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Publication numberUS3732846 A
Publication typeGrant
Publication dateMay 15, 1973
Filing dateJan 7, 1972
Priority dateJan 7, 1972
Publication numberUS 3732846 A, US 3732846A, US-A-3732846, US3732846 A, US3732846A
InventorsLukaszek T
Original AssigneeUs Army
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Crystal plating monitoring system
US 3732846 A
Abstract
This disclosure relates to crystals, and particularly to plating techniques for improving the quality of crystals. More particularly, this disclosure relates to the production of crystals having a minimum of unwanted, or inharmonic, modes of oscillation. This disclosure describes an apparatus and a method for taking a mode spectrograph to determine the impedance characteristic of a crystal in the frequency range of a desired harmonic while the crystal is in the process of being plated in a vacuum.
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United States Patent 1 1 Lukaszek 51 May 15, 1973 1 CRYSTAL PLATING MONITORING SYSTEM [21] App]. No.2 216,032

[52] U.S. Cl. ..118/8, 118/9, 118/49.l [51] Int. Cl ..C23c 13/08 [58] Field of Search ..118/7, 8, 9, 5, 48-495;

117/1 NQ; 204/1 NQ; 250/1 NQ; 324/56; 310/93, 8.2, 8.9; 73/67.1

3,017,525 l/1962 Wolfskill ..310/9.4 3,059,611 10/1962 Fury et al. ..118/8 3,382,842 5/1968 Steckelmacher et al. ..118/8 3,383,238 5/1968 Unzicker et a1 117/106 R 3,593,125 7/1971 Wilheim et al. ..324/56 3,600,675 8/1971 Primary ExaminerM0rris Kaplan Attorney-Harry M. Saragovitz [57] ABSTRACT This disclosure relates to crystals, and particularly to plating techniques for improving the quality of crystals. More particularly, this disclosure relates to the production of crystals having a minimum of unwanted, or inharmonic, modes of oscillation. This disclosure describes an apparatus and a method for tak- [56] R f en Cit d ing a mode spectrograph to determine the impedance characteristic of a crystal in the frequency range of a UNITED STATES PATENTS desired harmonic while the crystal is in the process of 1,649,828 11/1927 1111116 ..324/56 bemg Plated a vacuum- 2,470,737 5/1949 Bach ..324/56 X 2,621,624 12 1952 Chil0wsky.... ..118/8 3 Glam, 3 Draw; Fgures 2,906,235 9/1959 Hirsh ..118/8 FREQUENCY COUNTER J 25 SWEEP FREQUENCY ATTENUATOR Lgiflililrrl-ljlglc GENERATOR Q ELECTRONIC 0 sweep GENERATOR OSCILLOSCOPE Grenier ..324/56 X PATENTEUHAYI 1s 3,732,546

17 FIG. 1 3|A 46 FREQUENCY SUPPLY CouNTER 34 VACUUM 25 SWEEP YBR o OGARITHMIC FREQUENCY H 1 L SIGNAL ATTENUATOR BRIDGE AMpL| HER GENERATOR 1 Q ELECTRONIC SWEEP GENERATOR osc IL LOSCOPE 26 27 FIG 2 FIG. 3

CRYSTAL PLATING MONITORING SYSTEM BACKGROUND OF THE INVENTION Crystals are very well known and are very valuable, particularly in filters for frequency control. The techniques for grinding and preparing crystals to obtain the most accurate frequency, the highest possible Q," and the minimum of unwanted modes of oscillation are also well known. However, regardless of the care taken in processing crystals with present, state-of-the-art techniques, there is almost inevitably some degree of imperfection and even the finest crystals will have a certain amount of these unwanted modes of oscillation.

The shear mode of oscillation is through the thickness of the crystal and provides the strong fundamental and harmonic characteristics that are normally used in the upper high frequency, and very high frequency ranges. However, in addition to the harmonic modes of oscillation, there are always some unwanted, inharmonic modes of oscillation which, for example, may be propagated laterally through the crystal. These inharmonic or parasitic modes have a somewhat predictable, but not precisely predictable, relationship to the frequency for which the crystal is ground or to each other. These inharmonic modes may sometimes be strong enough to make the crystal, apparently, respond to frequencies other than those of its fundamental or its harmonic modes, and thereby produce errors or false settings of frequency in the equipment where these crystals are used. They are quite undesirable and the lower the ratio between these inharmonic modes and the desired harmonic modes of the crystal, the better its operation.

