US2261791A - Piezoelectric thickness-mode quartz crystal - Google Patents

Description

Nov. 4, 194-1. I s, BOKQVQY 2,261,791

PIEZOELECTRIG THICKNESS-MODE QUARTZ CRYSTAL Filed April 29, 1959 2 Sheets-Sheet l N0v.4,1941. A BOKOVOY 2 261391 PIEZOELECTRIO THICKNESS-MODE QUARTZ CRYSTAL I Filed April 29, 1939 2 Shams-Sheet 2 Ennentor %ttomeg Patented Nov. 4, 1941 PIEZOELECTRIC THICKNESS-MODE QUARTZ CRYSTAL Samuel A. Bokovoy, Merchantville, N. J assignor to Radio Corporation of America, a corporation of Delaware Application April 29, 1939, Serial No. 270,886

10 Claims.

This invention relates to the piezoelectric art and particularly to the cutting of quartz piezoelectric resonator and oscillator elements of the type designed to be vibrated at a frequency which is a function of the thickness dimension.

In the case of both thickness-mode and length-breadth or contour mode quartz crystals, several modes of vibration may be present and, as a consequence, the crystal blank or element may be vibrated at several different and apparently unrelated frequencies. This, of course, is undesirable, especially where the several frequencies are quite close to each other, in which case the operator may unwittingly cause the crystal to be vibrated at a frequency other than that at which it was designed to operate. This difficulty may be obviated in the case of a contour-mode crystal by so proportioning the length and width dimensions of the element that it will vibrate at only one of the several contourmode frequencies normally present when a random relation exists between the length and width. As to this, see U S. Patents 2,064,288, 2,073,046, 2,111,383 and 2,111,384.

The problem, as it relates to thickness-mode crystals, is not susceptible of the same solution and is apparently far more complex, since in such crystals there are present not only frequencies which are characteristic of the several thickness modes of vibration, but there also exists various spurious and apparently unrelated frequencies which may be said to constitute a secondary spectrum of responses. Each mode in the thickness family of modes appears to have its own family of secondary responses; in any event a secondary response exists irrespective of the orientation at which the crystal blanks are cut from the mother crystal.

Several theories may be advanced for explaining this characteristic difference between contour-mode and thickness-mode crystal elements. By way of example: (a) The usually greater activity of a crystal when vibrated at a thicknessmode frequency may bring out what would otherwise be highly damped motions in the quartz. In this connection, it is apparent that a very weak response not noticeable when a low voltage is applied to the crystal may become quite pronounced with increased voltage. (1)) Some of the secondary or spurious frequencies may be directly related to harmonic responses of lower fundamental vibrations inherently present in the quartz. Some of the secondary responses may be related to what is commonly known as coupling between the desired mode of vibration and other normal modes of vibration.

The problem of attenuating spurious frequencies present in thickness-mode crystals is one that has long engaged the attention of those skilled in the art and some small measure of success has been achieved by adopting very exacting standards in the cutting and finishing of the crystal blanks. By way of example: a crystal blank whose major surfaces are substantially optically flat and whose minor surfaces are at pre cisely a right angle thereto has heretofore been thought to exhibit fewer and less pronounced spurious frequencies than one cut to less exacting standards. It has also been thought that, when the length and breadth dimensions of thickness-mode crystals are as small as possible, the spurious or secondary responses are less pronounced than in the case of similar larger crystal elements These expedients, however, have never achieved wide commercial success either because of the increased manufacturing costs incident to the achievement of duplicate parallel optically-flat surfaces, or because it is extremely difiicult to clamp a very small thickness-mode crystal without excessive damping.

Accordingly, the principal object of the present invention is to provide a thickness-mode piezoelectric element which is free from spurious frequencies approaching that of the fundamental thickness-mode frequency at which the said element is designed to be operated.

A related object is to provide a quartz piezoelectric element possessing a substantially unitary freedom for its thickness-mode of vibration and, in the case of an element designed also to be operated, selectively, at a predetermined contour-mode frequency, a unitary freedom also for its contour-mode of vibration.

Another and important object of the present invention is to provide an accurate and simple method of cutting thickness-mode crystals to eliminate spurious or secondary response frequencies and one ensuring minimum wastage due to the production of nonusable crystal elements.

ther objects and advantages will be apparent and the invention itself will be more readily understood by reference to the following specification and to the accompanying drawings, wherein Figure 1 is a sectional view in perspective and Figures 2 to 18 inclusive are views in perspective of eighteen piezoelectric thickness-mode quartz elements cut in accordance with the principle of the invention to exhibit a substantially unitary freedom for their thickness-modes of vibration,

The foregoing objects are achieved in accordance with the invention by so cutting a quartz slab, blank or element that at least one of its electrode faces is beveled, inclined or slants off in opposite directions from a line or area of maximum thickness adjacent the center of said electrode face. When but one of the electrode faces is provided with such a slanted surface, the other electrode face should preferably be first rendered optically fiat or nearly so. This should not be construed, however, to mean that the fiat surface need necessarily be provided with a high polish or a mirror finish.

