US2254866A - Piezoelectric crystal element - Google Patents

Description

Sept. 2, 1941. Q BALDWlN ET AL 2,254,866

PIEZOELECTRIC CRYSTAL ELEMENT Filed March 28, 1956 3 Sheets-Sheet l Sept. 2, 1941- c. F. BALDWIN ETAL PIEZOELECTRIC CRYSTAL ELEMENT Filed March 28, 1936 3 Sheets-Sheet 2 Foe W/D 71/1 [/1/6 m fem 700,4 Mo 06) Sept. 2, 1941.

C. F. BALDWIN ET AL PIEZOELECTRIC CRYSTAL ELEMENT Filed March 28, 1936 3 Sheets-Sheet 3 Patented Sept. 2, 1941 PIEZOELECTRIC CRYSTAL ELEMENT Charles F. Baldwin, Schenectady, N. Y., and Samuel A. Bokovoy, Audubon,

N. J., assignors to Radio Corporation of America, a corporation of Delaware Application March 28, 1936, Serial No. 71,388

13 Claims.

This case is a continuation in part of application Serial No, 733,362 entitled Piezo-electric crystal elements to Samuel A. Bokovoy and Charles F. Baldwin, filed July 2, 1934, and assigned to the same assignee as the instant case.

This invention relates to the piezo-electric art, particularly to the cutting of quartz crystals and has special reference to the manufacture of piezoelectric elements of the type possessing a useful natural mode of vibration which is a function of length, or of width. Such crystal elements are known in the art as contour-mode or as length-mode or width-mode oscillators and resonators. Usually the fundamental frequency to which such crystal elements are cut is lower than that characteristic of so called thicknessmode" crystals.

The principal object of the present invention is to provide a quartz piezo-electric element which shall exhibit a zero or other desired low temperature coeflicient of frequency when vibrated at a frequency which is a function of its length or breadth.

Another object of the invention is to provide a simple, accurate and efficient procedure for the cutting of contour-mode crystals and to eliminate as far as possible any uncertainties with regard to the temperature coefficient of frequency and other operating characteristics of the finished elements.

Another object of the invention is to provide a system of orientation for use in the cutting of low temperature coefiicient of frequency piezoelectric elements which is applicable, without change, to quartz having a. crystalline structure c of either left-hand" or right-hand type.

Other objects and advantages will be apparent and the invention itself will be best understood by reference to the following specification and to the accompanying drawings, wherein:

Fig. 1 is plan view. of a natural or mother quartz crystal, the optic (Z) axis of which is perpendicular to the plane of projection; the relative location of the apex, the major and minor apex surfaces, an electric (X) axis, a mechanical (Y) axis, a reference W axis and a reference Y+0 axis are also marked as an aid to a clear understanding of the system of orientation followed in producing contour-mode quartz elements in accordance with the invention.

Fig. 2 shows in outline and in perspective a piece of natural quartz having a section cut and dividedto provide a rough bar having top and bottom surfaces lying in planes which are normal to the Z-axis and its long edges parallel to a W-axis which is 20 removed from an X-axis. Fig. 3 is an elevational view looking in the direction of the arrow of Fig. 2 showing the position of two blanks cut from the bar of Fig. 2,

one of these blanks is tilted towards parallelism with the plane of a major apex surface of the mother crystal and the other towards parallelism with the plane of a minor apex surface of the mother crystal.

Fig. 4 shows the left-hand blank of Fig. 3 removed and trimmed, the plane of projection being perpendicular to the plane of the paper.

Fig. 5 shows the right-hand blank of Fig. 3 removed and trimmed.

Fig. 6 is a cross-sectional view taken on the line WW of Fig. 1 showing the rotation of the blanks about a Y+0 reference axis of a mothercrystal and with respect to the major and minor apex surfaces thereof.

