US2505370A - Piezoelectric crystal unit - Google Patents

Description

April 25, 1950 R. A. SYKES 2,505,370

PIEZOELECTRIC CRYSTAL UNIT Filed Nov. 8, 1947 FIG! FIG 2 INVENTOR R A. S V/(E S A r TORNEV Patented Api 25, 1950 UNITED STATES PATENT OFFICE PIEZOELECTRIC CRYSTAL UNIT Roger A. Sykes, Fanwood, N. J., asslznor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application November 8, 1947, Serial No. 784,781

9 Claims.

This invention relates to piezoelectric crystal units, and particularly to a sealed crystal unit susceptible of frequency adjustment.

Crystal units mounted within small closed containers are well known in the art. Such units have been developed to insure greater frequency stability, to protect the mounted crystals from injury, and to make them easier to handle during the processes of supply, maintenance, and operation.

One of the best of the various types of containers for such crystal units utilizes an evacuated and sealed envelope of glass or metal similar to those employed in radio receiving tubes. The envelope is fixed for convenience in a base having projecting contact pins and arranged for removable mounting in a standard tube socket. Evacuated and sealed containers of this type are particularly advantageous for use in situations where extreme variations in temperature, humidity, and other climatic factors are encountered, which would normally cause changes in the frequency of the crystal. In contrast, the frequency of a crystal unit carefully manufactured and so mounted in a sealed envelope, may be depended upon to remain permanently fixed.

It may be necessary, however, to change the natural frequency during use, and often the change is desired under field conditions. In previously available crystal units an increase in frequency required dismantling the unit, and grinding or etching away some of the crystal or the plating, while to reduce the frequency it was necessary to add to the plating. Such operations require equipment not ordinarily available in the field, and necessitate removing the crystal unit from service for a considerable time.

It is an object of the present invention to enable decreasing or increasing the frequency of such a sealed crystal unit at will without requiring the disassembly of the crystal or its envelope, and without requiring the use of extensive auxiliary apparatus.

The invention contemplates, in brief, provision for mounting within a crystal-containing envelope a cathode from which metal may be evaporated onto the crystal to decrease its frequency, and means for removing metal from the crystal plating by sputtering when the frequency is to be increased. The sputtering process may 2 also be used instead of evaporation to apply metal to the crystal coating.

The invention has been illustrated in the drawings, in which:

Fig. l is a perspective view of the device, partially broken away to show the construction more clearly; and

Fig. 2 is a schematic circuit diagram illustrating the manner in which the invention is utilized.

A frequency-adjustable crystal unit, indicated generally as I, is shown in Fig. 1 with the various elements broken away to reveal the construction more clearly. A base 2, similar to the well-known octal type of vacuum tube base with a keyed alining plug 3, has secured thereto an evacuated envelope 4 in which is disposed the crystal 5.

Crystal 5 may be held by any of the known means for making good contact while affording a resilient support. The support is here accomplished by a spaced pair of vertically disposed wires 6, only one of which is visible in the figure. Each of the wires 6 terminates upwardly in a helix l, and external connection is provided thereto through contact pins 8. Helix I is spread axially to receive the corner 9 of the crystal between adjacent turns IO and I I thereof and retain it resiliently. Permanent connections may be obtained by soldering or cementing the helices to the metallic coatings of the crystal. For example, the coating l2 on the obverse face it of crystal 5 is joined by silver cement M to the helical turn ill. Similar connections, not visible in the figure, are provided to the coating on the reverse face of the crystal 5 at a point diagonally opposite corner 9. This type of mounting was disclosed and claimed in the United States Patent 2,392,429, issued January 8, 1946, to the present applicant.

Support wires 6 are held in alignment by virtue of their passage through a base plate I5 of mica or similar material. Base plate I5 is also bored to provide for the support and alignment of the shield I6, the filament i1, and the sputtering anode i9. A cooperatively bored mica top plate 20 completes the spacing structure. Suitable eye lets 2i may be used to protect the mica from chipping or fracture adjacent each of the borings.

