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.

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.

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.