US3917506A

US3917506A - Method of growing quartz crystals and seed plate therefor

Info

Publication number: US3917506A
Application number: US378282A
Authority: US
Inventors: Theodore E Lind; Richard J Burton
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1971-08-30
Filing date: 1973-07-11
Publication date: 1975-11-04
Anticipated expiration: 1992-11-04

Abstract

There is disclosed a method of growing monocrystalline quartz by use of a Z-plate seed. The Z-plate seed has facet inhibiting projections on certain edges to attenuate formation of major rhombohedral facets which limit the amount of useful material produced. The seed plate is of a generally rectangular shape having its length to width ratio of 10:1 or less.

Description

US. Patent Nov. 4, 1975 Sheet 1 of2 3,917,506

US. Patent Nov. 4, 1975 Sheet 2 of2 3,917,506

BACKGROUND OF THE INVENTION This invention relates to the growing of monocrystalline material and more particularly to the growing of quartz crystals.

The use of quartz crystals in piezoelectric oscillators and crystal filters led to a demand for monocrystalline quartz in relatively large quantities. Since quartz crystals of the natural occurring type are relatively rare and have non-uniform characteristics there has developed a new industry relating to the manufacture of monocrystalline quartz material to serve the electronics industry.

In the manufacture of quartz crystals a plurality of seed crystals are placed in the upper portion of a vertical autoclave. By maintaining the supply of silica and the seed crystals at a relatively high temperature and pressure for a period of time while maintaining a temperature differential between the supply chamber and the seed chamber, silica from the supply portion of the autoclave will be deposited upon the seeds thereby pro ducing monocrystalline quartz.

Quartz crystals, like most crystals, exhibit a preferential growth characteristic, that is growth takes place more rapidly on certain crystallographic surfaces as contrasted with other crystallographic surfaces of the seed. Thus, more rapid deposition takes place on a Z- surface (a surface perpendicular to the Z-axis of the quartz crystal) as contrasted with deposition on all other surfaces. Thus, a seed having its major surfaces perpendicular to the Z-axis of the quartz crystal is much preferred. A seed of this type is referred to as a Z-plate.

Certain crystallographic surfaces of a quartz crystal exhibit extremely low deposition or growth rates. In some cases, this deposition rate is virtually negligible such as to provide certain surfaces which may be referred to as growth inhibiting surfaces. For example. the natural prismatic faces of a quartz crystal (Y- plate" surfaces) exhibit virtually no growth during the deposition process. The X-axis of the quartz crystal is its polar axis; that is, the axis which exhibits electrical asymmetry. When a quartz crystal is tensioned along its X-axis a difference in electrical potential appears. The end at which the positive charge appears is designated the +X direction. It has been found that growth on faces perpendicular to the X-axis (X-plate surfaces) in the X direction is approximately /2 of the growth rate in the Z-direction, while growth proceeds even more slowly in the +X direction. Growth in the direction of the first order and second order rhombohedron faces is similarly relatively slow.

From the foregoing, it will thus be seen that a seed should be so dimensioned as to limit development of the prism and rhombohedral faces and not depend on growth in these directions or in the X-dircctions.

One of the more common crystal cuts used for oscillator plates is the AT cut. The AT cut has its major sur faces parallel to the X-axis and at an angle of 35 l4 to the Z-axis. Thus, the quartz crystal produced by a crystal growing process should be grown so as to yield the maximum number of AT cut plates or blanks. Many of the other cuts suitable for oscillator and filter plates,

the BT, CT, etc. cuts, also have major surfaces parallel to the X-axis and are generally referred to collectively as X-cuts. Thus, a crystal resulting from a crystal growing process having an orientation such that cuts parallel to the X-axis may be readily made is highly desirable.

SUMMARY OF THE INVENTION It is an object of this invention to provide an improved method and seed plate for the growth of monoerystalline quartz.

Another object of the invention is to provide an improved method and seed plate for the growing of quartz wherein the growth is primarily in a single direction relative to its crystallographic axis.

A still further object of the invention is to provide a method and seed plate for growing of monocrystalline quartz wherein the growth is primarily in the Z-axis direction.

Another object of this invention is to provide a seed plate shaped to inhibit growth of limiting rhombohedron faces and in the X-axis directions.