The source of the metal to be evaporated onto the crystal coating it? to decrease the frequency is a head or giobule 22 of gold or equivalent metal associated with filament 11. A loop 23 is formed in the filament, and the giobule 22 is deposited therein prior to assembly. The sizes of the loop 23 and the globule 22 are so related that the surface tension of the globule will hold it in position when heated to a molten condition for evaporation. The heating is produced by the passage of current through filament l1. Rigid legs 24 and 25, to which the filament I1 is fixed, extend through top plate 20 and bottom plate l5, and terminate in the usualpintype terminals 26 and 21 projecting from the base 2, as do terminals 3, for engagement with a conventional socket, not shown.

The portion of the obverse crystal face I3 on which the evaporated metal is deposited is limited by the conducting shield l6, through which a properly shaped bore 28 is formed. Shield I6 is positioned by a center pin 29 passing through a bore 30 in base plate l5, and by rivets 3| securing it to the top plate 20. The shield l6 may have side portions 32 and 34 extending at an obtuse angle thereto, to provide a partial enclosure of the bead 22 and prevent the evaporated metal from straying around the shield and depositing beyond the desired area on the crystal surface. Connection to the shield i9 is provided through center pin 29, which is linked to terminal pin 35.

The sputtering anode i9 is held by two legs 39 and 31 passing through top plate 20 and bottom plate l5. Leg 36 provides external connection to the sputtering anode through pin terminal 33. The two remaining terminals 39 of the standard octal base are not utilized in this embodiment. Sputtering anode l 9 may be a fiat or curved sheet of metal or other conductive material. It is aligned with the shield bore 28 and crystal coating i2, but its exact shape and position is not critical.

The circuit diagram is shown schematically in Fig. 2. In normal operation, the crystal is connected through terminals 9 to the circuit in which it is acting as a frequency control, here shown as an oscillator 40. The filament control switch 4i is either open or else connected to the sputtering anode lead 39; sputtering power supply switch 42 is open, while the position of shield switch 44 is immaterial.

When it is desired to decrease the natural frequency of the crystal, the filament control switch 4| is thrown to close the filament circuit through battery 45, rheostat 46, filament terminal 26, the filament I1, filament terminal 21, and a lead 41. The filament current then heats the bead 22 to a molten state, and evaporation proceeds. The filament resistance is so chosen that the normally available battery supply voltage -may be used, which may be, for example, 6 volts.

The evaporating metal is dispersed substantially evenly in all directions. Those portions of the crystal face l3 on which it is not desired to deposit additional metal are masked by the shield IS, with bore 28 permitting passage toward the area to be plated. The shield l6 may be grounded at 49 by shield switch 44. The rate of evaporation is controlled by the filament rheostat 45, and the sputtering power supply switch 42 is opened, since no high potential is to be applied to the unit I from the rectifier tube 49. Depositing of the evaporating metal is continued until the frequency of oscillator 40 reaches the desired value. This point may be determined by conventional methods, such as feeding the oscillator output into a comparison circuit and observing the beat note produced therein against the output of an oscillator of known frequency. Switch 4| is opened when the beat note frequency reaches zero, and the operation is complete. A change of several kilocycles in the natural frequency of the crystal may thus be accomplished in a few seconds. In the factory production of crystals using this system, the output per operator in the final phase of frequency adjustment has been raised from less than a dozen crystals per hour to over two hundred.

When the crystal frequency is to be increased, a portion of the crystal plating is removed. In previously known plated crystal units, increasing the frequency required, as mentioned above, grinding or lapping the crystal or the plating,

. or the removal of the latter by means such as etching in acid. These processes are obviously unsuitable for use in correcting a crystal sealed into an evacuated envelope.

The crystal unit herein described permits adjustment by increasing the crystal frequencies under service conditions, without requiring any disassembly of the unit, and the adjustment may be completed in a short interval.