Fig. 7 is a chart showing the correlation between certain angles W V, and W -V required to achieve a contour-mode oscillator having a zero or other low temperature coeiiicient of frequency,

Fig. 8 is a chart indicative of the characteristics of contour-mode quartz plates rotated throughout the entire 360 scale about the Z-axis.

The present invention involves a system of orientation in which (a) the major and minor apex surfaces of the mother crystal are em- .ployed as reference planes and (b) certain W- axis and (c) certain Y+0 axes are employed as reference axes. It is, therefore, necessary to identify these planes and axes.

As to (a), referring to Figs. 1 and 2 of the drawings and having in mind that all unbroken quartz crystals are uniformly shaped hexagonal bi-pyramids, it will be seen that certain of the terminal surfaces of the quartz extend to the apex of the pyramid. These surfaces are designated M and are the major apex surfaces. The terminal surfaces which do not touch the apex are designated N and are the minor apex surfaces of the mother crystal. Occasionally a mother crystal will be found in which more than three of the cap or apex faces extend to the tip of the pyramid, other crystals may have their pyramid ends broken off. No confusion, however, need exist as to the virtual location of the major and minor apex faces of a broken or otherwise abnormal crystal providing that the side faces "m and n, or one of them, is intact, for it will be apparent from an inspection of Fig. 2 that those side edges of the mother crystal which approach each other in the direction of its ends terminate in a major apex face, whfle those which diverge in this direction terminate in a minor apex face. This is so in the case of both left-hand and right-hand quartz.

As to (b) Fig. 1 is marked to show an electric, X-axis and, an adjacent mechanical or crystallographic, Y-axis. The optic or Z-axis, marked in Fig. 2, is perpendicular to the plane of projection in Fig. 1. The W-axes lie in the X-Y plane, 1. e. in a plane normal to the Z-axis. All axes which in intersecting a reference X-axis form W-angles of from to 30 therewith are reference W-axes within the purview of the invention. In accordance with this definition a 0 W-axis coincides with an X-axis and a 30 W- axis coincides with a Y-axis 30 removed (in either direction) from said X-axis. In Fig. 1, there is a W-axis marked which forms a W-angle of 20 with that X-axis which is designated XX.

As to (0), there is Y+0 axis for each W-axis, each Y+0 axis is normal to its own W-axis and normal to the Z-axis. It is about a Y+0 axis that the element is rotated or tilted to achieve a desired temperature coefficient of frequency.

In order to produce a quartz piezo-electric element or plate having a predetermined lowtemperature coefficient, a suitable angle of orientation (angle W) with respect to an X-axis, and a coordinated angle representing the inclination of the electrode (i. e. top and bottom) faces must be chosen. This latter angle will be referred to generally as the V-angle and more specifically as the V' angle or the V angle as determined by the direction of rotation, i. e. whether toward parallelism with the plane of a major apex surface (m) or a minor apex surface (12) of the mother crystal.

Referring now to Fig. 7 which shows the correlation between the temperature coefiicient and the angles W and V. In this chart an X-axis and an adjacent Y-axis are indicated by the vertical, correspondingly designated, lines and the space or region therebetween is calibrated in degrees from the X-axis, the calibrations being along the horizontal line which intersects the centrally marked optic axis, Z. In agreement with the previously given definition a line extending at right angles at any point along this 0-30 line will coincide with a W-axis.

It will be noted that there are two scales marked along the line representing the X-axis. That scale which reads in an upward direction from 0 represents a rotation of the electrode faces of the element about its Y+0 axis in a direction away from parallelism with the Z-axis and towards parallelism with the lane of a minor apex surface of the mother crystal, the calibrations V being in degrees from parallelism (0") with the Z-axis. That scale which reads in a downward direction from 0 represents a rotation of the electrode faces of the element about its Y+0 axis in a direction away from parallelism with the Z-axis and towards parallelism with the plane of a major apex surface of the mother-crystal; the calibrations V like those of V being in degrees from parallelism with the optic (Z) axis.

