Frequency adjustment of piezoelectric resonator utilizing low energy oxygen glow device for anodizing electrodes

A monolithic crystal filter formed by a multi-section piezoelectric resonator having anodizable electrodes is subjected to plasma from a low energy oxygen glow device to adjust the frequency response. The electrodes are selectively connected to potentials whereby the electrodes are individually anodized to increase the density thereof and change the frequency response of the resonator. The glow device includes anode and cathode rings separated by an insulator ring which holds a cathode screen across the cathode. The anode ring has a concave face and the inside diameter is generally the same as the diameter of the resonator so that the resonator can be placed close to the anode and plasma is localized on the resonator electrodes. The electrodes are connected in a frequency measuring circuit so that the response is monitored and the extent of anodization of the individual electrodes is controlled.

BACKGROUND OF THE INVENTION 
Piezoelectric resonators are used for accurate frequency response and 
control in many applications, such as for monolithic filters used in radio 
apparatus. Monolithic crystal filters include a flat quartz wafer having 
an array of thin film electrodes deposited on each side thereof. The 
frequency response of such resonators depends upon the thickness of the 
quartz wafer and the thickness of the electrodes. It is not possible using 
presently known, low cost, manufacturing techniques to provide the degree 
of accuracy required in many applications. It has been proposed to modify 
the characteristics of crystal filters by the use of external devices, but 
this increases the cost and size of the apparatus and has not been 
generally satisfactory. 
For use at frequencies in the VHF range, the electrodes for the crystal 
resonators are commonly formed of aluminum. It is known that the mass of 
aluminum electrodes can be adjusted by anodically growing an oxide 
thereon. However, a satisfactory method of implementing such anodization 
has not been known. A liquid anodizing bath has been used, but this has 
the disadvantage that the resonator must be rinsed and dried before 
testing, and repeated operations may be required to provide the desired 
frequency response. It is also known that anodization can be accomplished 
by use of oxygen plasma produced by a glow discharge, but as it has not 
been possible to localize this action, high energy plasma must be used. 
This produces substantial heat so that errors in measurements tend to 
result, and damage to the crystal and electrodes may take place. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a simple and effective 
process for adjustment of a piezoelectric resonator by anodizing the 
electrodes thereof. 
Another object of this invention is to provide a process for selective 
anodization of individual electrodes of a monolithic crystal filter, to 
thereby adjust the overall frequency response of the filter. 
A further object is to provide a process for selective anodization of a 
plurality of electrodes, wherein the frequency response is monitored 
during anodization, and potentials are selectively applied to the 
electrodes to individually control the anodization thereof in response to 
the measurements obtained. 
A still further object is to provide a process utilizing a low energy 
oxygen glow device for directing plasma to a localized region to anodize 
electrodes of a piezoelectric resonator in a dry atmosphere without 
producing substantial heat. 
Still another object is to provide a process and apparatus for selectively 
anodizing electrodes of a resonator which does not contaminate or 
otherwise damage the same. 
In accordance with the invention, the frequency response of a monolithic 
crystal filter, or other multisection piezoelectric resonator, is adjusted 
by selectively anodizing the electrodes to increase their mass or 
effective thickness. A low energy oxygen glow device applies plasma in a 
localized area adjacent the electrodes, and the electrodes are selectively 
connected to potentials to control the anodization action thereon. The 
frequency response of the filter is monitored during the anodization 
operation, and the measurements obtained are used to control the extent of 
the anodization of the individual electrodes. For use with aluminum 
electrodes, the glow device can also be made of aluminum so that there 
will be no material sputtered from the device which might contaminate the 
resonator. The glow device includes small anode and cathode rings 
separated by an anodized ring insulator which holds a cathode screen 
against the cathode ring. This device can be easily and inexpensively made 
of aluminum. The anode has a concave annular end adjacent the resonator to 
direct plasma on the electrodes, so that the low energy glow device 
provides effective anodization of electrodes which are connected in a 
current path. This process produces very little heat and permits 
continuous monitoring of the frequency response of the resonator so that 
anodization of the individual electrodes can be controlled to provide the 
desired frequency response.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1 shows an oxygen glow device or gun 10 for applying oxygen plasma to 
the electrodes 14, 15 and 16 of piezoelectric resonator device 11, which 
may be a monolithic crystal filter. The electrodes 14, 15 and 16 are 
positioned on one side of wafer 12 of piezoelectric material, such as 
quartz, with cooperating electrodes 13 on the opposite side to form three 
resonating sections coupled through the wafer 12. The gun 10 includes an 
annular cathode 20 having a screen 21 across the open end. An annular 
anode 24 is at the opposite end of the gun 10, with an insulating ring 22 
interposed between the anode and cathode. The cathode 20, ring 22 and 
anode 24 may all be formed of aluminum, with the surfaces of ring 22 being 
anodized to render the same non-conductive, and certain of the surfaces of 
cathode 20 and anode 24 also being anodized, as will be explained. The 
screen 21 can be formed of a fine mesh stainless steel. 