To obtain a strong, clean, main-mode response in quartz resonators, it is necessary to lower the resonant frequency of a portion of the wafer below the resonant frequency of the surrounding portions in the manner taught by the Energy-trapping theory. This has the effect of containing the vibratory energy in a limited region of the quartz wafer, thus enhancing the Q of the resonator. This also has the effect of containing the group of inharmonic modes of oscillation, which are functions of the lateral dimensions of the electrodes and plate and the mechanical characteristics of the frequency lowered electrode region. The technique for lowering the resonant frequency of a portion of the wafer is to plate the parallel surfaces of that portion of the crystal, after it has been ground, to a precise thickness dependent on electrode diameter and resonator frequency. The plating has the effect of introducing separate cut-off frequencies for the electrode and surrounding regions of the wafer, thereby controlling the wave propagation patterns.

However, even though it is established that the general characteristics of the crystal will be improved by plating, the effect of the plating is not linear for all the inharmonic modes, and may be different for the inharmonic modes around each of the harmonics. It is, currently, not possible to predict how the magnitude of each individual inharmonic mode will be effected by the given amount of plating. For critical applications, it is still necessary to take all available crystals, as now fabricated, and test each one, individually, over a band of frequencies around the harmonic of interest, to find the crystal that has the minimum or an acceptably low level of unwanted modes of oscillation.

This testing is obviously tedious, time-consuming, expensive, and, at best, it can only indicate the apparently-best crystal among those tested. Furthermore, there is no assurance that this is the best quality that these crystals, particularly the plated crystals, could achieve. There is always the question whether any of the crystals could have been improved by additional plating or could have had the same or better results with less plating. This procedure also raises the question of what to do with the less-desirable crystals.

It is therefore an object of this invention to provide an improved system for reducing unwanted modes within a given frequency range of a crystal.

It is a further object of this invention to provide an improved, dynamic means for indicating the characteristics of all of the modes of oscillation of a given crystal while decreasing the unwanted modes of the crystal by plating the crystal to obtain the optimum performance for the crystal.

SUMMARY OF THE INVENTION be carried on simultaneously, this shows the instantaneous effect of the plating on the modes of oscillation of the crystal and makes it possible to stop the plating process at a precise point whereat the unwanted modes of oscillation of the crystal have all been reduced below a given level or whereat the ratio of the desired fundamental or harmonic mode to the unwanted modes is within an acceptable proportion. The desired harmonic of the crystal can also be monitored so that the plating process can be stopped before there is a significant change in the desired harmonic.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 of the drawing shows a pictorial representation of the crystal plating device with a block diagram of the circuitry.

FIGS. 2 and 3 show typical examples of the frequency characteristics of a crystal, during the course of the plating of the crystal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, the vacuum, crystal-plating apparatus 10 has a base 11 that supports a bell jar 12, and a component mount 13 that supports a crystal 14. The crystal is electrically coupled through a rotary joint 15 to a coaxial tubing 16 that extends through a vacuum-tight connection 17 to a hybrid bridge 24 that is part of a sweep-frequency test circuit 20. The sweepfrequency test circuit includes a sweep-frequency signal generator 21 that connects through an attenuator 23 to the hybrid bridge 24. A logarithmic amplifier 25 connects the hybrid bridge to an oscilloscope 27 with a display tube 28. The sweep of the oscilloscope is synchronized with that of the sweep-frequency signal generator by an electronic sweep generator 26. A frequency counter 29 is connected to the sweepfrequency signal generator.

The crystal plating apparatus also includes plating filaments 31A and 31B connected through the base of the apparatus to a power supply 32. The bell jar has a vacuum connection 33 to a vacuum pump 34.

In operation, the sweep-frequency signal generator 21 is adjusted to sweep through a band of frequencies of interest that will, normally, include the frequency of the harmonic at which the crystal is intended to be used. The actual frequency of the generator can be indicated by the frequency counter 29. The output voltage level of the sweep-frequency generator signal will be constant. The attenuator 23 may be necessary to reduce the strength of the signal applied to the hybrid bridge and to the crystal to a constant level within the tolerance of the crystal.

The hybrid bridge applies the constant-level, attenuated, sweep-frequency signal voltage to the crystal, but the impedance of the crystal, connected to one of the arms of the hybrid bridge, results in a voltage across the crystal, at any specific frequency, that varies with the effective impedance of the crystal at that frequency. The voltage across the crystal will rise to a peak at the frequency of each of the harmonic or inharmonic modes of oscillation of the crystal. The relative heights of the peaks of the fundamental mode or any of its harmonic modes as compared with those of the inharmonic modes will indicate the quality of the crystal.

