U.S. patent number 3,599,123 [Application Number 04/860,821] was granted by the patent office on 1971-08-10 for high temperature ultrasonic device.
This patent grant is currently assigned to Bell Telephone Laboratories, Incorporated. Invention is credited to John T. Krause.
United States Patent |
3,599,123 |
Krause |
August 10, 1971 |
HIGH TEMPERATURE ULTRASONIC DEVICE
Abstract
Certain gold alloys containing from about 12 to about 67 percent
indium when used in ultrasonic bonds have been found to permit the
operation of fused silica delay lines at temperatures of from 200
to 550.degree. C. resulting in at least a 50 percent increase in
frequency of acoustic waves transmitted for a given loss level as
compared with room temperature operation.
Inventors: |
Krause; John T. (New
Providence, NJ) |
Assignee: |
Bell Telephone Laboratories,
Incorporated (Murray Hill, NJ)
|
Family
ID: |
25334104 |
Appl.
No.: |
04/860,821 |
Filed: |
September 24, 1969 |
Current U.S.
Class: |
333/147;
228/110.1; 228/122.1 |
Current CPC
Class: |
H03H
9/125 (20130101) |
Current International
Class: |
H03H
9/125 (20060101); H03h 007/38 () |
Field of
Search: |
;29/155.5,195,473.1,492,504 ;117/217 ;287/189.36 ;333/30,3R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lieberman; Eli
Claims
What I claim is:
1. A device for transmitting ultrasonic waves at elevated
temperatures, comprising a fused silica transmission medium having
at least one polished face, at least one electromechanical
transducer, a bonding medium for attaching said transducer to said
face, said bonding medium comprising an alloy of indium and gold, a
metal sublayer between the polished face of the transmission medium
and the bonding medium, and electrical input and output means
attached to said transducer, characterized in that said alloy
contains from about 12 to 22 atomic percent indium and further
characterized in that said transducers have fundamental resonances
between 70 and 1045 megahertz.
2. The device of claim 1 in which said metal sublayer is selected
from group consisting of chromium and nichrome.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to high temperature ultrasonic devices for
high frequency-low loss operation.
2. Prior Art
Recent developments in ultrasonics combined with recent
developments in integrated circuitry have greatly enhanced the
competitive position of ultrasonic devices in a number of fields of
application. For example, it has been recently shown that the use
of high frequency integrated logic circuitry with certain
ultrasonic delay lines permits the manufacture of digital delay
line memories at a cost per bit that is competitive with integrated
circuit memories.
In a delay line memory, the bits to be stored are inserted into the
delay line in the form of acoustic pulses, which appear after some
characteristic delay time at the output of the delay line. Where
storage times longer than the delay time are desired, the pulses
may be reinserted as often as necessary. An existing memory of this
type uses several ultrasonic delay lines, each having a delay time
of 707 microseconds and operated at a frequency or "bit rate" of 40
megahertz, thus storing 28,280 bits of information per line. Such a
memory typically has a total or insertion loss of about 40
decibels.
It would, of course, be highly desirable to be able to increase the
frequency of operation of these memories not only to increase the
bit rate or storage capacity, but also to reduce their size and
cost. Such a goal has been hindered by the fact that in general the
acoustic loss of the transmission medium increases with increasing
frequency. For example, although fused silica exhibits very low
acoustic loss at frequencies of a few megahertz (at 1.6 megahertz,
for example, the loss is equivalent to a Q of about 10.sup.6), as
frequency is increased, the loss also increases by an amount
approximately proportional to the square of the frequency. See
Applied Physics Letters, Vol. 11, No. 10, page 308 (1967).
It is known that loss decreases with increasing temperature for
certain ultrasonic transmission media, and that this decrease in
loss with temperature is marked for fused silica within the range
of about 200.degree. C. to 550.degree. C. It might be expected
therefor that some advantage could be gained by operation of a
delay line at these elevated temperatures. Unfortunately, few
materials are available to maintain an adequate bond between the
ultrasonic material and the input and output transducers at these
temperatures. For example, epoxy bonds fail at temperatures above
about 125.degree. C. Indium bonds melt at 156.degree. C.
