U.S. patent number 3,873,944 [Application Number 05/337,059] was granted by the patent office on 1975-03-25 for bonding of ferrite to metal for high-power microwave applications.
This patent grant is currently assigned to Varian Associates. Invention is credited to Dennis R. Nichols, Victor A. Vaguine.
United States Patent |
3,873,944 |
Vaguine , et al. |
March 25, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Bonding of ferrite to metal for high-power microwave
applications
Abstract
A ferrite-to-metal bond suitable for the environment of a
high-power microwave circulator is disclosed. The bonding surface
of a gyromagnetic ferrite or garnet button is metallized by a
sputtering process that deposits successive layers of nichrome,
copper and gold thereon. During the sputtering process, a flexible
stainless steel band surrounds the button to prevent sputtered
material from being deposited on other than the bonding surface of
the button. The metallized bonding surface is then soldered to a
metal wall of the circulator. The bond so formed is capable of
withstanding a peak power level in the circulator of 2.0 megawatts
and an average power level of 3.5 kilowatts under standing-wave
conditions.
Inventors: |
Vaguine; Victor A. (Palo Alto,
CA), Nichols; Dennis R. (San Jose, CA) |
Assignee: |
Varian Associates (Palo Alto,
CA)
|
Family
ID: |
23318942 |
Appl.
No.: |
05/337,059 |
Filed: |
March 1, 1973 |
Current U.S.
Class: |
333/1.1;
228/124.1; 204/192.2 |
Current CPC
Class: |
H01P
1/383 (20130101); C04B 37/026 (20130101); C04B
2237/72 (20130101); C04B 2237/40 (20130101); C04B
2237/122 (20130101); C04B 2237/34 (20130101); C04B
2237/125 (20130101); C04B 2237/124 (20130101); C04B
2237/12 (20130101); C04B 2237/86 (20130101); C04B
2237/708 (20130101) |
Current International
Class: |
C04B
37/02 (20060101); H01P 1/383 (20060101); H01P
1/32 (20060101); H01p 001/32 () |
Field of
Search: |
;333/1.1,24G,24.1,24.2
;29/473.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gensler; Paul L.
Attorney, Agent or Firm: Cole; Stanley Z. Morrissey; John
J.
Claims
1. A microwave circulator comprising a metal wall, a gyromagnetic
ferrite button, and a bond affixing said button to said wall, said
wall comprising at least three metal layers, each of said layers
comprising a different metal from the others of said layers.
2. The microwave circulator of claim 1 wherein said button is made
of garnet.
3. The microwave circulator of claim 1 wherein one of said metal
layers comprises a first layer in contact with said button, said
first layer comprising a metal selected from the group consisting
of nichrome, molybdenum and chromium.
4. The microwave circulator of claim 3 wherein another of said
metal layers comprises a second layer in contact with said first
layer, said second layer comprising copper.
5. The microwave circulator of claim 4 wherein another of said
metal layers comprises a third layer in contact with said second
layer, said third layer comprising gold.
6. The microwave circulator of claim 5 further comprising a layer
of solder in contact with both said third layer and said metal
wall.
7. The microwave circulator of claim 6 wherein said solder
comprises indium.
8. The microwave circulator of claim 6 wherein said solder
comprises a mixture of lead and tin.
9. The microwave circulator of claim 4 wherein another of said
metal layers comprises a final layer comprising a mixture of solder
and gold, said final layer being in contact with both said second
layer and said metal wall.
10. The microwave circulator of claim 9 wherein said solder
comprises indium.
11. The microwave circulator of claim 9 wherein said solder
comprises a mixture of lead and tin.
12. In a microwave transmission system, a circulator comprising a
metal wall, a gyromagnetic ferrite button, a bond affixing said
button to said wall, said bond comprising at least three metal
layers, each of said layers comprising a different metal from the
others of said layers, and means for applying a magnetic field to
said button.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is a further development in the high-power microwave
art, and in particular provides a ferrite-to-metal bond that will
tolerate standing-wave conditions in a microwave circulator for an
extended length of time.
2. Description of the Prior Art
One method for achieving non-reciprocal transmission of power in a
microwave system is by means of a circulator having a pair of
symmetrically disposed gyromagnetic ferrite or garnet buttons
mounted therein to concentrate the flux lines of an externally
produced magnetic field. Interaction of the microwave with the
externally magnetized gyromagnetic ferrite or garnet buttons will
cause transmission of the microwave power only in a particular
direction. Prior to the present invention, such ferrite or garnet
buttons had been bonded to opposing metal walls of a circulator by
a dielectric bonding material such as an epoxy or a mixture of
epoxies. It has been found, however, that for high-power levels
that are frequently encountered in microwave systems, dielectric
bonding material is likely to melt, evaporate, bubble or boil out
thereby causing the bond to fail. Until the present invention, a
metallizing technique had not been developed which could provide a
bond capable of withdstanding high-power levels such as would occur
under standing wave conditions caused by a frequency mismatch
between, for example, a resonant linear accelerator load and a
magnetron power source.
