US2470737A - Method and means of finishing piezoelectric crystals - Google Patents

Description

n E WNGM DE C OV PAL No OMP N E o -Ho M men M w. L w n u 1 IA 0D N r w 6 oC/ flmu I ...n F PCE F IG. Z

INvENTo/v- HENRY M- BACH -rroluvey May 17, 1949.

75 VAcwM PUMP Patented May 17, 1949 METHOD AND MEANS OF FINISHING PIEZOELECTRIC CRYSTALS l-Ienry M. Bach, Lawrence, N. Y., assignor to Premier New York, N. Y.

Crystal Laboratories, Incorporated,

Application August 29, 1944, Serial No. 551,738

olaims. 1

This invention relates to a new and improved method of manufacturing piezo-electric crystals.

An object of this invention is to manufacture piezo-electric crystals having desirable performance characteristics.

A further object of this invention is to provide an improved method of manufacturing predimensioned plated crystals.

A still further object of this invention is to provide a method of manufacturing plated crystals wherein aging effects will be reduced to a minimum.

A still further object of this invention is to provide a method of manufacturing crystals Which have been either pre-dimensioned or which have been empirically formed to give Optimum performance over a specified temperature range and Which have been etched to minimize aging effects.

Further objects of this invention will be apparent from the following description and claims, and from the drawings illustrative of the invention, wherein:

Figure 1 is a set of curves illustrating the performance characteristics of a typical piezo-electric crystal.

Figure 2 is a simplified diagrammatic view showing a preferred arrangement of an apparatus for practicing this invention.

For the successful fabrication of piezo-electric oscillator plates on a mass production scale it is advantageous to employ a pre-dimensioning technique. This technique has been heretofore practiced on the assumption that the oscillator plates will not be loaded or plated. By predimensioning, that is, the selection of optimum values of the dimensions of the plate for a particular frequency, it is possible to produce large quantities of plates for this frequency having substantially uniform performance Characteristics.

The advantages of plated crystals are well known in the art. It is also known that oscillator plates finished by conventional methods have a microscopically thin layer of disturbed quartz adhering loosely to the lapped surfaces thereof. This disturbed layer of 'disoriented material is insecurely held, and progressively detaches itself from the crystal during service, producing so-called 'aging effects, generally causing an increase in frequency over the original normal removing the disturbed layer, either by etching or by very fine lapping, and then depositing plating material on the treated surface to stabilize it.

The etching (or fine lapping) and plating processes respectively affect the pre-determined performance Characteristics of the pre-dimensioned oscillator plates to a measurably serious degree. To demonstrate the effects of plating, for example, on a typical pre-dimensioned unetched blank, reference is made to Figure 1, wherein the temperature Characteristics of the main mode relative to the mechanically coupled modes of vibration of the blank are graphically illustrated The main frequency characteristic of the quartz plate 'before plating, assuming the plate to have proper dimensions, is shown by the typical curve 0 in Figure 1. As shown in this figure, the dimensions have been selected so that for a particular frequency f at room temperature To the frequency characteristic between the specified temperature limits Ti and Tz is Within the points X and Y, said points representing regions of interference with mechanically coupled mdes, whose frequency characteristics are represented by lines I and 2, respectively. The mechanically coupled modes cause activity dips at temperatures TX and Ty which for the unplated, unetched crystal, lie outside the specified temperature range.

It may be added that the above holds true as well for plates whose dimensions have been empirically determined by a cut-and-try edge working technique.

When plating is applied to the crystal, the original frequency-temperature characteristic of the main mode of vibration is slightly lowered due to the loading effect of the plating. The loading effect of the plating is practically negligible as far as the temperature Characteristics of the mechanically coupled modes are concerned since these secondary mode Characteristics are determined mainly by the crystal dimensions. This is illustrated in Figure 1, where it is seen that the shifting of the main mode characteristic due to plating with respect to a relatively stationary secondary mode characteristic such as I may now cause a point of interference such as X1, to be included in the main mode characteristic between the specified temperature range ilmits T1 and Ta.

In this same manner, if a correctly dimensioned plate is lightly etched to remove quartz dust and disoriented surface material to thereby minimize its tendency to agefi the frequencyemperature characteristic of the main mode andra? of the plate is slightly raised, and since the dimensions which produce mechanically coupled modes are afiected only to a negligible degree, this may cause a point of interference X2 betWeen the main mode characteristic and a secondary mode characteristic 2, to be included in the main mode characteristicfbetween the specified temperatur'erangeflimits4 yTr' and T2.

