US2493461A - Means and method of forming piezo-electric crystals - Google Patents

Description

Jan. 3, 1950 c. w. MMCONNELL MEANS AND METHOD OF FORMING PIEZOELECTRIG CRYS TALS 2 Sheets-Sheet 1 Filed May 4, 1944 INVENTOR.

CHARLES W MAc Colman.

ATTORNEY v 2 Sheets-Sheet 2 c. w MWCONNELL MEANS AND METHOD OFFORMING PIEZOELECTRIC CRYSTALS Jan. 3, 1950 Fi led May 4, 1944 F Q INVENTOR CHARLES W Macaw/ELL ATTORNEY PM has, 1950 MEANS AND METHOD OI FORMING PIIZO- CRYSTALS Charles W. MaoOonnell. Bouthbridge, Mala, as-

' signer to Harvey-Wellsflommnnioations, Ina, Southbridm-Mm, a corporation of Massachusetts Application May 4, 1944, Serial No. 584,187 r Claims. (curl-w) This invention relates to piezo-electric crystals and more particularly to emcient and economical means and method Of processing said crystals whereby their resultant frequency and output characteristics may be more simply and positively controlled.

This application is a continuation in part of my copending application, Serial No. 423,017, flied December 15, 1941 and which has matured into Patent 2,353,171 issued on July 11, 1944.

One of the principal objects of the invention is to provide improved means and method of processing piezo-electric crystals whereby the said crystals may be made to oscillate properly and may be more positively and economically controlled as to frequency of oscillations and may be more permanently stabilized as to activity and as to said frequency of oscillations.

Another object is to provide more stable or aged piezo-electric crystals and method of making the same.

Another object is to provide improved means and method of lapping piezo-electric crystals whereby substantially parallel surfaces may be formed on the opposed sides of said crystals in a simple and efiicient manner and of subsequently subjecting said crystals to acid etching for rendering said crystals more permanently activity and frequency control.

Another object is to provide novel means and method of economically processing precision crystals of given frequency values and of aging or stabilizing said crystals as to activity and frequency output.

Anoth'er object is to providenovel means and method of changing the frequency of a'crystal which oscillates properly, but which is slightly off as to desired frequency, without materially altering its oscillating properties and for simultaneously rendering said crystal stable as to said desired frequency. Other objects and advantages of the invention will become apparent from the following description taken in connection with the accompanying drawings and it will be apparent that many changes may be made in the details of construction, arrangement of parts and steps of the methcdshown and described without departing from the spirit of the invention as expressed in the accompanying claims. It is therefore desired that the invention not be limited to the exact details of construction, arrangement of parts, and steps of the method shown and described as the preferred forms only have been given by way of illustration. H

Referring to the drawings:

stable as to Fig. I is a perspective view of the device embodying the invention setting forth one step of the process of manufacture;

Fig. II is a sectional view taken as on line 1I-II of Fig. I and looking in the direction indicated by the arrows;

Fig. III is a sectional view taken as on line III-m of Fig. II looking in the direction indicated by the arrows and showing the sectional parts in full:

Fig. IV is a diagrammatic view illustrating a step of a modification in the method of the inventlon;

Fig. V is an exaggerated diagrammatic view of a crystal surface subsequent to the abrading of said surface;

Fig. VI is a view generally similar to Fig. V of said surface subsequent to controlled acid etching; and

Fig. VIII is a greatly exaggerated diagrammatic view similar to Figures V and VI.

It has been usual in processing pieao-electric crystals to reduce the crystals to the proper size. working characteristics and frequenc by very delicate hand controlled operations which depended entirely upon the skill of the operator.

One of the difficulties in processing piezo-electric crystals of the character described has been to obtain a substantially parallel relation of the side surfaces of the crystals while simultaneously reducing said crystals to the required uniform thickness. The uniform thickness is essential in obtaining the working characteristics or activity and frequency values desired of the crystals. Because of the necessity of exceptionally skilled laher in processing said crystals, in the past, and because of the delicate hand operations required to produce crystals of this nature, such crystals have been extremely difficult and costly to make.