SUMMARY OF THE INVENTION
This invention provides a technique for metallizing a gyromagnetic
ferrite or garnet button so that the button can be soldered to a
wall of a microwave circulator, and thereupon function in a
high-power environment within the circulator for an extended length
of time without being fractured and without suffering diminution in
its capacity to effect non-reciprocal transmission of microwave
power. A sputtering process is utilized to metallize the button by
depositing an adherent layer of nichrome onto the bonding surface
of the button, and thereafter a layer of copper onto the nichrome
layer, and finally a layer of gold onto the copper layer. During
the sputtering process, a flexible stainless steel band surrounds
the ferrite button to prevent sputtered material from being
deposited on any portion of the button other than its bonding
surface. Flexibility of the band is achieved by providing a scallop
in the band.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 shows in schematic form a microwave system comprising a
three-port circulator having a pair of gyromagnetic ferrite buttons
disposed to achieve non-reciprocal transmission of power.
FIG. 2 shows in plan view a stainless steel band as used in the
sputtering process of this invention.
FIG. 3 shows the band of FIG. 4 surrounding a ferrite button, and
indicates appropriate dimensions.
FIG. 4 shows a cross-sectional view of a ferrite button metallized
according to the technique of this invention.
FIG. 5 shows a cross-sectional view of a ferrite button bonded to a
metal wall according to the technique of this invention.
FIG. 6 shows a cross-sectional view of a ferrite button bonded to a
metal wall by an alternative bond to the bond shown in FIG. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In many microwave systems, power reflected from the working load
cannot be tolerated by the power source. For example, where the
working load is a resonant apparatus such as an electron
accelerator and the power source is a magnetron, reflections of
power from the accelerator can be caused by frequency mismatch
between the accelerator and the magnetron and/or by transient
processes inherent in pulsed operation of the accelerator. Even
small power reflections from the accelerator will tend to drive the
magnetron off frequency. If the magnetron is off frequency, all
power will be reflected from the accelerator thereby causing
further detuning of the magnetron. It is therefore necessary in
such systems that the power source be isolated from the resonant
load.
Referring now to FIG. 1, a three-port circulator 1 is shown which
isolates a power source 2, which may be a magnetron, from a
resonant load 3, which may be a linear accelerator. Broken line 4
shows the direction of transmission of microwave power from the
power source to the resonant load, and dotted line 5 shows the
direction of transmission of power reflected from the resonant load
to a dummy load 6. The power source is isolated from the resonant
load because the circulator achieves non-reciprocal transmission of
power within the circulator structure. A pair of gyromagnetic
ferrite or garnet buttons, indicated by reference number 7', is
disposed within the circulator structure. The wave generated by the
power source interacts with the externally magnetized gyromagnetic
ferrite or garnet buttons in such a way that power from the power
source port of the circulator can be transmitted only in the
direction of the resonant load port, as shown by broken line 4.
Similarly, any power reflected from the resonant load can be
transmitted only in the direction of the dummy load port, as shown
by dotted line 5. The dummy load 6 is intended to absorb
substantially all of the reflected microwave energy. Techniques
exist for utilizing small reflections from the dummy load, which
the ferrite buttons cause to be transmitted only in the direction
of the power source port, in order to stabilize the frequency of
the power source at the frequency of the resonant load. For
example, see U.S. Pat. No. 3,714,592, assigned to Varian
Associates, assignee of the present invention.
For low power levels of operation, i.e., where peak power remains
below 1.7 megawatts and the average operating power level is below
2.0 kilowatts, the ferrite or garnet buttons can successfully be
located in proper position within the circulator by the dielectric
bonding technique of the prior art, which essentially involves
bonding a button 7 to a wall of the circulator with an epoxy or a
mixture of epoxies. It has been found, however, that for high power
operation, dielectric bonding material is likely to melt,
evaporate, bubble or boil out, thereby causing the bond to fail.
Where the resonant load is an apparatus designed to operate at a
sharply defined resonance frequency, even slight frequency mismatch
between the power source and the resonant load can produce
substantially total reflection of power, and can thereby cause a
standing wave condition within the circulator. The occurrence of
such standing waves must be anticipated in a microwave system that
comprises, for example, a linear accelerator. Under standing wave
conditions, the electric field strength will be double that of
travelling wave conditions. The microwave power in the circulator
under standing-wave conditions will therefore be four times higher
than under travelling wave conditions. It has been found that
dielectric bonding material cannot withstand power levels that are
attained under such standing-wave conditions.