To provide an oxygen glow at the anode 24 of the gun 10, the anode 24 is 
held at a positive potential with respect to the cathode 20, with a 
potential of the order of 350 volts being found to be suitable. The anode 
24 has a concave or tapered end surface 25 adjacent to the piezoelectric 
device 11 and acts to direct the plasma onto the electrodes 14, 15 and 16. 
It is pointed out that the piezoelectric device 11 can have any number of 
electrodes thereon, more or less than the three electrodes which have been 
illustrated. The concave end 25 of the anode 24 makes it possible to place 
the resonator device very close thereto. The inside diameter of the anode 
24 is substantially the same size as the diameter of wafer 12, so that the 
glow is localized on the electrodes 14, 15 and 16. 
The piezoelectric device 11 can be supported on any standard socket, as 
when it is actually used in electronic apparatus. This is illustrated 
schematically in FIG. 1 by an insulator 30 having sockets 32 therein for 
receiving conductors 34, 35 and 36 connected to electrode 14, 15 and 16, 
respectively, and conductors 38 which are connected to electrodes 13 which 
are opposite the electrodes 14, 15 and 16 on the wafer 12. Connections are 
provided to the sockets 32 engaged by conductors 38 to complete the 
circuits from the electrodes 13 through by-pass capacitors 39 to ground. 
The circuit from electrode 14 extends through conductor 34 and through 
resistor 40 and switch 41 to a bias potential provided by potentiometer 
42. Similarly, circuits are provided from electrodes 15 and 16 through 
resistors 46 and 47 and switches 48 and 49 to potentiometers 50 and 51. 
The potentiometers 42, 50 and 51 are all connected to a terminal 44 to 
which a potential of the order of +50 volts applied. The leads 34, 35 and 
36 from the electrodes 14, 15 and 16 are also connected through D.C. 
blocking capacitors 52 to a frequency measuring device 54. 
To provide the anodization action, the gun 10 and resonator device 11 are 
placed in a dry oxygen atmosphere at approximately 1.7 Torr. The chamber 
about the gun 10 and resonator device 11 is schematically represented by 
the dotted lines 29 together with the base 28 and insulator 30. The anode 
24 of the glow gun 10 can be grounded through post 26 connected thereto, 
and the cathode 20 can be connected to a negative potential through post 
23. The posts 23 and 26 can also form supports for the gun, as will be 
further explained. The potential applied between the anode 24 and the 
cathode 20 will produce a glow current of the order of 0.25 to 2.0 
milliamps. This produces plasma flow between the screen 21 and the face 25 
of the anode 24, so that the plasma is localized on the electrodes 14, 15 
and 16 of the resonator device 11. 
The extent of the anodization of the electrodes 14, 15 and 16 is 
selectively controlled by the selective application of potentials thereto 
through the switches 41, 48 and 49 and potentiometers 42, 50 and 51. 
Anodization depths of the order of 400 Angstroms, with a resolution of 
.+-.1 Angstrom, can be obtained, because the process is very uniform and 
stable. As no anodization takes place when an electrode is not connected 
in a return circuit, the electrodes can be anodized to different extents. 
A positive potential of the order of 50 volts is applied to terminal 44, 
and the potentiometers 42, 50 and 51 can be set to apply a portion of this 
potential, such as 30 volts (for example), to the electrodes. The 
penetration of the anodization of each electrode depends on the potential 
applied, and the length of time the electrode is connected. 