The invention is applicable to quartz crystal blanks of various orientations. It is preferable, however, to start with a blank which has been so cut with respect to the natural axes of the mother crystal that it will exhibit a zero or some low temperature coefiicient of frequency when vibrated at a frequency characteristic of its thickness dimension. Such orientations are described, by way of example, in British Patent No. 441,438 (1936). The blank selected for finishing may be square, oblong, circular, elliptical, or other shape, and of any convenient dimensions suitable for the particular type of mount or holder with which the finished crystal is to be used.

In the event that a plural frequency element is desired, the blank should preferably be initially so proportioned, for example in the manner described in U. S. Patents 2,064,288, 2,073,046, 2,111,383 or 2,111,384, that it will exhibit a unitary freedom for its contour-mode of vibration.

The unfinished blank at the outset should, in any event, be somewhat thicker than is calculated (from the thickness-mode constant peculiar to blanks of the selected orientation) to be necessary to obtain the desired thickness-mode frequency in a finished blank having duplicate parallel electrode faces. In carrying the invention into efiect with such a blank, one of the major surfaces is selected as a reference plane and is rendered optically fiat, or nearly so, by any of the known methods of grinding and lapping. The blank may then be mounted in a clamp with its unfinished surface slightly askew with respect to the axis of movement of the grinding wheel or other instrument, so that, when it is presented to the wheel, the grinding action is confined to one side of the center of the blank, to produce a face which slants off from the center toward one edge of the blank. The blank and clamp are then moved with respect to the grinding wheel to produce a substantially duplicate slant on the other side of the center of the said unfinished surface.

Instead of using a clamp and grinding wheel, the blank may be placed, with its unfinished face down, on a surface upon which a grinding compound has been spread and then manually lapped by simply confining the lapping force, exerted by the operators fingers, first on one side of center and then on the other.

It is this slanting off by beveling operation which results in the attenuation or entire disappearance of the secondary response spectrum. During the beveling operation, the fundamental thickness-mode frequency will be increased because the effective or overall thickness of the blank is decreased. (Conversely, for practical purposes, it may be considered that the frequency constant K has been increased at the central portion of the blank to a degree determined by the angle of the bevel.) In order to attenuate the spurious frequencies to a high degree, it is usually necessary to alter the bevel angles during the grinding operation. If freedom from spurious thickness-mode frequencies is achieved before the exact desired frequency response is reached the lapping may be continued, taking care not to substantially alter the bevel angle until the exact desired thickness-mode frequency is achieved. Alternatively, a narrow area adjacent the line or region of maximum thickness may be ground down to further reduce the effective thickness of the blank until the desired frequency is obtained. In cases where a relatively large change in frequency is desired after the blank has been freed of spurious responses, it may be necessary to change the bevel angle as the thickness, adjacent the center of the blank, is reduced.

It is not possible to lay down any specific rule as to exact bevel angles required to free the crystal blanks from spurious frequencies. This is so because the bevel angles change not only with orientation and with frequency but also with the area of the blanks. Further, it appears that freedom from secondary responses may be achieved in a given blank at several bevel angles, which may or may not be exact multiples of each other. Having the above factors in mind, it may nevertheless be said that the depth of the bevel in a slab of ordinary dimensions (say 1.5 x .75" x .125") out at any of the known low temperature angles will seldom exceed onehalf the thickness of the slab and will ordinarily be of the order of one-quarter of the thickness of the finished element.

As previously set forth, the invention may be applied to quartz crystal elements of substantially any desired shape and orientation. Thus, referring to Fig. l, the invention is shown applied to a crystal slab or blank of circular contour. In this figure, la designates the optically fiat electrode face and lb and I0 designate beveled faces which slant oif from a diameter Id of the blank. The thickness of the blank is uniform when measured at points along the diameter ld. Reference axes Z and X are marked in Fig. 1 to indicate an orientation calculated to endow the element with a low temperature coefficient of frequency. The axes Z and X may be reversed if desired, that is to say, the slant of the faces lb and I0 may be in the direction of the X-axis.

The crystal blanks of Figs. 2 to 18, inclusive, are parallelepipeds in which one of the limiting planes (i. e., the top plane) has been replaced, in accordance with the invention, with a pair of beveled faces (b and 0, respectively) which slant off in opposite directions from the central region, area or line of maximum thickness. Oblong or square shapes are preferred to circular or elliptical shapes in cases where the greater dimensions of the blank are to be so proportioned (for example, in the manner disclosed in the previously identified U. S. patents) as to endow the finished elements with a unitary freedom for their contour-mode of response.

Fig, 2 shows a finished element having a plane face 2 which caps the slanting faces 2b and 20. As previously described, a cap face may be provided in instances where freedom from spurious frequencies is achieved before the exact desired thickness-mode frequency is obtained. In this case there was no necessity of changing the bevel angles of the faces 21) and 20 (with a consequent danger of initiating spurious responses) in making the final frequency adjustment.

In the oblong blank of Fig. 3, as in the circular blank of Fig. l, the beveled faces 3b and have the same bevel angle and their planes intersect along the center line 3d.