A still further object of the invention is to provide a method and seed crystal for the growing of monocrys talline quartz in a rapid and economic manner to produce monocrystalline quartz giving an optimized yield for oscillator and filter plates.

In accordance with these objects there is provided a seed for growing monocrystalline quartz from a supersaturated medium which comprises a plate of monocrystalline quartz having its major generally rectangular surfaces perpendicular to the Zaxis, its minor surfaces perpendicular to respective X- and Y-axes and a facet inhibiting face on at least one of the surfaces parallel to the said Y-axis, and method utilizing the aforesaid seed plate.

THE DRAWINGS Further objects and advantages of the invention will be understood from the following complete description thereof and from the drawings wherein:

FIG. 1 is a pictorial representation of a quartz crystal depicting its major crystallographic faces;

FIG. 2 is a cross section thereof depicting a seed plate crystal in accordance with an embodiment of the invention;

FIG. 3 is a perspective view of a seed plate;

FIG. 4 is another embodiment of a seed plate crystal in accordance with the invention; and

FIG. 5 is a crystal resultant from the use of the seed plate of FIG. 3.

DETAILED DESCRIPTION As shown in FIG. I a quartz crystal 10 in accordance with conventional trigonal nomenclature has a Z-axis extending parallel to its major hexagonal prism faces 11, a Y-axis perpendicular to the Z-axis and to the major prism faces II and an X-axis orthogonal therewith. The )(axis therefore passes through the intersection between adjacent major prism faces II. A quartz crystal normally exhibits major rhombohedral faces 12 and minor rhombohedral faces 13. Trigonal bipyramidal faces, trigonal trapezohedral faces and other faces sometimes exhibited by natural quartz are not pertinent to the present invention and are not shown. Also. no distinction need be made between left-handed and right-handed quartz since growth occurs in the same sense as the seed. The quartz crystal is sometimes hexagonally described in terms of its Z-axis and the three axes through the intersection of the major prism faces (X,, X X However, in the electronics field the orthogonal X. Y, Z nomenclature is more common with the Z-axis being the axis of crystal symmetry and the X-axis being the polar axis, which nomenclature is used herein.

In accordance with the invention a seed plate 16 is oriented with respect to the crystallographic axes as shown in FIG. 2. The seed plate 16 has end edge surfaces l7 perpendicular to the Yaxis, hence having its major length in the Y dircction. The side edge surfaces 18 parallel the Y-axis, with its minor length in the X direction. The thickness of the plate is in the Z- direction, hence providing a seed plate having its major surfaces 19 perpendicular to the Z-axis for primary growth in the direction of the Zaxis. Preferable dimensions for a Z-plate in accordance with the invention is one having a thickness of 0.080 of an inch, a length of approximately 6 inches along the Y-axis and a width of approximately 2 inches in the X-direction. Since growth in the Xdirection is not desired, the Y to X ratio should be less than :1 and is preferably about 3:1.

in accordance with the invention the crystals growth is enhanced by an edge 18 of the seed plate 16 configurated to result in triangular projections 20 (HO. 3) adjacent ends 17. These projections result in an increase in usable quartz by effecting an inhibition of the formation of rhombohedral faces which faces limit growth and forcshorten the resultant quartz crystal seed plate. End tabs 21 are useful for mounting the seed plates in the autoclave. They may be eliminated if desired. providing some other suitable holding means is provided,

thus effectively increasing the overall length of the seed plate for growth of "2 material. 2" material. as used herein, is meant to be that material which is basically developed on a surface normal to the Z-axis. Z material is the most desirable material from both electrical and production standpoints because this material has more consistent electrical characteristics and grows most rapidly.