This was due to a great percentage of spoilag which inherently resulted from the hand controlled procedure of fabrication and is one of the many disadvantages which the present invention obviates.

Although the reducing of the crystals to uniform thickness by grinding or abrading has been found to be very practical from the viewpoint of saving time and cost, it has been found that crystals reduced to required frequency by this method possess surface irregularities and incipient fractures which during use or when stored for eventual use will break off and destroy the activity of the crystal. 1

The efl'ect is similar to that of drawing a diamond across a piece of glass hardly leaving a entirely through the piece of glass causing cracksto occur. a I

It has also been found that if acrys'tal has bad on the shelf from incipient fractures, iine quartz powder will be found .inside of the electrode holder if the crystal is put to use and forced to oscillate. Buch fractures also alter the frequency of the crystal tending to cause thefrequency of the crystal to increase due the particles breaking off.

A- major feature, therefore. of the present invention, in addition to reducing the surfaces of the crystals to parallelism by abrading, is to etch the crystals by dipping them in acid such ashydroiluoric acid or some other similar etching media to cause relatively pronounced protuberances, fractures and strains produced by the abrading operation to be'etched sumciently so asto prevent or avoid their breaking off or causing parts of the crystal to break off when stored or subsequently put to use. This removal of the fractures, etc. set forth above, causes the crystals to be more stable or produces a so-called aging of said crystals whereby their activity and frequency of oscillation will be more permanent and will not alter after a relatively long period of storage and when then put to actual use. This stabilizing also tends to increase the activity of the crystals and also permits a more positive fretherein communicating with the longitudinal.

quency control to be obtained. This removal by. I

etching is brought about simultaneously to the reducing of the crystal to the desired frequency of oscillation, the abrading being halted at a point whichwill permit removal of the material by etching to obtain the desired frequency and simultaneously stabilise, the said frequency as to activity and frequency of oscillation by removing parts of the crystal which might have'a tendency to break 01! through subsequent jarring,

storing, or during actual use.

It is known in the art that crystals have been acid etched butsuch etching mainly has been for the purpou of initially exposing the twinning with said crystal being thereafter further etched to obtain the frequency desired. I

It has been found by applicant that crystals etched to this extent are not desirable or practical for use. The etching according to applicant's findings on one hand must be sufficient to cause the bottoms of the fractures to become cupped or curved whereby the tendency for parts to break off is greatly reduced or eliminated and,

on the other hand, must not be sufiicient to cause crystals and simultaneously produce the frequency desired but not of an amount suilicient to cause too much of the more readily dissolvable portions of they crystal to be removed.

Referring more particularly to the drawings wherein like characters of reference designate like parts throughout the several views the device which performs one of the essential steps of face of the lap. The piers-electric crystal 4. which is to .be lapped, is held in a holder I and is moved with said holder'over the relatively flat surface I of the-lap i. crystal with the holder and lap can best be seen inl"ig.1I. Theholderlcomprisesamainupright portion I having a base I with a relatively. flat surface I thereon. The main upright portion i has a longitudinal opening I extending throughout the major portion of the length thereof with a closed end I. adjacent the base I. The closed end has a reduced opening ll opening Internally of the opening 0 there is a pair of angularly' disposed plates II. The said plates I2 form a slideway for a slide plunger ll. The said slide plunger is is preferably of a triangular cross section and is held in desired frictional contact with the slide plates II by means of a pressure plate ll. The said pressure plate It is resiliently urged into engagement with the slide plunger is by pins I I pressed toward the plate by backing springs it. The said pins are slidably mounted within the threaded lonlitudinal bores ll of spaced blocks it. The blocks are attached to the upright 6 by suitable means such as pins or screws not shown. The tension of the respective backing springs It is varied by means of screws is threaded with the internally threaded bore of the blocks II.