The voltage across the crystal is applied to the logarithmic amplifier 25 that effectively extends the scale of the signals to be applied to the oscilloscope 27 and provides greater sensitivity to the lower, inharmonic peaks that are to be reduced by the plating process.

The sweep of the oscilloscope is established and synchronized with that of the signal generator by the electronic sweep generator 26. This sweep is combined with the signals from the amplifier 25 to provide a trace on the oscilloscope screen representing the impedance of the crystal with respect to frequency over the range of the signal generator sweep.

FIG. 2 shows an oscilloscope display tube 28, corresponding to that of FIG. 1, with a typical example of the display of the impedancevs. frequency characteristics of a crystal over a band of frequencies including one of its harmonics 42. The oscilloscope display tube may be provided with a grid having an ordinate 40 and an abscissa 41. The strength of the signal across the crystal is indicated with respect to the ordinate 40 and the frequency is shown with respect to the abscissa 41. The impedance of the crystal at any frequency is, of course, inversely proportional to this signal strength. The harmonic frequency of the crystal will have the lowest impedance which, will produce the highest peak voltage 42. The inharmonic modes of oscillation also affect the impedance of the crystal as shown by the lower voltage peaks 43. These are normally less than those of the fundamental or harmonic frequency 42, but still may be high enough to cause erroneous functioning of the crystal under critical conditions.

The crystal, mounted in a vacuum in the bell jar, can be plated in a well known manner. The filaments 31A and 31B are charged or loaded with any desired plating material, such as aluminum, gold, copper or silver. Energy is applied to the filaments through suitable connection to the power supply 32. The plating continues on both sides of the crystal at a predictable rate until the filament is disconnected from the power supply.

FIG. 3 shows the oscilloscope display tube 28 with another, typical example of the impedance vs. frequency characteristics of the same crystal after an interval of plating. The oscilloscope display tube 28 has the same ordinate 40 and abscissa 41 covering the same ranges of voltages and frequencies as shown in FIG. 2. The desired harmonic frequency is now 52. It will be shifted, almost imperceptibly, but it will not be substantially attenuated.

The plating can be continued until the inharmonic mode peaks 53 have been reduced to an acceptable level, or until there are no dominant peaks that could cause erroneous function of the crystal. The filament is then disconnected to shut off the plating process and to retain the improved characteristics of the crystal. The crystal is ready for use without further testing.

It should be noted that variations in the sweep frequency test circuit are possible. For example, the attenuator may be omitted if it is not needed. Other types of test circuit and other types of impedance and frequency indicators may also be substituted.

In another variation of this crystal plating monitoring system, a peak sensing device may be connected to the output of the hybrid bridge or logarithmic amplifier, and gated to detect peaks within certain bands of frequencies including those of the most serious of the inharmonic modes of oscillation. This peak sensing device may be set to disconnect the filament and terminate the plating process when the peaks of the unwanted, inharmonic modes fall below a given level. Alternatively, the desired harmonic frequency may also be monitored by a peak sensing device, and its peak may be compared with the output of the peak sensing device for the unwanted, inharmonic modes of oscillation to detect when the ratio between the two reaches a given, acceptable proportion before disconnecting the filament.

In practice, there may be only a few frequencies that include inharmonic mode peaks of any serious magnitude. These may be monitored individually, with or without the oscilloscope display, to ascertain when the levels of these particular, unwanted peaks are reduced to an acceptable level.

Perhaps the most ideal system that could be used here to obtain the optimum quality for each individual crystal, would be to record the progress of the characteristics of the crystal, with respect to plating, at least for the most dominant inharmonic modes, during the entire plating process, and to plate the crystal to a thickness beyond the apparent optimum. This would provide a record of the changes in the characteristics of the crystal with respect to the thickness and a means for determining the precise thickness of plating that would provide the optimum characteristics for any given crystal for any given purpose. This would also establish the minimum plating thickness that would provide acceptable characteristics. The plating could then be etched off and replated to the optimum or desired level.

Each crystal of an identically ground batch may retain a certain degree of individual characteristics, or respond to plating with certain individual differences. However, for practical purposes, all of the crystals in a given batch will follow the same general pattern during plating. Once an optimum level of plating is established for typical crystals of an identical batch the same level can probably be used for the others as well.

In a typical embodiment of this invention, a crystal ground to a 30Ml-lz fundamental frequency with an intended use at its 150MHz (5th) harmonic would have a family of inharmonics, extending about 300kHz from the 5th harmonic, and having an apparent strength, only -15db below that of the 5th harmonic. When plated with an aluminum electrode of 1.5mm diameter to a thickness whereat the frequency of the 5th harmonic is reduced SOkHz, the apparent strength of the inharmonics are reduced to a level at least 35db below that of the 5th harmonic.