A seed plate I6 is depicted in FIG. 5 with a resultant mono-crystalline quartz crystal superimposed therearound. The crystal 30 is an elongated bar with the primary growth surfaces 3l and 32 perpendicular to the Z-axis having a generally flat surface which would be termed a basal pinacoid since it is parallel to the XY plane. However, these faces are of the non-dominant type and have a rippled surface. The top surface 33 and the bottom surface 34 lie in the YZ planes and are in verse prism surfaces, i.e., surfaces parallel to the Y-axis rather than perpendicular thereto as in a natural crystal. Thus, the manufactured crystal has an overall shape in which the Y-axis appears to be the axis of crystal symmetry for a tetragonal class crystal. Hexagonal prism faces I la appear at the ends of the crystal. However, two of these prism faces are restricted by the fact that the seed crystal was held at these end faces during growth and the other four prism faces do not appear or are restricted by the minor amount of growth in the X- dircction. The latter four prism faces if they appear, lie in planes that are at 60 to the X2 plane. The other facets at the end of the grown bar are the low and high angle major

rhombohedral faces

12a and 12b and the high angle minor rhombohedral faces 13b. Certain minor faces 35 and 36 appear which are parallel to the Y-axis and at an angle to the X and Z axis along the surface 33. It will be thus seen that much of the end of the bar structure is terminated by the high angle rhombohedral faces 12b and 13b which lie in planes that are at approximately 30 to the X2 plane rather than being dominated by the low angle rhombohedral faces which are at angles of approximately 60 to the X2 planes. There is thus more useful quartz in a bar grown in accordance with the invention than in the prior art wherein the low angle major rhombohedral faces greatly foreshortened the end structure of the monocrystalline quartz crystal. By so controlling growth in the X-direction at least 25% more usable quartz length may be obtained in a crystal of practical length.

The dimensions of the seed plate are selectable based on various criteria, such as growth rate, end use and electrical characteristics. The growth rate is dependent on the development of growth limiting faces such as the prismatic faces, the major rhombohedral faces and the minor rhombohedral faces. Because of the fast growth rate in the Z-direction, the plate may be as thin as may be practically cut so that a maximum number of seed plates may be obtained from any given stock. The width in the X-direction must be sufficient for development of the high angle rhombohedral faces 12b and 13b which lie in planes at 30 to the X2 plane and intersect the Z-plate surfaces in a line parallel to the X-axis so that minimized foreshortening of the bar results. The length in the Ydirection should be sufficient for practical handling for cutting purposes.

The overall size is determined by the amount of 2- material that may be grown in a practical amount of time, for example, 60-75 days. Within this period of time, approximately one-half inch of growth occurs in both Z-directions, hence, yielding a bar approximately one inch thick in the Z-direction. Since the rhombohedral surfaces 120 intersecting the X and Y axes of the crystals are limiting, the seed should be at least two inches in one of these directions to obtain the basal pinacoids 31 and 32 and

inverse prisms

33 and 34. Since the Y-direction is greatly affected by the low angle limiting rhombohedral faces 120, it is preferable that the seed have the two inch dimension in the Y-axis. However, so long as the dimension in the X-axis of the seed is greater than two inches, a shorter length in the Y- direction may be considered since the sides would be dominated by the high angle rhombohedral faces 12b and [3b. The crystal must be long enough in the X- direction to insure formation of the high angle rhombohedral faces l2b and 13b which form intersections with the Z-plate surfaces parallel to the X-axis. These will appear at the four ends of the Z-plate surfaces provided the seed plate is at least one inch in the X-direction, two of these faces being major rhombohedral faces 12!) and two of the faces being minor rhombohedral faces 13!). These minor rhombohedral faces will not necessarily appear in a regular rectangular plate without the use of the X-growth inhibition projection 20 unless the plate is approximately one inch and one-half in the X- direction. With the use of X-growth inhibiting configuration the formation of these

rhombohedral faces

12b and 13b is enhanced and the X-direction dimension need be only one inch.

Another criteria for the dimensions of the seed plate in accordance with the invention is the end use, i.e., the oscillator or filter plates to be cut therefrom. Many of the cuts suitable for oscillator and filter plates, the AT,

BT, CT, etc. cuts. have major surfaces parallel to the X-axis. Therefore, it is highly desirable to use a crystal seed which results in a bar of quartz crystal which has its width parallel to the X-axis and hence perpendicular to the Y-axis, therefore its length should extend in the Y-axis direction. Since most of the cuts parallel to the X-axis are at some angle to the Z-axis. it is preferable that the length of the plate be made as long practical in the Y direction to increase percentage yield. For example, an AT plate is at 35 to 14' to the Z-axis. Because of this angle. the effective length of bar in the Y- direction which produces usable quartz is proportional to the tangent ofthe angle, or approximately 7/10 of an inch per inch of thickness in the Z-direction. Thus. if one assumes a 1 inch by 1 inch rectangular bar of quartz (ignoring for the moment the faccting at the ends of a synthesized monocrystalline bar) its Y-axis length must be more than 7/10 of an inch to produce any useful quartz. Since the losses at the ends are fixed. yield increases with length, cg. a 2-inch bar would produce approximately a 50% yield, a 3-inch bar approximately 66'72. a 4inch bar 75%. etc.