Now, having in mind that there is a Y+0 axis for and normal to each W-axis, then, from an examination of this Fig. 7 it will be seen that for each W-axis which forms a 0 to 30 angle (W) there is a definite angle of rotation (V 0 V required to achieve a desired temperature coefficient of frequency. For example, assuming that a piezo-electric element is required which shall exhibit a substantially zero temperature coefficient of frequency, assuming further that suchv element is to be so cut that its electrode faces are tilted in a direction away from parallelism with the Z-axis and towards parallelism with the plane of a, minor apex surface of the mothercrystal, Now, selecting a W angle on the upper scale, say 20, (read along the horizontal line which intersects the Z-axis) then, the correlated V angle of rotation of the electrode surfaces about the Y+0 axis is substantially 43", this latter reading being obtained by observing at what point on the upper left hand scale a line will fall projected at right angles to a 20 W-axis from a point (p) at which the said W axis crosses the zero temperature-coeificient line.

An element exhibiting a zero temperature coefiicient of frequency may likewise be achieved by tilting the electrode surfaces about a Y+0 axis towards parallelism with the plane of a major apex surface. Thus, assuming as in the above example that a 20 W-axis is selected, then, reading the lower left-hand scale at the point (p') where a 20 W-axis crosses the zero temperature coefiicient line it will be seen that the correlated V angle of rotation is substantially The curves of Fig. 7 show clearly that the invention is not limited in its scope to the cutting of piezo-electric plates having an exactly zero temperature coefiicient of frequency. For certain uses the characteristic to be sought is a definite positive or negative temperature coefllcient. Such crystal elements have great utility in oscillator circuits which inherently have certain temperature coefficients of frequency irrespective of the crystal frequency control therefor. Thus it becomes advantageous at times to use a crystal control element having a temperature coeflicient of frequency which is of opposite sign to, and substantially compensates for, the natural temperature coefiicient of frequency of the oscillator network per se.

To determine the V or the V angle of rotation required to achieve a piezo-electric element having a particular coefficient of frequency the left-hand scale of Fig. 7 should be read at the point where the selected W-axis crosses the line specific to the temperature coefiicient desired. But three temperature coeflicient curves are marked in each of the charts of Fig. '7. There is a zero temperature coeflicient curve (the center curves, each marked (:0) for each direction of tilt. The curves on opposite sides of the zero line of the upper chart embrace the W and V angles, required to achieve an element exhibiting a temperature coefficient of frequency of +5 cycles and -5 cycles, respectively, per million, per degree centigrade. The curves on the opposite sides of the zero line" of the lower chart embrace the W and V angles required to achieve an element exhibiting a temperature coefficient of frequency of 5 cycles and +5 cycles, respectively, Der million, per degree centigrarle. It is obvious that other curves representative of other temperature coefiicients of frequency may be derived from the ones illustrated.