Activity dips Within the specified temperature range may therefore be caused by either plating of fmaking all-'owance foretchingfby finishing the blank a little low: frequency 'prior-to etching.

Both in the case of the first above-described Vm'etliodfof: finishfingZ crystals; i. e.,= by 'plating to frequencyf and in' i thel 'case -of- =the second -abovedescribed 'method ofdfinishi-ng- .'crysta-ls, i. e., by 'etc'hin'g' to'frequency,-:no provision has vbeen -made fory :maintai-nfingthe relationship of the temperature ychaitmteristioiPcurve for the desiredy mode 'constant AWitli-A respectfto: the characteristics -of the b'oupled modes'so as to' prevent the imovement Vinto the 'specifiedfi-temp'erature:.range of activity dipsroaused: by .'sh-ifi'ing'ly oflthermain: mode" fre- 'quency-temperaturelcharacteristic.

i fIn =tlie1 present: invention it is desired to take -advantagef of'fl-the?L opposite "shiftingiof the 'mainI mode vfrequencyl-ten*flperat-'t'lre 4'ch'aracteristic caused lrespe'ctivel-y-by: the etching and! plating of a' *correctlyf dimensioned crystalwblankf to vrestore vthe original roomfwtemperature frequency and to' maizritaihfthe originalirel-ationship of the frequency-temperature ich-aracteristic of -the desired mode o'f Tvilo'ration-'v 'of the-plate 'fwitlr 'respect to -the temperature rcharacteristicsi of- -the mechanicallycouple'd'modes. It -ha-s'ibeen I'foundtby-- experiment.- by etching and 'subsequentlyplatin'g as disclosed herein,-that very :stable 4fop'drating. conditions' ofthe plate can be achieved and that the plate -Willhave very f satisfactory ffreedom '-f-romffaging'eiects together With' substantially'other-same efieotive i geometry a-s nthefinitially'v-correctly dimensione'd blank.

In a preferred embodiment'ofithis invention ing vpressurealt-hougltthe lplating devices are' kept inactive Standard: "electrodes withraised corner's-flas'hshown in: applicant's' U, S. Patent-No. 2,329-,321- are disposedinf-working relationship to H the blank 23-y yand-said electrodes are connected toanexternal conventional tunad-plate oscillator having a rectified D: C. gridl current ammeter connected so= as tomeasure the-f'activity of the lorystaL-vflAn externa-lfvariablevcapacitor Cp is connectedv in-ipa-rallel' With the crystal andV capacitance thereof is adjusted to bringy the crystal back'y to frequency f -fromthe'fvalue resulting from ythe I evacua-tionof f-thefbell ij ar to 'plating pressure,I Thew-adjustment. of fpE at'this point is-noted Als'o xnoted are-thefadius-tment values ofCp at which'factivity dipsfarefobserved. (These values correspond to points along the temperature characteristic at which mechanically coupled modes cause activity dips. A correctly dimensioned blank will exhibit activity dips only at adjustment values corresponding to temperatures outside the specified Working range.) The blank 23 is then removed from 'bell jar'Z-O and etched the desired amount to remove the layer of disturbed quartz Which might tend to cause "aging" of the blank. The etched blank 23 is then replaced in bell jar 20. Frame-Shaped electrodes 2i, suchv as' disclosed in applicant's U. S. Patent 'ticles. -What frequency increase Willv be caused by this deposited I*on each plate.

No. 2','327,487 are disposed in working relationiship to'the blank23, the blank faces are each lightly platedlandsaid electrodes are connected to the external'conventional tuned-plate oscillator having a rectified D. C. grid current ammeter connected so as to measure the 'activity" of the crystal. The frame-Shaped electrodes Zl are of such external length and width as to eX- tend beyond the' edges of the'blank 23 in a manner to prevent :plating lmaterial from-being deposited onth'e edges of the -bl-ank. Pl'ating is'deposited thereon until the original adjustment'values of CD are duplicated.

conventional -means f of accurately measuring the output frequency of the oscillator at any time are employed in the :aboveprocedure.