By adjustment of the screws ll proper tension may be directed tothe resilient backing springs ii, thereby setting up-a pressure on the pins it suitable to retain the slide plunger II in different positions of adjustment longitudinally of the slide plates i2. p

The slide plunger l3, adjacent the lower end thereof. is provided with a protruding portion ll fitting within the opening i I. The protruding portion 20 has an end surface Ii which is substanopening I I and the contour thereof depends upon.

the size and shape of the crystal which is to be abraded and is preferably made slightly larger than the crystal.- The procedure of processing the crystal is substantially as followsLThe crystal is first cut from the mother crystal in proper desired relation to the optical axis, electrical axis and one of the crystallization axes of the mother crystal, depending upon the characteristics desired of the completed crystal. A suitable abradant such as an ordinary mixture of emery and kerosene is placed on the surface 2 of the lap. The kerome is preferably sprayed on the surface of the lap or applied thereto by an ordinary oil cup and a the emery is in powder form and is shaken onto the surface of the lap. A second lap or means simulating the lap l is then placed on the surface of the lap with its-effective abrading surface engaging the effectlve abrading surface of the lap l and is moved in a sidewis direction over said lap to work the abradant into the surtbemetbod.asshOwn inl'igs;IthroughIlI,com- 7.6 faceofthelap i andtocausetheexcessiveabradprises broadly a lap i having a relatively flat su'rfacelthereon. 'lhesaidsurfaoelhasa' plurality of communicatim longitudinal groovu" I therein which provide means into which ex- I cessive abrasive may flow from the effective sur- The relation of the 5 ant to flow intothe longitudinal grooves 8 of said lap. If there is an excessive amount of kerosene or abradant on the lap this may be blotted oil through the use of suitable blotting paper or the like. a

Thecrystallisthenplacedintheopening ii and is rested on the relatively flat end 2| of the portion 20. The said crystal is held in position while the holder 8 is slid onto the surface ofv the lap in a sidewise direction. After the said crystal and holder have been placed on the surface 2 of the lap I a pressure is exerted on the knoblike member 22 to cause the slide plunger II to move downwardly and urge'the crystal I into engagement with the surface 2 of the lap. The

tension resilient means It is such that the plate ll, functioning cooperatively with the slide plates I 2, will frictionally hold the slide plunger It in I adjusted position. The pressure is then released on the knob 22 and the holder 5 together with the crystal 4 is moved into a sidewise direction over the surface 2 of the lap I. This causes the abradant on said surface 2 to remove a given amount from the engaging surface of the crystal. Because of the fact that the surface 2| on the end of the plunger l3 and the surface 8 on the base I are substantially parallel with each other, substantially equal pressure will be distributed throughout the area of the crystal and will cause the opposed surfaces thereof to be abraded substantially parallel with each other. Periodically during the abrading the crystal is removed and cleansed and is tested in a suitable testing device for determining its oscillating characteristics and frequency. If the crystal is of a slightly lower frequency than desired it is again-placed in the holder 5 on the lap I and an additional amount is removed. By following this procedure the crystal may be reduced to the proper thickness to obtain the frequency and oscillating characteristics desired. The crystal, because of the par- 'ticular arrangement of the holder 5 and lap I,

will more positively have its opposed surfaces formed in substantially parallel relation with each other and will therefore be of substantially constant thickness throughout, thereby insuring good working or oscillating characteristics. During the abrading the crystal is first abraded on one side for a given period of time and is then removed from the holder, cleansed, reversed, and again placed in the holder and abraded on the second side for a given period of, time. This procedure more positively insures forming parallel surfaces on the crystal. During said periodic removal of the crystal from the holder it is tested to determine its frequency and also to determine its oscillating characteristics. The procedure is somewhat of a cut and try method.