As a matter of manufacturing convenience it is often desirable to select standard W-angles from the substantially infinite number available within the 0-30 range. Recommended standard W-angles and their correlated V-angles required to achieve a contour-mode piezo-electric quartz element of a desired low temperature coeflicient of frequency (as derived from the direction of tilt and with the temperature co chart of Fig. 7,) are: efficient. Furthermore, it is possible to produce Electrode faces tilted towards a minor (n) apex surface Degrees Degrees Degrees Degrees Degrees Degrees Degree; 0 T. C. V 57 53 49 46 43 3S. 6 +5 T. C. V- 70 63 58 53 40 42 5 T. C V 45 42 40 38 37 86 35 Electrode faces tilted towards a major (m) apex surface faced crystals have proven very satisfactory as 0 0 well as those of more or less oblong proportions. The thickness dimension is of almost negligible new Degree, Degree, Degree, moment in determining the length-and-breadth T. o v 57 5c 54 mode frequency characteristic. +2 X: 3g 20 As illustrative of a preferred manner of cutting contour-mode quartz piezo-electric elements having a zero or some other low temperature Any X-a S m y be Selected as the referemfe coeflicient of frequency reference is had to Figs. axis from which the W-axis is measured. This 1 to 6jnc1u iv iS brought Out in the chart Of 8 Shows 25 Referring fir t to 2 a section 2 ay one the correlation requi ed between P W and V inch thick, is first sliced from the body of the angles for zero temperature coefficient contourmother crystal, the thickness dimension f this mode quartz plates. thrOughOllt the l' section is parallel with, and the length-breadth scale about the -a S- dimension of this section is normal to the Z-axis. Refe in detail to i there are three 30 A bar 3, say one inch wide, is then cut from this X-axes and th ee Y-aXeS passing through the section 2, preferably at substantially the exact Z-axis and d vid the 360 scale into twelve 30 W-angle selected as a reference axis. In the ilse me s. Each X-axis coincides with a lustrated embodiment the long dimensions of the axis, each Y axis coincides with a 30 W-axis, F bar 3 coincide t a w which is 20 all of these X and Y X88 d all Of the moved from that X-axis (and hence 10 removed stantially fi t number of axes lying in the from that Y-axis) marked in both Figs. 1 and 2. same XY plane are -a and any one Referring to Figs. 3 and 4, the blanks 4 and 5 may b s t d s a reference s in carrying from which the finished elements are formed are the invention into effect. There is a zero temt sliced from bar 2 t either a v angle or vn pe tu e c ifi ie t i e whi h p es through angle dictated by the temperature coefficient deeach (W) axis in the XY p This line has sired. The electrode faces of blank 4, as shown dissimi ar Opp e h pcd Segments. which are have been rotated substantially 43 (the V anmarked for purposes of illustration, by the curved le) about its Y+0 axis in a direction away from arrows V and V Wherever a W-axis touches parallelism with the Z-axis toward parallelism one of thes segments V or V of the zero temwith the plane of that minor apex surface of the perature coeflicient line, there, is the second or th r crystal hi h i designated N1 in Figs 1 V-angle of rotation required to be known in the d 4. I F 2, N1 i opposite t major apex cutting of contour-mode quartz crystal e emen surface designated M1.) This 43 V angle is possessing a zero temperature coefficient of freshown in Figs. 7 and 8, to be the precise angle qu y- The Scale o reading -a les is marked 59 required to achieve a substantially zero temperain degrees in two directions along that X-aX S ture coefficient of frequencywhen the selected which divides the main 360 scale horizon a y. -axis forms a W-angle with an Xaxis of 20. The exponents m and are indicative of the The electrode faces of blank 5 of Figs. 3 and 4 direction of tilt with respect to the plane of a have been rotated substantially 55 (the V anmajor (m) and minor (n) apex surface of the 55 gle) about its Y+0 axis in a direction away from mother c ys l- The readings obtained from this parallelism with the Z-axis towards parallelism scale will be found to correspond to those of the with the plane of that major apex surface of chart of Fig. 7. the mother crystal which is designated M1 in The dimensions of the finished elements neces- Figs. 1, 2 and 4. This blank 5, like blank 4, will sary to produce a desired frequency may be de- (3 exhibit a substantially zero temperature coeffitermined according to the usual methods. One cient of frequency when vibrated at a frequency preferred method, however, is that taught in which is a function of one of its greater dimen- U. S. Patent 2,064,288 to Samuel A. Bokovoy. In sions. that patent it was shown how a dimension (other The proper length-width dimensions required than thickness) perpendicular to the optic axis G3 to achieve adesired contour-mode frequency havmay be chosen according to known frequency ing been determined, it remains only to grind constants, and the other of the two greater and lap these blanks to size, and to trim the edges dimensions may gradually be reduced until the (e, e Fig. 4, e, e Fig. 5) so that planes of desired frequency is obtained. the electrode faces Z+V are normal to the W+V It is not deemed essential to a complete disthickness dimension. It is preferable to bevel the closure of the present invention that specific conedges and round the corners of the otherwise finstants be given with respect to the relation beished elements of Figs. 4 and 5 in order to retween dimensions and frequency characteristics. move minute irregularities in the cutting and to These constants vary extensively not only with 7 prevent chipping in case the crystals should be the W-angle and with V-angle but also with the satisfactory crystal elements having a considerable range of length-to-breadth ratios. Square excited at very great amplitudes of oscillations.