Fig. 4 shows that it is not always necessary that the beveled faces 42), 4c meet at the center line of the crystal blank nor indeed that the bevel angles need be the same.

The bevel angles of the crystal of Fig. 5 are the same (as in Figs. 1 and 3) and the slanting faces 5?), 5c meet at a line which is off center (as in Fig. 4) causing the crystal to be thicker at one end than at the other.

Fig. 6 is similar to Fig. 3 but the sloping faces 61), have different bevel angles.

The blank of Fig. 7 is a replica of Fig. 3 but here the two sloping faces Tb, 1c are inclined toward the observer, resulting in an element which is thinner along one long edge than it is at corresponding points on the opposite long edge.

In the blank of Fig. 8, the slope toward the observer is confined to one of the sloping faces.

Referring to Fig. 9, it is not always necessary that the beveled faces 91), comprise plane surfaces. In this case, the faces 5b, to are non-1in ear, with the curvature decreasing as the ends of the crystal are approached.

In Fig. 10, the inclined faces lob, Hlc comprise sections of toroid which intersect along the line ltd.

The blank of Fig. 11 is similar to the one shown in Fig. 10 but is capped by a plane face I If which is substantially parallel to the optically flat elec trode surface I la.

The curvature in the transverse direction of the inclined faces lilb, I2c of the blank of Fig. 12 is in the opposite direction to those of Fig. 11.

As shown in Fig. 13, the slope on the opposite sides of the center of the crystal need not be linear, in which case the curvature should preferably increase as the opposite ends of the blank are approached. This element may be described as a parallelepiped, one of the limiting planes (i. e. the top plane) of which has been replaced by a cylindrical face whose generatrix (l3d) is substantially parallel to one (i. e. the bottom) of the other limiting planes of the parallelepiped.

In crystals cut to respond to predetermined thickness and contour mode frequencies, various forms of cap faces may be resorted to in achieving the desired frequencies and freedom from the secondary spectrum of responses. Thus in Figs. 14 to 17, inclusive, the blanks have been provided with cap faces which slope in directions normal to the slope of the principal faces I) and c.

The crystal element of Fig. 18 is similar to the one shown in Fig. 3 but here both electrode faces comprise beveled surfaces 12, b, c, c which slant in opposite directions from the central region of maximum thickness.

Although certain specific ways and means for accomplishing the objects of the invention have been set forth, it will be understood that they have been given by way of example and should not be construed as limitations to the scope of the invention.

It is well known in the art that in order to obtain a desired frequency characteristic in a piezoelectric element with the precision that is required, frequent tests should be made between successive stages of the grinding operation. The invention is, therefore, not to be limited except insofar as is necessitated by the prior art and by the spirit of the appended claims.

What is claimed is:

1. Method of cutting a piezo-electric element so as to exhibit a substantially unitary freedom for its thickness mode of vibration, said method comprising grinding a major surface of a quartz blank in the form of a surface which slants off in opposite directions from the central region of said blank.

2. Method of cutting a piezo-electric element so as to exhibit a substantially unitary freedom for its thickness mode of vibration, said method comprising grinding a major surface of a quartz blank in the form of a surface which slants off in opposite directions from the central region of said blank and then changing the'bevel angles of said slanting surface until spurious frequencies related to the said thickness mode of vibration are attenuated.

3. Method of cutting a piezo-electric element so as to exhibit a substantially unitary freedom for its thickness mode of vibration, said method comprising grinding one of the major surfaces of a quartz blank whose thickness is slightly greater than is required to achieve a desired thickness-mode frequency to substantially optical flatness, grinding the opposite major surface of said blank in the form of a surface which slants off in opposite directions from the central region of said blank, changing the bevel angles of said slanting surface until spurious frequencies related to the said thickness-mode of vibration are attenuated, and finally reducing the thickness of said blank until the desired thicknessmode frequency is achieved.

4. A piezo-electric quartz element, one of the electrode faces of which comprises a beveled surface which slants off in opposite directions from the central region of said electrode face.

5. The invention as set forth in claim 4 wherein said beveled surface is capped by a substantially plane surface.

6. A piezo-electric quartz element having one substantially optically flat electrode face and an oppositely located electrode face comprising a beveled surface which slants off in opposite directions from the central region of said electrode face.

'7. The invention as set forth in claim 6 and wherein said beveled surface is capped by a substantially plane face which is substantially parallel to said optically fiat electrode face.

8. A piezo-electric quartz element one of the electrode faces of which comprises a plurality of surfaces each comprising a portion of the surface adjacent the inner circumference of a toroid.

9. A piezo-electric quartz element having a substantially optically flat electrode face and another electrode face comprising a pair of surfaces each comprising a portion of the surface adjacent the inner circumference of a toroid. said portions being capped by a plane surface which is substantially parallel to said optically fiat face.

10. A piezo-electric element comprising a parallelepiped, one of the limiting planes of which has been replaced by an electrode surface which slants off in opposite directions from a region of maximum thickness which extends across substantially the center of said element.

SAMUEL A. BOKOVOY.

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