Removal of the required amount of plating is accomplished by sputtering, that is, by applying to a sputtering anode in the evacuated envelope a positive potential of sufficient magnitude to cause a glow discharge, in which minute particles of the metal are driven out of the plating, and then attracted toward a sputtering anode on which they are collected. For the plating metals commonly used, such as gold, an attracting potential of the order of 1500 to 2500 volts is adequate to accomplish this aim in a short time. While for efficient evaporation, evacuation should be carried as far as possible, a lesser degree of evacuation is required for most efficient sputtering, as well understood by those skilled in the art. It is dependent, among other factors, upon the material being sputtered and the rate at which it is desired to carry out the operation. For gold plated crystals, the optimum pressure to which evacuation must be carried is about .011 millimeter of mercury, with comparable pressures for platinum or silver. If the crystal is plated with copper, the sputtering should be carried on in an atmosphere of hydrogen. Other plating materials may operate best at different pressures and in other atmospheres.

During the sputtering process, the crystal will 'be heated to a temperature which may be from 200 to 250 C. by positive ion bombardment. In consequence the operator must, in adjusting the frequency, apply a correcting factor relating the observed frequency to that which will be experienced at the normal operating temperature of the crystal. In contrast, during the evaporation plating, the time required is so short that the crystal temperature is not increased significantly above normal operating temperature. It is, of course, necessary that those elements which receive the sputtering potentials should be adequately insulated, in accordance with standard practice.

Sputtering potential is obtained from a step-up transformer 59, primary 51 of which is connected to an alternating current source 52 through sputtering power switch 42. Transformer 50 is of a conventional type, with a low voltage secondary winding 53 supplying heating current to the rectifier tube filament 54, and a high voltage secondary winding 55 connected to the rectifier tube anode 56 by lead 51 and supplying negative potential to the plated face l2 of the crystal through aooasvo lead 89. The plated face I! is to act as the cathode during the sputtering process. The positive potential from the rectifier filament is applied through lead 6'! to the crystal unit filament 23, and through switch 4! and terminal 38 to the sputtering anode I9.

The shield l6 may be used to control the rate at which the sputtering process is carried out. For this purpose, switch 44 is shifted from ground connection 48 to the adjustable arm 50 of a potentiometer 6i bridged across the leads 4! and 58, and hence between the sputtering anode and cathode. The potential of shield it may thus be set at any desired fraction of the potential dinerence between the plated face l2 and the sputtering anode.

In some cases it may be desirable to use a variant of the embodiment illustrated, eliminating the elements for plating by evaporation, and employing the sputtering process both for plating and deplating the crystal. The changes in the basic circuit of Fig. 2 required to accomplish this aim will be obvious to those skilled in the art; the

crystal plating itself must be connected as the anode when plating is to be done by sputtering,

and the sputtering anode [9 then acts as the cathode.

While the invention has been illustrated as applied to use with a crystal designed for vibration in the thickness shear mode, it is equally applicable to crystal vibrating in other modes, and particularly at the higher frequencies. In such cases, the location of the electrodes from which metal is to be applied to the crystals, the placing of the necessary shields, the sizes and shapes of the apertures in the shields, and the disposition of the sputtering electrodes, will be determined by the shape of the crystal, and the portions to which the plating is to be applied. It is to be understood that the embodiments shown and described herein are illustrative only or the invention, and that the invention includes all other equivalent forms within the scope of the appended claims.

What is claimed is:

l. in combination, a piezoelectric crystal, a metal coating on said crystal, means for oscillat= ing said coated crystal at its natural frequency under low pressure, means comprising an anode electrode spaced from and having a positive potential with respect to said coating for removing metal from said coating on said crystal while said coated crystal is oscillating for thereby increasing the natural frequency of said coated crystal during said oscillating thereof, and means applicable to the indication of the natural frequency of said crystal during said oscillation.