The above procedure is followed to reduce the crystals to a desired predetermined thickness which preferably is not the final thickness required. The crystals are then preferably reduced in thickness by etching with a suitable acid, such as hydrofluoric acid. This may be accomplished by grasping the sides of the crystal by a suitable holder 23 and by dipping the crystal into a bath 24 of hydrofluoric acid fora given time interval, which is sufficient to obtain the desired frequency. After each dipping, the crystal is washed in water or other suitable liquid to remove the acid. This avoids the danger of altering the oscillating characteristics of the crystal and provides a means of obtaining the desired-frequency without further abrading. It is to be understood that any knownacidmaybeused 6 J border for crystals to oscillate properly and at the desired frequency. the dimensional characteristics must be very carefully. controlled. In

following theteaching of the present invention, the crystals are preferably initially abraded to reduce the surface to substantially parallelism and to reduce the thickness to within controlled limits whereby'a given required amount of the material of the crystal will thereafter have to-be removedby acid etching in order'to obtain the desired activity and frequency of oscillations. The amount allowed to remain on the crystal at the completion of the abrading is controlled so that it does not require undue etching. For example, the preferred amount to be removed must be sufllcient to permit the stabilizing of the crystals and simultaneously avoid altering] by glazing or altering the structure of the crystal body so as to bring out the quartz structure thereof too predominantly.

For example, the amount of material to be removed from the crystal is preferably determined by frequency change. The frequency change which is found to be most practical as produced by etching should be more than one-tenth percent of the frequency and less than one percent of the frequency of the crystal, the preferred amount being sufficient to bring about a frequency change of from about five-tenths percent to eight-tenths percent for any crystal. For example, for a crystal which is to have a frequency of 5000 kc., it is advisable to stop the grinding when the crystal has approximately 4970 kc. and to thereafter etch the crystal until it reaches 5000 kc. or preferably stop the grinding at 4980 kc. and stop the etching at 5000 kc. It is not advisable to grind to 4995 kc. and stop the etching at 5000 kc. because this would not be sufficient etching to stabilize the crystal. This same principle will apply to other frequencies. The etching is controlled preferably to remove the fractures due to grinding, that is, to cause the bottom of the fractures to become rounded whereby subsequent breaking oil of parts of the crystal whenstored or during actual use will be greatly reduced or obviated. The remov- II must be of .an amount sufficient to simultaneously remove fractures of the type which will pro- 7 gross upon use of the crystal or upon subsequent jarring or storage and'must not be sufflcient to bring out too predominantly the quartz crystal U structure.

these fractures simultaneously to the obtaining of desired frequency of oscillation greater stability or aging is accomplished and more desirable working characteristics or activity is obtained. That is, crystals formed in this manner remained more permanently stable and can be stored for subsequent use over considerably long periods without loss of activity and with substantially no change as to frequency of oscillation and said 70 crystals tend to maintain a more constant frequency of oscillation during use. The present method of etching quartz crystals does not in any way alter the working characteristics of the crys tals but rather has a tendency to improve the activity. It is to be understood, of course, that it is As a result of acid etching to remove neeenary ha 'thequartsforfliesecrystals to vs freefromalltypesoftwinnin'gasiscommonly known in the art and that several different cuts with respect to the orientation of the mother crystal may be formed as is commonly known. The present invention'may be employed with any of the above cuts. v i

The word. activity. .as set forth herein is in tended to mean the starting ability of the crystal to oscillate or its ability to quickly respond to electrical exciting.

In Fig. v of the drawings, there is diagram-' matically v illustrated a crystal 2. having an abraded surface 21 thereon. It is particularly pointed out that the surface has an irregular formation resulting from the abrading operation when greatly magnified. The difficulty. as found in the past, is that such abraded surfaces have a plurality of fractures therein such as diagrammatically illustrated at ll. These fractures, when the crystals are stored or under conditions of usehave 'a tendency, as illustrated by the dashline I! in Fig. V. to cause portions of the crystal to break away thereby altering the activity of the crystal and its frequency of oscillation. It is quite apparent that should a plurality of such pieces,

as diagrammatically illustrated at 30, break away surface 32 whereby the tendency for parts, such as diagrammatically illustrated at 30, breaking off is greatly reduced or completely obviated.

Fig. 'l is a much greatly magnified view illustrating the features "diagrammatically shown in Figs. and 6. By etching the abraded surface thereof to remove the fractures as set forth above,

a more stable or aged crystal'is' obtained. By

aging, it is meant stability as to permanency of activity and frequency.