The electrodes 14, 15 and 16 of the resonator device 11 are connected to a 
frequency measuring device 54, which may be of known construction. The 
electrodes can be connected to separate measuring circuits, or connected 
in turn to a single measuring circuit. The measuring circuits are 
completed from the resonator device 11 to ground through the capacitors 39 
which are connected to the electrodes 13. As is well known, a monolithic 
crystal filter includes a plurality of sections which are acoustically 
coupled through the wafer, and the overall response of the filter depends 
upon the individual responses of the sections. By measuring the responses 
and selectively controlling the potentials applied to the electrodes, each 
of the electrodes can be anodized to the depth required to provide the 
desired response. As the anodization takes place in a dry vacuum, 
measurements are accurate, and can be taken continuously, if desired. 
Details of the structure of the oxygen glow gun 10 are shown in FIGS. 2 to 
5. FIG. 2 is a plan view of the anode 24 looking at the concave face 25, 
and FIG. 3 is a cross-section view along the lines 3--3 of FIG. 2. FIG. 2 
shows a specific form of the connecting and mounting post 26 and the 
insulating sleeve 60 coupled thereto. The sleeve 60 is formed of a plastic 
insulating material, such as Delrin, and has internal threads engaging 
threads on the post 26. A connector tip 62 within sleeve 60 has a pointed 
end 63 mating with a recess in the post 26 and held thereagainst by spring 
64. The connector tip 62 has a recess 65 for receiving a conductor 66 for 
making an electrical connection through the tip 62 and post 26 to the 
anode 24. As previously stated, the anode 24 can be formed of aluminum, 
and the inner surface 27 and face 25 thereof are conductive to form the 
active anode surfaces. The remaining surfaces of the anode can be anodized 
to render the same insulating. The glow region is confined to the interior 
of the gun 10, and directly adjacent to the concave face 25 of the anode 
24. 
FIG. 4 is a cross-section view of the insulator 22 which is positioned 
between the anode 24 and cathode 20. All the surfaces of this aluminum 
ring 22 are anodized so that it forms an insulator. The surface adjacent 
to the cathode 20 has an annular projection 31 which engages the cathode 
screen 21, as shown by FIG. 1. 
FIG. 5 shows the cathode 20 of the glow gun 10 and shows post 23 for 
supporting and connecting the same. This post may be identical to post 26 
connected to the anode 24, and an insulating sleeve identical to sleeve 60 
and a connector tip identical to connector tip 62 (FIG. 2) can be used 
therewith. The cathode 20 has a recess 19 for receiving the screen 21, 
which may be a fine mesh stainless steel screen. The projection 31 on 
insulator 22 engages screen 21 and extends into recess 19 to hold the 
screen 21 in place. The inside surface 18 of cathode 20, and the recess 19 
thereof, are conductive to provide a current path to the screen 21, and 
the remainder of the cathode 20 can be anodized. Openings 17 are provided 
in the closed end of cathode 20 for the flow of oxygen therethrough. 
Where potentials are applied to the glow gun 10 as shown in FIG. 1, a glow 
region is established from the screen 21 to the concave face 25 of the 
anode. This region may be of the order of 20 millimeters long and 20 
millimeters in diameter. The overall length of the gun 10 can be less than 
50 millimeters. This forms a small, low energy, oxygen glow device which 
localizes the oxygen plasma at the face 25 of the anode 24, so that it can 
be directed on the electrodes of the resonator device 11. The gun 10 
produces a very small amount of heat so that it does not damage the 
resonator, or impede the monitoring of thefrequency characteristics 
thereof. 
It will be apparent that the measurements obtained by the measurement 
device 54 may be used to control the individual anodization of the 
electrodes of the resonator device 11 in different known ways. For 
example, an operator can observe the measurements and control the switches 
41, 48 and 49, and the settings of potentiometers 42, 50 and 51 to produce 
the desired action. Also, a general purpose computer can be used to 
receive the measurements and activate the controls in accordance with a 
preset pattern. 
The process which has been described has been found to be highly successful 
for adjusting the frequency responds of a multi-section piezoelectric 
resonator, such as a monolithic crystal filter. As the response can be 
continuously monitored, and the anodization of the electrodes can be 
individually controlled, very accurate adjustment can be easily 
accomplished. The process utilizes low energy plasma so that there is 
substantially no danger of damage to the resonator device which is being 
adjusted.