In addition to thermal stability, a good ultrasonic bond must
exhibit adequate mechanical strength and chemical stability, and
good adherence both to the transducer and to the transmission
medium. Sodium silicate bonds, while adhering well and being
thermally stable at the required temperatures, nevertheless are
mechanically fragile and susceptible to moisture attack.
There appear to be few reliable bonds which would permit reduction
of losses by operation of ultrasonic devices at elevated
temperatures.
SUMMARY OF THE INVENTION
According to the invention certain alloys containing indium have
been found to exhibit excellent ultrasonic bonding characteristics
up to temperatures as high as 647.degree. C. thus enabling reliable
operation of ultrasonic devices at elevated temperatures. According
to a preferred embodiment, a fused silica delay line is operated at
temperatures of from 200.degree. to 550.degree. C., enabling at
least a 50 percent increase in frequency of operation (and a
corresponding increase in the bit rate for delay line memories) for
a given loss level, as compared with room temperature
operation.
For example, where insertion losses of up to 40 decibels are
tolerable, operation of a thousand microsecond delay line may be
increased from about 70 to about 130 megahertz. Where much shorter
delay lines are desired, as for example, in rapid access memories
in which delay times of about 10 microseconds are attractive,
frequencies of up to 1045 megahertz are possible.
The alloy bonds of the invention include gold alloys containing
from about 12 to about 67 percent indium. Such alloys are not only
thermally stable at the required temperatures, but also exhibit
excellent adherence to the transducers and delay lines, good
electrical conductivity and mechanical and chemical stability.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic diagram of an ultrasonic delay line in
accordance with the invention; and
FIG. 2 is a graph of shear attenuation in decibels per microsecond
versus frequency in megahertz both plotted on a log scale showing
loss versus frequency of a typical fused silica material at
25.degree. C. and 250.degree. C., respectively.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to FIG. 1 of the drawing, there is shown one
embodiment of an ultrasonic device in accordance with the
invention, including ultrasonic transmission medium 10,
piezoelectric crystal transducers 11a and 11b attached to opposite
parallel faces of medium 10 by means of bonding layers 12a 12b,
which layers also act as electrodes. Additional electrodes 16a and
16b are attached to the external faces of transducers 11a and 11b.
Electrical input to and output from transducers 11a and 11b is
through leads 13 and 14, each attached to an electrode pair and
connected to appropriate circuitry, not shown. The device is
enclosed by schematically depicted heating means 15.
In accordance with a preferred embodiment of the invention, delay
medium 10 is fused silica. Although depicted as having a
rectangular shape, it may be of any shape, such as cylindrical or
polygonal, and size to give delay times consistent with the
intended device application. Typical delay times are from 10 to
1000 microseconds. At least one face of the delay medium should be
smooth and preferably polished so that the input and output
transducer may be affixed thereto in such a manner as to minimize
distortion of the acoustic pulses.
The transducers 11a and 11b may be any of a number of piezoelectric
crystals known to be useful for converting electric energy to
acoustic energy, such as sodium potassium niobate and quartz. As is
known by those skilled in the art, these transducers are cut to
give a preferred crystallographic orientation, shape, and size
consistent with the particular device requirements envisioned. It
is essential for minimal distortion that the transducer faces which
are to be bonded to the delay medium are smooth and flat, and
preferably polished. The thickness of the transducers in the
direction normal to the faces to be bonded must be such that the
device will operate at the high frequencies intended, that is, the
thickness must be such that the fundamental resonance of the
transducers is between 70 and 1045 megahertz. This thickness may
easily be calculated by use of the following relationship,
t=.lambda./2 in which t is the thickness of the transducer and
.lambda. is the wavelength of the stationary wave that is excited
in the transducer.
The transducers are affixed to the delay medium by a bonding layer.