It is to be understood, of course, that when the crystals have been initially abraded that they are cleansed and checked as to their frequency subsequent to the ilnalietching. This cleansing can be accomplished by a quick dip in'the acid and a rinse in water to remove oil, grit, etc. from the crystal so that a fairly accurate check as to its frequency can be obtained prior to the final acid etching.

Although applicant has shown and described the abrading of only one surface of the quartz crystal at a time, it is to be understood that the said opposed surfaces of the crystal or crystals may be simultaneously abraded. This is accomphshed by placing the crystals in a suitable holder between an upper and lower lap or abrading tools having flat surfaces in substantially parallel relation with each other and causing the crystals to be moved relative to the abrading surfaces of the laps or tools in the presence of a suitable abrasive suchas oil, kerosene or other suitable liquid'having powdered abrasive means therein such as emery, carborundum, crushed diamonds, etc.

In addition .to moving the holder and crystals between abrading surfaces of the laps, one or both of the laps could be moved relative to each other while maintaining the abrading surfaces substantially parallel and while permitting one of, the

laps to move towards the other as the material is 8 removed from the In this manner a plurality of crystals may be reduced to desired thickness by abrading the opposed sides of the crystal simultaneously.

Itis to be understood that the crystals are finished by acid etching as set forth above.

'I'he acidusedinetchinsmaybeofanydesirable type which will attack the crystals such as hydrofluoric acid.

From the foregoing description, it will be seen that-simplaeillcient and economical means have been provided for accomplishing aliof'the obiects and advantages of the invention.

Having described my invention, I claim: 1. A method of forming stabilized piezo-electric crystals comprising the steps of abrading to substantial parallelism the opposed surfaces of a crystal cut from the mother quartz to approxito introduce incipient fractures in the abraded surfaces of controlled depth, continuing the abrading action until the frequency value of the crystal has been increased to a value of from 0.1 to 1.0% lower than the flnalfrequency required for the crystal. cleansing the surfaces of the crystal to remove remaining abrasive and loose particles of quartz and subsequently sub- Jecti'ng the cleansed crystal to an acid bath of a given concentration for an interval of time controlled according to the concentration of the bath suflicient to reduce said crystal to its required frequency, with said bath simultaneously rounding out the bottoms of said incipient fractures present in the abraded surfaces while avoiding the glazing ofjsaid surfaces of the crystal and the development of its crystalline structure.

2. A method of forming stabilized piezo-electric crystals comprising the steps of abrading the opposed surfaces of a crystal to substantial parallel.- ism while subjected to a substantially uniform pressure throughout the area of said surfaces, said crystal having been cut from the mother quartz to approximately required dimensions and s simultaneously permitting the abradin action of said surfaces to introduce incipient fractures in the abraded surfaces of controlled depth, continuing the abrading action until the frequency value of the crystal has been increased to a value 60 of from 0.5 to 0.8% lower than the final frequency required for the crystal, cleansing the surfaces of the crystal to. remove remaining abrasive and loose particles of quartz and subsequently subjecting the cleansed crystal to an as acid bath of a given concentrationfor an interval of time controlled according to the concentration of the bath sufficient to reduce said crys. tal to. its required frequency with said bath simultaneously rounding out the bottoms of said '60 incipient fractures present in the abraded surfaces while avoiding the glazing of said surfaces of the crystal and the development of its crystalline structure.

3. In'the method of forming stabilized piezoelectric crystals of' quartz, the steps comprising subjecting the crystal to an acid bath of a given concentration, said crystal having opposed surfaces thereon abraded to. substantial parallelism, with said surfaces containing incipient fractures of controlled depth resulting from the abradingaction to which said surfaces have been subjected, and having been cleansedof abrasive and loose particles of quartz. said crystal having a frequency value of from 0.1 to 1.0% lower than the final frequency required for the crystal, and

" mateiy required dimensions while simultaneously,

permitting the abrading action on said surfaces 9 causingsaidcrystaltoremainsubiectedtothe acid bath for an interval of time controlled according to the concentration of the bath and sumcient to reduce said crystal to its required frequency, with said bath simultaneously rounding out the bottoms of said incipient fractures present in the abraded surfaces while avoiding the glazing of said surfaces of the crystal and the development of its crystalline structure.