As in prior art contour-mode oscillators and resonators the thickness-dimension has little or no effect upon oscillation performance.

Although certain specific ways and means for accomplishing the objects of the invention have been set forth it is to be understood that they have been.given for the purpose of explaining the inventive concept and should not be construed as limitations to the scope of the invention. Neither is it to be understood that any statements herein made in regard to the value or relationships between angles or orientation and "temperature coefficients of frequency are other than close approximations.

The invention, therefore, is 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. The invention as set forth in claim 2 wherein the angle formed by the intersection of said electric (X) axis and said W-axis is at least 1 and not more than 29.

2. A quartz piezo-electric element cut from a mother crystal having a major apex surface, a minor apex surface, an optic (Z) axis, an electric (X) axis, a mechanical (Y) axis, a W-axis and a Y+ axis, said W-axis lying in the X--Y plane and said Y+0 axis being normal to said W-axis, said element having its electrode faces so tilted about said Y+0 axis in a direction away from said optic (Z) axis and away from the plane of one of said apex surfaces, toward parallelism with the plane of the other of said apex faces, that it possesses a temperature coefiicient of frequency within substantially :5 cycles per million per degree centigrade when vibrated at a fundamental contour-mode frequency, the direction and the angle of tilt of said element about the Y+0 axis necessary to achieve said temperature coefficient of frequency being such a function of the angle formed by the intersection of the W-axis and the electric (X) axis as indicated by the curves of Fig. '7.

3. A quartz piezo-electric element cut from a mother crystal having major and minor apex surfaces, said element having its electrode faces parallel to a plane which is rotated from substantially 70 to substantially 35 about a Y+0 axis in a direction away from parallelism with the Z-axis and towards parallelism with the plane of a minor apex surface, said Y+0 axis being normal to a W-axis which lies in a plane containing an X-axis and a Y-axis the angle formed by the intersection of said W-axis and said X-axis being between the limits of substantially 0 and not more than 30, whereby said element exhibits a temperature coefiicient of frequency within substantially :5 cycles per million per degree centigrade when vibrated at a fundamental contour-mode frequency.

4. A quartz piezo-electric element cut from mother crystal having major and minor apex surfaces, said element having-its electrode faces parallel to a plane which is rotated from substantially 70 to substantially 42 about a Y+0 axis in a direction away from parallelism with the Z-axis and towards parallelism with the plane of a minor apex surface, said Y+0 axis being normal to a W-axis which lies in a plane containing an X-axis and a Y-axis the angle formed by the intersection of said W-axis and said X-axis being between the limits of substantially 0 and not more than 30", whereby said element exhibits a temperature coeflicient of frequency of substantially +5 cycles per million per degree centigrade when vibrated at a fundamental contour-mode frequency.

5. A quartz piezo-electric element cut from a mother crystal having major and minor apex surfaces, said element having its electrode faces parallel to a plane which is rotated from substantially 45 to substantially 35 about a Y+0 axis in a direction away from parallelism with the Z-axis and towards parallelism with the plane of a minor apex surface, said Y+6 axis being normal to a W-axis which lies in a plane containing an X-axis and a Y-axis the angle formed by the intersection of said W-axis and said X-axis being between the limits of substantially 0 and not more than 30, whereby said element exhibits a temperature coefiicient of frequency of substantially -5 cycles per million per degree centigrade when vibrated at a fundamental contour-mode frequency.