The composition of this bonding layer is critical for the obtaining
of reliable high temperature operation of the device and includes
gold alloys containing critical amounts of indium. It is essential
for the efficient transmission of acoustic waves, and in particular
acoustic shear waves (the dominant high frequency transmission mode
for ultrasonic delay lines) that the bond remain substantially
solid at the operating temperature. In accordance with this
requirement, the gold alloy may contain from about 12 up to 67
atomic percent indium (corresponding to a gold to indium volume
ratio of from 5 to 0.3) and still remain completely solid at
temperatures up to about 450.degree. C. For higher temperature
operation an indium content of from 12 up to 22 atomic percent (a
gold to indium volume ratio of from 5 to 2.5) results in an alloy
which is solid at temperatures up to 647.degree. C.
The bond may be formed by a suitable method known in the art and
may be formed either before the alloy itself is formed or as the
alloy is being formed. For example, suitable layers of gold and
indium may be vapor deposited to the respective transducer and
delay medium faces and a bond formed by cold compression.
Subsequent heating of the bond to above the melting point of
indium, 156.degree. C., would allow alloy formation by dissolution
of the solid gold in the liquid indium. Such formation could
advantageously be carried out during heating of the device to its
operating temperature, thus avoiding repeated cycling between room
temperature and elevated temperatures, and consequently avoiding
stresses due to possible thermal expansion mismatches between the
transducers and the delay medium.
Poor adherence of vapor deposited metal layers such as gold to
vitreous surfaces may be improved by predepositing a thin layer
(typically a few hundred angstroms) of a metal such as chromium or
nichrome.
The relative thicknesses of the layers should be such that the
required ratio of indium to the higher melting metal of the alloy
is obtained. Typical conditions for the achieving of an
acoustically reliable bond include the optical polishing of the
surfaces to be bonded to within a half wavelength of light (sodium)
and depositing successive metal layers such that their total
thickness is about 3000 to 5000 angstroms. The fact that the end
members of the alloys of the invention generally exhibit good
conductivity permits the use of the bond as an electrode to which
electrical leads can be attached.
The metal or metals chosen for the opposing electrode on the
external faces of the transducers are not critical, but must in
general exhibit suitable conductance, good adherence to the
transducers, and stability at the operating temperature. Such
metals include a chrome-gold alloy or aluminum.
It is a principal advantage of the invention that an ultrasonic
device having fused silica as the delay medium when operated at
elevated temperatures exhibits lower losses than would be the case
for room temperature operation. This is due to the fact that fused
silica exhibits an acoustic loss minimum at a temperature between
about 225.degree. C. and 500.degree. C., the exact temperature of
this minimum being dependent upon the method of production of the
fused silica and the impurities present therein. While such loss
minimum may easily be determined using the high temperature
ultrasonic bonds of the invention, it has been observed that in
general impurities usually present in commercially available fused
silicas, such as hydroxyl and chloride ions, tend to shift the loss
minimum to lower temperatures. For example, it has been observed
that a fused silica containing 900 parts per million of hydroxyl
ions exhibited a loss minimum at about 225.degree. C., while
another fused silica containing about 5 parts per million of
hydroxyl ions exhibited a loss minimum at about 500.degree. C.
Operation of the acoustic device may be at temperatures of about
50.degree. C. above or below the loss minimum, beyond which a
minimal advantage will result due to the rapid rate of increase in
loss above or below the minimum point.
Referring to FIG. 2, there is shown a graph of acoustic loss for a
shear wave (shear attenuation) per unit of delay time expressed as
decibels per microsecond versus operating frequency in megahertz,
both on the log scale, for a device operated at 25.degree. C. and
250.degree. C., respectively. The temperature of 250.degree. C.
represents the temperature at which the loss minimum occurs for the
fused silica upon which this data is based. Assuming that the upper
level of total tolerable loss is 40 decibels, the maximum frequency
of operation at either temperature may be found for a delay line
having any given delay time by dividing the delay time into 40
decibels, and drawing a line corresponding to the resultant decibel
per microsecond value through lines A and B. Choosing, by way of
example, a thousand microsecond delay line and a 10 microsecond
delay line corresponding to lines C and D respectively, it is seen
that increasing the temperature of operation from 25.degree. C. to
250.degree. C. for the first case results in a frequency increase
of from about 70 to about 130 megahertz, and in the second case
from about 760 to 1045 megahertz, each a substantial increase in
the frequency.
* * * * *