition, the high stability values mentioned at the beginning are only available with selected, evaporated under extremely clean conditions in an ultra-high vacuum Single and multilayer electrodes, for example, made of Ti + Au or Pd + Au or Au, achieved. The same applies in particular to the quartz surfaces that are later covered with electrode coatings to be cleaned by complex and correspondingly expensive vacuum bakeout processes.

It has also been found that thermal loads of the Net electrode film, or the interface between quartz and metal electrodes, allegedly significantly to the considerable frequency changes, which are several orders of magnitude move above the already mentioned values during the running-in period after commissioning of the thermostat, so that conventional crystals only after several weeks show their optimal frequency stabilization properties. Above all, own research finally also confirm a considerable contribution of the metal covering to the long-term aging.

The invention was therefore based on the object, as a frequency normal indicate usable electromechanical resonator arrangement, which after commissioning faster than previously known arrangements achieved high frequency stability values and also realizes long-term frequency stability values that are currently known Stand out. In addition, it should be ensured that the manufacturing costs for Highly stable quartz crystals can be kept lower than currently usual without compromising to have to make the also required high vibration quality.

This object is achieved according to the invention in that the resonator area is designed in the form of a raised structure, the surface of which is partially with metallic electrode issues is covered.

The vibrating resonator area is preferably made by a structure etching process carved out of the quartz disk and then to a small extent with metallic electrode coatings for acoustic excitation. With the appropriate The height of the raised structure can have very high relative frequency reductions A of this Area in relation to its surroundings, d. H. compared to the thinner disc area will be realized. Is d the distance from the edge of the raised structure to a Loss process with the absorption factor 6 and has the raised resonator area the proper 40 is the quality Q actually achieved for a flat pane given by (1) Q = (Q0 -1 + #. e-2γ-d) -1 γ is the attenuation constant the thickness shear wave excited in the resonator area. Since such loss processes practically only occur on the edge of the pane and on the mounting elements, is d fijr these processes closely approximate the distance between the disc edge and the resonator structure edge.

For the attenuation constant γ applies to the pane thickness t in the area outside the raised resonator (2) γ = a. tri n whereby a one of the crystallographic position of the direction of propagation under consideration dependent constant and n represents the order of the overtone used.

As the height of the raised structure increases, the relative decrease in frequency increases A and according to equation (2) the attenuation constant>, so that the actually achieved quality Q in equation (1) with sufficient step height practically approaches the intrinsic quality QO of the resonator region.

all from an acoustic excitation within the raised resonator The resulting vibrational energy then remains concentrated within it and can do not escape into the area of smaller pane thickness.

The disruptive influences are then even with small pane dimensions the edge and the bracket on the vibration quality eliminated. The same applies accordingly also for the disturbing influences on aging.

For acoustic excitation of the resonator it is necessary to use the raised Resonator area to a small extent with electrode coatings, preferably made of Aluminum, to be provided, which via connection contacts with the holding and contacting elements are connected. Since in this case the electrodes only have the function of an acoustic Suggestion, but not perform a simultaneous energy capture need, their thickness can be kept very low. In practice it is only limited by the conductivity of the electrodes.

The interfering influence of the electrode material, which increases exponentially with the layer thickness this does not affect the vibration quality.

The advantages of the solution according to the invention can be seen in the fact that high quality values even without expensive rush contouring with such structured planar crystals can be realized, as the influence of loss mechanisms increases the electrodes is reduced by several orders of magnitude.

Since the excitation electrodes are given a considerably smaller area can be called the oscillating a; uarz area, the influence of diffusion and tension processes at the interface between the electrode material and the quartz Aging according to the ratio of the areas of the excitation electrode and the vibrating one Quartz area reduced. The proportion of volume-effective contributions in the electrode material on the quartz aging is reduced even more.

Figure 1 shows schematically an embodiment with a round quartz disk 1, round resonator area 2 and a strip-shaped electrode 3. Both disc thickness t as well as the step height At of the resonator are drawn in a greatly exaggerated manner.

This is practical with the slice thicknesses t im in question Usual frequency range between a few NHz and 50 NHz according to the overtone order this is the case with resonator areas that are a few to a few 10-1 µm thicker are than the non-excited disk area, Figure 4 shows a section A - Å ' through the resonator arrangement. Figure 2 shows the course of the resonance frequency along this cut. In the raised resonator is the frequency of the thickness shear oscillation lower by the contribution Af than in the surrounding quartz disk. Because of the mass of the electrode coating results in a slight amount even with low electrode thicknesses additional frequency lowering among these contributions, so that a total of one plan disk three frequencies must be considered: the frequency f a outside of the raised resonator, the frequency for Af lower within the raised Resonator area and the frequency e of the resonator area provided with electrodes.

FIG. 3 shows the distribution of the vibration intensity along the section AA '. The highest intensity is measured under the excitation electrodes. As a result Due to the very low frequency jump at the edge of the electrode, the vibration energy spreads only with a slightly decreasing intensity within the resonator area. First at the edge of the resonator, due to the high frequency jump, af occurs to the thinner quartz disk areas a drastic drop in vibration intensity, so that practically no Influencing with the edge of the pane takes place more. The preferred embodiment the invention will therefore be flat through the condition Af = a fr> to - e. the mentioned above and in the Angels, ego quartz literature as "plateback" The designated quantity A is given by Af # Af.

a Figures 5, 6 and 7 show further embodiments with U-shaped, circular and annular electrodes.

FIG. 8 shows the impedance curve of a structure constructed according to the invention Resonator in the vicinity of its resonance frequency. The impedance curve is practical identical to that of a conventional quartz resonator. Just the distance between Series resonance frequency fs and parallel resonance which are a measure of the acoustic Represents the coupling factor, is the same resonator area as the conventional resonator and overtone order reduced by a factor approximately equal to the quotient from raised resonator surface and the excitation electrode area is. However, this does not mean a restriction in the application, since the inventive Solution with the same resonator and disk dimensions as a conventional quartz operated in lower overtone order n due to the more perfect energy capture and the acoustic coupling factor h / 1 increases with n.

n

Claims (7)

P a t e n t a ri s p r ü c h e Can be used as a frequency standard electromechanical resonator arrangement with a planar or convex disc piezoelectric material, preferably made of quartz, which has at least one resonator area contains, which on average has a lower resonance frequency than its surroundings, characterized in that the resonator region is in the form of a raised structure is formed, the surface of which is partially covered with metallic electrode coatings is. 2. resonator arrangement according to claim 1, characterized in that the area covered by the metallic electrode coatings is much smaller than the surface of the raised structure. 5, resonator arrangement according to claim 1 or claim 2, characterized in that that the frequency difference between the areas covered with electrode coatings and the electrode-free areas of the raised structure is smaller than the frequency difference between the electrode-free areas and the areas in the vicinity of the raised Structure. 2 e, sonator arrangement according to one of claims 1 to 3, characterized in that that the resulting from the edge steepness width of the transition zone between the raised structure and its surroundings are at most in the order of magnitude of the wavelength the vibrations that can be excited in the raised structure. Esonator arrangement according to one of Claims 1 to 4, characterized in that that the electrode coatings on both sides of the disc either on the edge or in the middle of the raised structure. 6. resonator arrangement according to one of claims 1 to 5, characterized in that that the raised structure by a chemical or physical structure etching process is carved out of the disc. 7. resonator arrangement according to one of claims 1 to 5, characterized in that that the electrode material consists essentially of aluminum.