6. A quartz piezo-electric element cut from a mother crystal having major and minor apex surfaces, said element having its electrode faces parallel to a plane which is rotated from substantially 57 to substantially 38.6 about a Y+0 axis in a direction away from parallelism with the Z-axis and towards parallelism with the plane of a minor apex surface, said Y+0 axis being normal to a W-axis which lies in a plane containing an X-axis and. a Y-axis the angle formed by the intersection of said W-axis and said X-axis being between the limits of substantially 0 and not more than 30, whereby said element exhibits a substantially zero temperature coefiicient of frequency when vibrated at a fundamental contour-mode frequency.

'7. A quartz piezo-electric element cut from a mother crystal having major and minor apex surfaces, said element having its electrode faces parallel to a plane which is rotated from substantially to substantially 45 about a Y+0 axis in a direction away from parallelism with the Z-axis and toward parallelism with the plane of a major apex surface, said Y+9 axis being normal to a W-axis which lies in a plane containing an X-axis and a Y-axis the angle formed by the intersection of said W-axis and said X-axis being between the limits of substantially 0 and not more than 30 whereby said element exhibits a temperature coefficient of frequency within substantially :5 cycles per million per degree centigrade when vibratedat a fundamental contour-mode frequency.

8. A quartz piezo-electric element cut from a mother crystal having major and minor apex surfaces, said element having its electrode faces parallel to a plane which is rotated from substantially 70 to substantially 59 about a Y+0 axis in a direction away from parallelism with the Z-axis and toward parallelism with the plane of a major apex surface, said Y+0 axis being normal to a W-axis which lies in a plane containing an X-axis and a Y-axis the angle formed by the intersection of said W-axis and said X- axis being between the limits of substantially 0 and not more than 30 whereby said element exhibits a temperature coefiicient of frequency of substantially +5 cycles per million per degree centigrade when vibrated at a fundamental contour-mode frequency.

9. A quartz piezo-electric element cut from a mbther crystal having major and minor apex surfaces, said element having its electrode faces parallel to a plane which is rotated from substantially 45 to substantially 49 about a Y+0 axis in a direction away from parallelism with the Z-axis and toward parallelism with the plane of a major apex surface, said Y+ axis being normal to a W-axis which lies in a plane containing an X-axis and a Y-axls the angle formed by the intersection of said W-axis and said X- axis being between the limits of substantially 0 and not more than 30 whereby said element exhibits a temperature coeflicient of frequency of cycles per million per degree centigrade when vibrated at a fundamental contour-mode frequency.

10. A quartz piezo-electrlc element cut from a mother crystal having major and minor apex surfaces, said element having its electrode faces parallel to a plane which is rotated from substantially 57 to substantially 54' about a Y+0 axis in a direction away from parallelism with the Z-axis and toward parallelism with the plane of a major apex surface, said Y+0 axis being normal to a W-axls which lies in a plane con-v taining an X-axis and a Y-axis the angle formed by the intersection of said W-axis and said X- axis being between the limits of substantially 0 and not more than 30 whereby said element exhibits a substantially zero temperature coeflicient of frequency when vibrated at a fundamental contour-mode frequency.

11. The invention as set forth in claim 2 wherein the angle formed by the intersection of said electric (X) axis and said W-axis is less than 1.

12. A quartz piezo-electric element having its electrode faces substantially parallel to an electhic (X) axis and inclined substantially 38 degrees with respect to the optic (Z) axis in the positive direction or toward parallelism with a minor apex face of the mother crystal from which said element is cut, whereby said element exhibits a substantially zero temperature coeflicient of frequency when vibrated at a fundamental frequency which is a function of one of its greater dimensions.

13. A quartz piezo-electric element having its electrode faces substantially parallel to an electric (X) axis and inclined substantially 53 degrees with respect to the optic (Z) axis in the negative direction or toward parallelism with a major apex face of the mother crystal from which said element is cut, whereby said element exhibits a substantially zero temperature coefficient of frequency when vibrated at a fundamental frequency which is a function of one of its greater dimensions.

CHARLES F. BALDWIN. SAMUEL A. BOKOVOY.

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