I wish it to be understood that I do not desire to be limited to the exact details of procedure shown and described, for obvious modifications will occur to a person skilled in the art.

What is claimed is:

l. A device for plating the surfaces of a crystal while monitoring the electrical characteristics of said crystal to improve the ratio between the impedance of said crystal at a given harmonic mode of oscillation and the impedances of said crystal at the adjacentinharmonic modes of oscillation within a given frequency range comprising;

a plating apparatus and an electrical apparatus;

said plating apparatus comprising a vacuum-tight enclosure;

means for evacuating said enclosure;

means for'jmounting said crystal in said enclosure;

a first and second filaments positioned on opposite sides of said crystal, said filaments being charged with a plating material;

said electrical apparatus comprising a constantvoltage source of signals that covers said given frequency range;

a vacuum-tight electrical connection through said enclosure;

means for connecting said source of signals through said vacuum-tight electrical connection to said crystal;

means for measuring the voltage of the signal across said crystal over said given frequency range;

a source of electrical power;

means for connecting said source of electrical power to said first and second filaments to start said plating;

means for disconnecting said source of electrical power from said first and second filaments when said characteristics of said crystal achieve a given desired condition; means for determining the ratio between the voltage of said signal across said crystal at the frequency of said given harmonic mode, and the voltage of said signal across said crystal at the frequencies of said adjacent, inharmonic modes, and said means for disconnecting said source of electrical power from said first and second filaments operative when said ratio between the voltage of said signal across said crystal at the frequency of said given harmonic mode and the voltages of said signals across said crystal at the frequencies of said adjacent inharmonic modes reaches a given value;

said source of signals comprising a sweep-frequency signal generator covering said given frequency range;

said means for connecting said source of signals through said vacuum-tight electrical connection to said crystal including an attenuator and a hybrid bridge;

said attenuator being connected between said sweepfrequency signal generator and said hybrid bridge, and said crystal being connected in one arm of said hybrid bridge; and

said means for measuring the voltage of the signal across said crystal including an oscilloscope having a calibrated display tube, means for connecting the output of said crystal to one of the inputs of said oscilloscope and means for applying a sweep voltage, synchronized with said sweep frequency signal generator, to the other input of said oscilloscope to indicate the voltage across said crystal at any frequency.

2. A device for plating the surfaces of a crystal while monitoring the electrical characteristics of said crystal as in claim 1 wherein said vacuum-tight enclosure is a bell jar and said means for evacuating said enclosure includes a vacuum pump.

3. A device for plating the surfaces of a crystal while monitoring the electrical characteristics of said crystal as in claim 1 having a frequency counter connected to the output of said sweep frequency signal generator to determine the frequency of the output of said sweep frequency signal generator.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US1649828 *Sep 30, 1925Nov 22, 1927Wired Radio IncMethod of preparing piezo-electric plates
US2470737 *Aug 29, 1944May 17, 1949Premier Crystal Lab IncMethod and means of finishing piezoelectric crystals
US2621624 *Jul 19, 1948Dec 16, 1952Constantin ChilowskyApparatus for manufacture of piezo-electric elements
US2906235 *Mar 22, 1957Sep 29, 1959Bulova Res And Dev Lab IncFrequency adjustment plating control
US3017525 *Nov 26, 1956Jan 16, 1962Wolfskill John MMounting support for piezoelectric crystal units
US3059611 *Jul 5, 1960Oct 23, 1962IbmMonitoring apparatus
US3382842 *Oct 14, 1964May 14, 1968Edwards High Vacuum Int LtdApparatus for controlling vapour deposition in a vacuum
US3383238 *May 27, 1965May 14, 1968Eugene Unzicker ArlynMethod and apparatus of controlling thin film deposition in a vacuum
US3593125 *Jul 16, 1969Jul 13, 1971Shover Harry TCrystal testing apparatus for use with an oscilloscope
US3600675 *Dec 30, 1969Aug 17, 1971Western Electric CoMethod and system for adjusting electrical components using alternately applied signals
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US4121537 *Mar 21, 1977Oct 24, 1978Hitachi, Ltd.Apparatus for vacuum deposition
US7680614 *Oct 27, 2004Mar 16, 2010Sartorius AgMethod for determining the humidity and density of a dielectric material
US20080234958 *Oct 27, 2004Sep 25, 2008Klaus KupferMethod for Determining the Humidity and Density of a Dielectric Material
Classifications
U.S. Classification118/668
International ClassificationC23C14/54
Cooperative ClassificationC23C14/546
European ClassificationC23C14/54D6B