4. In the method of forming stabilized piezoelectric crystals of quartz. the steps comprising subjecting the crystal to a chemical bath having characteristics for etching quartz and of a given concentration, said crystal having opposed surfaces thereon abraded to substantial parallelism, with said surfaces containing incipient fractures 'of controlled depth resulting from the abrading action to which said surfaces have been subjected, and having been cleansed of abrasive and loose particles of quartz, said crystal having a frequency value of from 0.5 to 0.8% lower than the final frequency required for the crystal, and causing said crystal to remain subjected to the acid bath for an interval of time controlled according to the concentration: of the bath and sumcient to reduce said crystal to its required frequency, with said bath simultaneously rounding out the bottoms of said incipient fractures present in the abraded surfaces while avoiding the glazing of said surfaces of the crystal and the development of its crystalline structure.

5. The method of processing a piezo-electric crystal comprising placing the crystal within a recess in a holding member, said crystal having been initially cut from the mother quartz to approximately the required dimensions, placing said holding member and crystal on the abrad- T ing surface of a lapping tool, imparting a substantially uniform pressure on a side surface of said crystal throughout the area thereof while in said holder to cause the opposed surface of said crystal to engage the abrading surface of the lapping tool, moving said holder and crystal over the abrading surface of the lapping tool, and periodically reversing said crystal during said lapping so as to cause ,the opposed surfaces of the crystal to be reduced to substantial parallelism while permitting the abraded surfaces of the crystal to contain incipient fractures of controlled depth, continuing the lapping action on said surfaces until the frequency value of the crystal has been increased to a value of from 0.1 to 1.0% lower than the final frequency required for the crystal, cleansing the surfaces of the crystal to remove remaining abrasive and loose particles of quartz and subsequently subjecting the cleansed crystal to an acid bath of a given concentration for an interval of time controlled accordin to the concentration of the bath and suflicient to reduce said crystal to its required frequency, with said bath simultaneously rounding out the bottoms of said incipient fractures present in the abraded surfaces while avoiding the glazing of said surfaces of the crystal and the development of its crystalline structure.

6. The method of processing a piezo-electric crystal comprising placing the crystal within a recess in a holding member, said crystal having been initially cut from the mother quartz to approximately the required dimensions, placing said holding member and crystal on the abrading surface of a lapping tool, said abrading surface having abrasive material uniformly distributed thereon, imparting a substantially uniform pressure on a side surface of said crystal throughout the area thereof while in said holder to cause the opposed surface of said crystal to engage the abrading surface of the lapping tool, moving said holder and crystalover the abrading surfuce of the lapping tool, and periodically reversing said crystal during said lappin so as to cause the opposed surfaces of the crystal to be reduced to substantial parallelism while permitting the abraded surfaces of the crystal to contain incipient fractures of controlled depth, continuing the lapping action on said surfaces until the frequency value of the crystal has been increased to a value of from 0.5 to 0.8% lower than the final frequency required for the crystal, cleansing the surfaces of the crystal to remove remaining abrasive and loose particles of quartz and subsequently subjecting the cleansed crystal to an acid bath of a given concentration for an interval of time controlled according to the concentration of the bath and suflicient to reduce said crystal to its required frequency, with said bath simultaneously rounding out the bottoms of said incipient fractures present in the abraded surfaces while avoiding the glazing of said surfaces of the crystal and the development of its crystalline structure.

CHARLES W. MscCONNELL.

namannons crrnp The following references are of record in the file of this patent:

'rmrran s'rs'rss PATENTS Number Name Date 1 9 1,869,169 Marrison July 26, 1932 2,323,599 Hawk July 8, 1943 2,353,171 MacConnell July 11, 1944 2,364,501 Wolfskill Dec. 5, 1944 2,376,219 Winslow May 15, 1945 2,411,298 Shore Nov. 19. 1949 2,417,179 Ross liar. 11, 1947

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