2. In a crystal unit, comprising a plated piezoelectric crystal mounted in an envelope evacuated to an optimum sputtering pressure and arranged for frequency adjustment, means comprising a sputtering anode spaced from said plated crystal for removing plating material from said crystal, means for maintaining said plated crystal in oscillation at its natural frequency while removing therefrom plating material, and means for frequency-determining connections during said oscillation.

3. In a metal plated piezoelectric crystal mounted in an evacuated container, means for adjusting the natural frequency of said plated crystal, comprising means including an anode electrode disposed within said container for sputtering metal off said metal plated crystal, means for oscillating said metal plated crystal at its 6 natural frequency of oscillation during said sputtering of said metal, and means for indicating the natural frequency of oscillation of said plated crystal during said sputtering of said metal.

4. An adjustable frequency piezoelectric crystal unit comprising a base, contact pins extending through said base, an evacuated and sealed envelope secured to said base, leads extending from said contact pins into said envelope, and elements disposed within said envelope and connected to said leads, comprising a coated crystal, means for masking a portion of said crystal, at source of material, means comprising said source constituting an anode electrode for receiving ma- .terial transferred from said coated crystal, and

means comprising said masking means for defining the coated portion of said crystal from which coated material on said crystal may be transferred to said source.

5. An adjustable frequency piezoelectric crystal unit comprising a base, contact pins extending through said base, an evacuated and sealed envelope secured to said base, leads extending from said contact pins into said envelope, and elements disposed within said envelope and con nected to said leads, comprising a coated crystal, a source of metal, means for shielding all except a desired coating portion of said crystal from said source, and means including said source disposed adjacent said shielding means and constituting an anode electrode with respect to said coated crystal for collecting metal from said coated crystal and thereby increasing the value of said frequency characteristic for said coated crystal.

6. An adjustable frequency piezoelectric crys tal unit comprising a base, contact pins extending through said base, an evacuated and sealed envelope secured to said base, leads extending from said contact pins into said envelope, and elements disposed within said envelope and connecteol to said leads, comprising a coated crystal, a source of metal, means for masking a portion of said coated crystal from said source, and means including said source and said last-mentioned means constituting anode electrodes with respect to said coated crystal for attracting metal from said coated crystal and thereby increasing the value of said frequency characteristic for said coated crystal.

7. An adjustable frequency piezoelectric crys tal unit comprising a sealed envelope evacuated to a pressure of the order of .011 millimeters of mercury and elements disposed within said evacuated envelope comprising a coated crystal, a source of metal selected from the group including gold, platinum, and silver, a shield disposed between said coated crystal and said source and arranged to mask desired portions of said coated crystal from said source, means comprising said source and said shield adaptable to produce a removal of metal from a desired coated portion of said coated crystal for raising to a desired value said frequency characteristic thereof, and electrodes sealed through said envelope connecting individually to said coated crystal, to said source of metal, and to said shield.

8. A piezoelectric crystal unit, comprising a sealed envelope containing an atmosphere of hydrogen at optimum sputtering pressure, a copperplated crystal, a separate source of additional copper, a, shield disposed between said crystal and said source and apertured to mask certain portions of said crystal from said source; and elec- 7g trodes sealed through said envelope arranged to provide external individual connection to said REFERENCES CITED crystal, said source, and s d shield.

9. In a piezoelectric crystal unit, the combine.- y: gs ii are of record in the tion of a sealed envelope, an atmosphere of hydrogen disposed within said envelope at optimum 5 UNITED STATES PATENTS sputtering pressure; a copper-plated crystal. a Number Name Dat source or additional copper, an apertured mask 1,765,413 Furth June 24, 1930 disposed between said source and said crystal; 2,164,595 Siebertz July 4, 1939 and electrodes sealed through said envelope ar- 2,241,228 Weinhart May 6, 1941 ranged to provide individual external connection 10 2,363,781 Ferguson Nov. 28, 1944 to said crystal, said-source, and said mask.

ROGER A. SYKES.

Images