This invention provides a metallizing technique whereby a
gyromagnetic ferrite or garnet button can be soldered to a wall of
a microwave circulator. The bond formed by soldering a ferrite or
garnet surface that has been metallized according to this invention
to a wall of a microwave circulator has been shown experimentally
to be capable of withstanding peak power levels as high as 2.0
megawatts and average power levels of 3.5 kilowatts for intervals
of time in excess of 30 minutes under standing-wave conditions.
These experimental limitations are not due to any discovered or
anticipated failure of the bond at the specified power levels, but
rather represent merely the maximum available peak power limit
(i.e., 2.0 megawatts) of the magnetron used in conducting the test
and the maximum repetition rate of the modulator used which
resulted in a maximum available average operating power of 3.5
kilowatts. It was determined that under identical experimental
conditions, a dielectric bond will fail within 2 minutes at 2.0
kilowatts average power or if peak power rises above 1.7 megawatts
under standing-wave conditions.
The metallizing approach was not an obvious solution to the
high-power bonding problem. Gyromagnetic ferrites are known to
undergo certain irreversible changes in their electromagnetic
properties at soldering temperatures, i.e., at temperatures above
175.degree.C. Consequently, it was to be anticipated that the
soldering process might cause a ferrite button to lose its property
of causing non-reciprocal transmission of power in a microwave
circulator. Furthermore, in any metallizing process it is essential
that the metal layer be permenently bonded to the bonding surface
of the button so that the metal layer cannot be removed by flaking
or otherwise in the high-temperature environment of high-power
microwave operation. Such permanent bonding can generally be
achieved only by a sputtering technique, where the particles of
metallizing material are driven into the substrate surface with an
average energy of 20 electron volts, whereupon the atoms of the
metallizing layer form a common interstitial structure with the
atoms of the substrate material. It has been found that unless a
special masking technique is used, sputtered metallizing material
will be deposited on portions of the ferrite button other than the
bonding surface. In particular, the side edge of the button is
likely to receive a relatively heavy coating of metallizing
material. Where a metallized surface of the ferrite button is
exposed to a microwave field, even where the "metallization"
consists of only a discontinuous deposition of minute amounts of
sputtered metal along the side edge of the button, the capacity of
the ferrite to function as a non-reciprocal wave guide is
significantly reduced. Furthermore, metallization of the side edge
of the ferrite button appears to promote arcing between the button
and the walls of the circulator. It is therefore essential that an
appropriate masking technique be used during the sputtering
operations.
A band of masking material covering the side edge of the ferrite
button during the sputtering operation would serve as a mask to
prevent the deposition of metal on the side edge surface. During
sputtering, however, the surface temperature of the ferrite button
will typically reach 250.degree.C, at which temperature the
coefficient of thermal expansion for ferrite is approximately 10
.times. 10.sup.-.sup.6 per degree C. A band enclosing the ferrite
button will either fracture the ferrite or be itself fractured,
unless the band expands at substantially the same rate as the
ferrite. Numerous masking materials exist which have coefficients
of thermal expansion approximating that of ferrite. However, at
sputtering temperatures, such materials will fuse with the ferrite.
Materials such as stainless steel, tungsten and tantalum will not
fuse with ferrite at sputtering temperatures, but have coefficients
of thermal expansion which vary so significantly from the
coefficient of thermal expansion for ferrite that fracturing of
either the ferrit button or the masking structure would seem
inevitable during the sputtering process.
According to the present invention, a masking technique has been
developed for metallizing the bonding surface of a ferrite button
while protecting those surface portions of the button other than
the bonding surface from the deposition of metallizing material. As
shown in FIG. 2, a band 10 of 304-stainless steel, tungsten or
tantalum, having a scallop 11 in its periphery, has been found to
provide adequate masking. The uneven rates of expansion of the
ferrite and the band material with respect to each other can be
accommodated by the scallop which allows the band to flex as the
ferrite expands at a faster rate than the band. Typical dimensions
for a ferrite button and for an appropriate stainless steel masking
band are shown in FIG. 3. A ferrite button for use in a microwave
circulator is typically in the form of a circular wafer having a
diameter of approximately 29 millimeters and a thickness of
approximately 4 millimeters. The side edge of the button is
typically faired into the surface of the button which faces the
microwave field. A typical radius of curvature for the convex
portion of the continuously faired surface of the button is 3
millimeters. It has been found that a typical ferrite button will
be adequately masked during the sputtering operation by a circular
stainless steel band having an inner diameter just large enough to
tightly accommodate the diameter of the button, an outer diameter
approximately one millimeter larger than its inner diameter, and a
scallop (as shown by reference number 11 in FIG. 2) extending
radially outward about 5 millimeters beyond the outer diameter of
the band with a separation of 2 millimeters between points on
opposite sides of the scallop opening on the inner periphery of the
band. A suitable thickness for the band is 2.5 millimeters. The
stainless steel will not fuse with the ferrite button during the
sputtering process, and will not fracture the button despite
unequal coefficients of thermal expansion because of the
springiness introduced into the band by the scallop. Tungsten of
tantalum could be used in place of stainless steel, but stainless
steel (and in particular 304-stainless) is especially preferred
because of its mechanical workability.