Another criteria for the dimensions of the seed plate is the electrical characteristics of the resultant quartz material. If growth is permitted in the X-direction. the seed crystal contains what might be termed X-material and Z-material. Thus. certain blanks cut from a bar grown in this manner will contain X-material and Z material with an interface therebetwecn. The electrical characteristics of the X-matcrial may not be the same as the characteristics of the [material and the interface makes an electrical discontinuity. Blanks with this interface are unsuitable for oscillator or filter plates. Therefore. the dimension of the seed in the X-direction should be at least one blank width. For example. if /2 inch blanks are to be formed, the dimension should be at least /2 inch, or a multiple thereof, for example. one or two inches. With all of the dimensional criteria taken into account, it is preferable that the seed plate 16 have a length to width ratio of less than 10. and preferably approximately 3.

The facet inhibiting projections need be provided only on the -X edge of seed plate to attenuate the two largest low angle rhombohedral surfaces 12a. However. some further improvement of effective length is attained by recessing of the +X edge 18 as well. A seed plate of this type is shown in FIG. 4. The seed plate 16' has end edges 17' and side edges 18 configurated to provide projections 20' in both of the X-directions. The four projections 20' will attenuate all four limiting rhombohedrons 12a. While the invention hsa been disclosed by way of the preferred embodiments thereof. it will be apparent to one skilled in the art that suitable changes may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

I. A method of growing monocrystalline quartz by the transfer ofquartz from an aqueous growing solution onto a crystal seed which comprises the steps of provid ing an elongated quartz crystal seed plate having its length extending substantially parallel to a crystallographic Y-axis and its thickness extending parallel to the Z-axis, forming said plate to provide a pair of spaced apart integral projections having a generally triangular portion on a side parallel to the crystallographic Y axis and extending outwardly from said side in the crystallographic X-direction with one side of the triangular portion forming an obtuse angle with the side. each projection being positioned adjacent an end of said side with said side extending substantially continuously thercbetwccn. and exposing said seed plate under growing conditions at elevated temperature and pressure to an aqueous solution in contact with a supply of silica to affect the transfer of quartz onto the seed bar.

2. A seed for growing monocrystalline quartz comprising a plate of monocrystalline quartz material having its thickness extending generally parallel to the Z- axis and major generally rectangular surfaces perpendicular to the Z-axis, the longitudinal sides of said plate extending in a crystallographic Y-axis direction. and the transverse ends of said plate extending in the crystallographic X-axis direction; and a pair of spaced apart integral projections having a generally triangular portion on at least one ofthe sides of said plate parallel to the Y-axis and projecting outwardly from the one side in the X-axis direction with one side of the triangular portion forming an obtuse angle with the side. each projection being positioned adjacent an end of the one side with the one side extending substantially continu ously therebetwcen to provide facet inhibiting charac teristics when said seed plate is placed in a crystalgrowing environment.

3. A seed as recited in claim 2 and further including a pair of projections on the other side parallel to the Y- axis.

4. A seed recited in claim 3 wherein the length to width ratio of said seed is less than 10:].

S. A seed as recited in claim 4 wherein said length to width ratio is approximately 3:].

6. A seed as recited in claim 2 and further including support tabs integrally formed at the ends of said seed plate.

7. A seed for growing monocrystalline quartz comprising a plate of monocrystalline quartz material having major generally rectangular surfaces perpendicular to its Z-axis and its width extending in a direction parallel to a major surface of a standard oscillator cut and longitudinal edges ofthc plate extending perpendicular to its width direction and a pair of spaced apart facet inhibiting integral projections having a generally triangular portion extending from one of the longitudinal edges in the width direction with one side of the triangular portion forming an obtuse angle with the edge. each projection being positioned adjacent an end thereof.