The initial adjustment value ofy C'p corresponds to kTo in- Figure 1; andthe adjustmentvalues for activity dips correspohd: to TX and Ty 'respectively, in Figure 1. The etched blank in the bell jar Zt before vplating willihavea frequency f' cor- Vres'ponding to point- Bin-Figure 1., at the Tu adjustm'ent of -Cp.--'As plating is=applied-, the frequency Will-decre'ase andthe 'plating is continued 'until the original-frequency-f is reached. i i To test for determiningzthatthe originalIeffective geom- .etry' yhas beenf restoredi Cp isf'varied to locate activity dips. -The activitydipsshouldappear approximately atflthe Vacljustment values of Cp obtained with the unetched blank.

InV mass production, following the teachings of this invention, a quantityV of "fquartz plates are adjusted to the desired ultimate frequency'by tionship are carefullycontrolled) for a' predetermined period of time corresponding to a depth of etch necessary toc'rem'overall disoriented'par- It has been accuratelydetermined exactly mass etch. The plates, 'after this etch, arethen massplated in a controlledn platingapparatus until a predeterminedfmassl of plating has been The predetermined mass is the mass'required to lower the vibrating frequency by the same amount that the mass etching increased the frequency.

While the above-*specific 'arrangement and provcedure has'b'een described as a preferred embodiment o'f-this invention it should be understood that the invention can be practiced in many different Ways and i's not limitedto anetching and plating sequence of operations. Any method of -removing the disturbed layer of materiall other than by etching, such as by fine lapping; for example, can* beused, and any method of loading vback the crystal to-restore its original effective geometry 'other than by plating, such as by X- r'ay irradiatiomfor exampleycan beL used as the means yof reducing i frequency.

This method'also contem-plates'locatingthe activity dips by other methods than by the above described method of varying a shunt capacitance; any method of varying the crystal input capacitance may be used. Furthermore, for greater accuracy in locating activity dips, conventional temperature runs may be made instead of relying on the Variable input capacitance method.

Also contemplated Within this invention are repeated cycles of unloading and loading the crystal to produce the desired effective geometry, that is, an Overplated crystal may be unloaded, as by etching or grinding, to restore the desired effective geometry. In this procedure if it should happen that too much material is unloaded, the desired effective geometry can be achieved by again depositing plating on the crystal. This method thereby provides a means of obtainng the desired effective geometry of any substantially completed crystal without edge Work thereon.

Numerous modifications will be apparent to those skilled in the art. This invention is therefore not to be limited except insofar as is necessitated by the prior art Or by the scope of the appended claims.

What is claimed is:

l. A method of processing a piezo-electric plate comprising the steps of varying the input reactance across the plate until a value of reactance is obtained causing the plate to have an activity dip, etching the plate to remove surface material therefrom, and then loading the plate until the activity dip of the plate occurs at substantially the same value of input reactance as that at Which the activity dip Was Originally 5:3

obtained.

2. A method of processing a piezO-electric plate comprising the steps of varying the shunt capacitance across the plate until a value Of shunt capacitance is obtained causing the plate to have an activity dip, etching the plate to remove surface material therefrom, and then loading the plate until the activity dip of the plate occurs at substantially the same value of shunt capacitance as that at Which the activity dip Was Originally obtained.

3. A method of processing a piezo-electric plate comprising the steps of varying the shunt capacitance across the plate until a value Of shunt capacitance is obtained causing the plate to have an activity dip, etching the plate to remove surface material therefrom, and then depositing plating material on the etched surface until the activity dip of the plate occurs at substantially the same value of shunt capacitance as that at Which the activity dip Was Originally obtained.

4. A method Of processing a piezO-electric plate comprising the steps of varying the shunt capacitance across the plate until a value of shunt capacitance is obtained causing the plate to have an activity dip, etching the plate to remove surface material therefrom, and then irradiating the plate with X-ray radiation until the activity dip of the plate occurs at substantially the same value of shunt capacitance as that at which the activity dip Was Originally obtained.

5. A method of processing a piezO-electric plate comprising the steps of varying the input reactance across the plate until a value of reactance is obtained causing the plate to have an activity dip, removing surface material from the plate, and then loading the plate until the activity dip of the plate occurs at substantially the same value of input reactance as that at which the activity dip Was Originally obtained.

HENRY M. BACH.

EEFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Nmnber Name Date 1.848.630 I-Iulburt Mar. 8, 1932 1,869,160 Marrison July 26, 1932 2,123,227 Bieling July 12, 1938 22041763 Mason June 18, 1940 2,27219911 Mason Feb. 10, 1943 2,320,080 Hight May 25, 1943 FOREIGN PATENTS Number Country V Date 445,046 Germany May 30, 1927

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