Having developed a suitable masking technique, the choice of the
particular metallizing material or materials to use remains
unobvious. The coefficients of thermal expansion for some materials
typically used for microwave circulator structures are as follows:
stainless steel, 16.4 .times. 10.sup. .sup.-6
(.degree.C).sup..sup.-1 ; aluminum; 23 .times. 10.sup. .sup.-6
(.degree.C).sup..sup.-1 ; and copper, 16 .times. 10.sup.-.sup. 6
(.degree.C).sup..sup.-1 ; whereas the coefficient of thermal
expansion for ferrite is only 10 .times. 10.sup.-.sup. 6
(.degree.C).sup..sup.-1. The bond between the ferrite button and
the metal wall of the circulator must therefore be able to
accommodate a relatively large difference between these
coefficients of thermal expansion, without subjecting the button to
such great mechanical stress that the button will be likely to
fracture or to suffer displacement from its proper position within
the circulator at high temperatures.
By a series of experiments, it has been found that a suitable
metallizing bond can be formed by sputtering successive layers of
nichrome, copper and gold onto the bonding surface of the ferrite
button. FIG. 4 shows a ferrite button metallized according to the
present invention. The ferrite button 7 has a layer of nichrome 20
sputtered onto its bonding surface, a layer of copper 21 sputtered
onto the nichrome layer, and a layer of gold 22 sputtered onto the
copper layer. The nichrome layer is approximately 10,000 angstroms
thick. Nichrome is chosen because it forms a particularly strong
oxide bond with ferrite or garnet. Other materials which form
strong oxide bonds with ferrite or garnet and are suitable for this
first sputtered layer include molybdenum and chromium.
Copper is chosen for the second sputtered layer because of its
excellent thermal conductivity, which is important in removing heat
from the button to the wall of the circulator during high-power
microwave operation. In addition, copper will not be dissolved in
the solder material during the soldering of the metallized button
to the wall of the circulator. The copper layer is relatively
thick, being about 30,000 angstroms.
A thin layer of gold, no more than 10,000 angstroms being
necessary, is then sputtered onto the copper layer. Gold is chosen
for the third sputtered layer because it is chemically inert. The
purpose of the gold layer is to preclude oxidation of the copper
layer. If an oxide were to form on the outer metallized layer of
the button prior to soldering, the ability of the solder to wet the
outer metallized layer would be seriously diminished and the bond
formed by the solder would consequently be weakened. An oxide layer
would also inhibit thermal conduction from the button to the wall
of the circulator. The gold layer, therefore, serves as a
protective coating on the copper layer. During soldering, the gold
layer might dissolve, either partially or totally, depending upon
the soldering temperature, into the solder material. This
dissolving of the gold into the solder, however, is not harmful to
the bond. It has been found that ordinary commercially available
solder comprising a mixture of tin and lead is a satisfactory
soldering material. A preferable soldering material would be
indium, which has a better heat transfer capability and, being a
softer material, provides better stress relief when cooling than a
tin-lead mixture.
FIG. 5 shows a wall 8 of circulator 1, with a ferrite or garnet
button 7 bonded thereto according to the technique of this
invention. An analysis of the metallizing bond between the ferrite
or garnet button and the circulator wall would reveal a first layer
20 comprising nichrome, molybdenum or chromium deposited upon the
bonding surface of the button, a second layer 21 of copper, a third
layer 22 of gold, and a fourth layer 23 comprising the solder
material. The fourth layer might comprise a mixture of tin and
lead, or it might be a layer of indium. To the extent that the gold
has dissolved into the solder material, the fourth layer will also
contain this dissolved gold. FIG. 6 shows a layer 24 sandwiched
between the copper layer 21 and the wall 8 of the circulator. Layer
24 comprises the soldering material with the gold protective layer
completely dissolved therein.
It is clear that changes could be made in particular details of the
preferred embodiment of the invention disclosed herein without
departing from the scope of the invention. Therefore, it is
intended that the above description and the accompanying drawing be
interpreted as illustrative only and not as limiting. The scope of
this patent shall be limited only by the following claims What is
claimed is:
* * * * *