8. A seed for growing monocrystallinc quartz as recited in claim 7 wherein said standard oscillator cut is an AT cut.

9. A seed for growing monocrystalline quartz as recited in claim 8 and further including a pair of projections extending from its edges in the other width direction.

10. A seed as recited in claim 9 and further including support tabs integrally formed at the ends of said seed.

11. A seed for growing monocrystalline quartz as re cited in claim 8 wherein the length to width ratio ofsaid seed plate is less than 10:1.

12. A seed for growing monocrystallinc quartz as recited in claim 1] wherein said length to width ratio is approximately 3:].

Claims

1. A METHOD OF GROWING MONOCRYSTALLINE QUARTZ BY THE TRANSFER OF QUARTZ FROM AN AQUEOUS GROWING SOLUTION ONTO A CRYSTAL SEED WHICH COMPRISES THE STEPS OF PROVIDING AN ELONGATED QUARTZ CRYSTAL SEED PLATE HAVING ITS LENGHT EXTENDING SUBSTANTIALLY PARALLEL TO A CRYSTALLOGRAPHIC Y-AXIS AND ITS THICKNESS EXTENDING PARALLEL TO THE Z-AXIS, FORMING SAID PLATE TO PROVIDE A PAIR OF SPACED APART INTEGRAL PROJECTIONS HAVING A GENERALLY TRIANGULAR PORTION ON A SIDE PARALLEL TO THE CRYSTALLOGRAPHIC Y-AXIS AND EXTENDING OUTWARDLY FROM SAID SIDE IN THE CRYSTALLOGRAPHIC -X-DIRECTION WITH ONE SIDE OF THE TRIANGULAR PORTION FORMING AN OBTUSE ANGLE WITH ONE SIDE, EACH PROJECTION BEING POSITIONED ADJACENT AN END OF SAID SIDE WITH SAID SIDE EXTENDING SUBSTANTIALLY CONTINUOUSLY THEREBETWEEN, AND EXPOSING SAID SEED PLATE UNDER GROWING CONDITIONS AT ELEVATED TEMPERATURE AND PRESSURE TO AN AQUEOUS SOLUTION IN CONTACT WITH A SUPPLY OF SILICA TO AFFECT THE TRANSFER OF QUARTZ ONTO THE SEED BAR.

2. A seed for growing monocrystalline quartz comprising a plate of monocrystalline quartz material having its thickness extending generally parallel to the Z-axis and major generally rectangular surfaces perpendicular to the Z-axis, the longitudinal sides of said plate extending in a crystallographic Y-axis direction, and the transverse ends of said plate extending in the crystallographic X-axis direction; and a pair of spaced apart integral projections having a generally triangular portion on at least one of the sides of said plate parallel to the Y-axis and projecting outwardly from the one side in the X-axis direction with one side of the triangular portion forming an obtuse angle with the side, each projection being positioned adjacent an end of the one side with the one side extending substantially continuously therebetween to provide facet inhibiting characteristics when said seed plate is placed in a crystal-growing environment.

3. A seed as recited in claim 2 and further including a pair of projections on the other side parallel to the Y-axis.

4. A seed as recited in claim 3 wherein the length to width ratio of said seed is less than 10:1.

5. A seed as recited in claim 4 wherein said length to width ratio is approximately 3:1.

7. A seed for growing monocrystalline quartz comprising a plate of monocrystalline quartz material having major generally rectangular surfaces perpendicular to its Z-axis and its width extending in a direction parallel to a major surface of a standard oscillator cut and longitudinal edges of the plate extending perpendicular to its width direction, and a pair of spaced apart facet inhibiting integral projections having a generally triangular portion extending from one of the longitudinal edges in the width direction with one side of the triangular portion forming an obtuse angle with the edge, each projection being positioned adjacent an end thereof.

8. A seed for growing monocrystalline quartz as recited in claim 7 wherein said standard oscillator cut is an AT cut.

11. A seed for growing monocrystalline quartz as recited in claim 8 wherein the length to width ratio of said seed plate is less than 10:1.

12. A seed for growing monocrystalline quartz as recited in claim 11 wherein said length to width ratio is approximately 3:1.