Metal working – Piezoelectric device making
Reexamination Certificate
1999-04-02
2001-02-06
Hall, Carl E. (Department: 3729)
Metal working
Piezoelectric device making
C029S840000, C310S31300R, C310S348000
Reexamination Certificate
active
06182342
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to acoustic wave devices, such as SAW devices, which are useful e.g. as filters and oscillators, and is more particularly directed to a technique of packaging these devices which is both economical and rugged.
The invention is more specifically with a packaged device which is highly ruggedized to withstand high-G accelerations without failure. The invention is also concerned with a process for packaging these devices in which a plurality of devices are manufactured and packaged as a single wafer, and are not singulated until after packaging, thus avoiding much of the reasons for high costs in fabrication.
The invention particularly concerns an encapsulation technology for ultrasonic devices for electronic signal processing, whose primary mode of wave propagation is surface-related, as in SAW and STW, there by providing improved performance during mechanical vibration and shock, and also allowing an outer enclosure to be formed in place wet, as in polymer molding.
SAW (Surface Acoustic Wave) devices, STW (Surface Transverse Wave) devices, or similar acoustic wave devices are commonly used in various applications, such as in the spectral filtering of electronic signals. The most common type of these are the SAW devices. In the SAW device, any of various techniques are used to launch a Rayleigh wave, and subsequently receive it after it has traveled along a predetermined path along a prepared region of a substrate material, such as quartz crystal, lithium niobate, or lithium tantalate. Usually, there are metal electrodes connected to some metallized termination, using wire bonds. The surface along which the acoustic wave travels is quite sensitive, and this sensitivity becomes extreme as the frequency of operation increases.
The surface also has to satisfy specific boundary conditions for the ultrasonic wave to propagate correctly, and this includes such practical considerations as cleanliness. A Rayleigh wave has an elliptical particle motion as it travels along the surface and is accompanied by an electric field above the surface that penetrates about one wavelength in the direction normal to the surface. Material on the surface attenuates the wave, and this is especially the case for viscous materials or materials having a significant dielectric constant. The optimum environment for this sensitive surface would be a vacuum, but a head space filled with dry neutral gas, e.g., nitrogen, is often employed. The purpose of the head space is to ensure that essentially nothing touches the active surface. This has restricted prior packaging techniques. Also, as the die and the bonding wires are not embedded in any sort of solid material, large or sustained mechanical vibrations or shock can degrade or destroy the SAW device.
Currently, SAW devices are placed in pre-made packages of ceramic or metal. This means that the devices themselves are singulated prior to that time and have to be inserted into their individual packages. A spot of mounting adhesive on the underside holds the SAW in place. In any practical application, a head space of air or nitrogen is required above the upper surface. This head space is required, as aforesaid, to ensure that essentially nothing touches the active surface so that the acoustic waves can travel on that surface. The need for head space limits the ruggedness of the packaging, and limits the mechanical severity of the environment in which the devices may be used. After the head space is filled with gas, an enclosure lid is attached, and is hermetically sealed to the rest of the package.
These packages are typically pre-fabricated by an outside supplier. Because each of the dies has to be installed individually into its own package, the step of packaging can be quite high. Typically, the cost of the finished SAW device is limited by the cost of the package. Also, the degree to which the size of the device can be reduced is also limited by the requirement for head space.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide a package and packaging technique that avoid the drawbacks of the prior art.
It is another object to provide a packaged SAW or other acoustic device in which there is substantially continuous contact with the sensitive active surface, but which interferes only minimally with the device's acoustic properties.
It is yet another object to provide a packaging with a dielectric constant near unity, i.e., vacuum, to achieve a minimal acoustic loading, and at the same time to achieve structural rigidity, and also to allow for secondary coverings or coatings for molding or sealing the device.
It is a further object to provide a ruggedized, packaged, acoustic wave device that functions over a wide frequency range, i.e., from below 1 MHz to substantially above 3 GHz.
It is still another object of this invention to achieve this improved packaging at a reduced manufacturing cost.
In accordance with an aspect of the present invention, an acoustic wave device is combined with a package therefor. A planar acoustic wave device, e.g., a SAW device, is formed of a die having an active area on its upper surface that is capable of supporting acoustic waves. A back surface of the acoustic wave device is bonded to a supporting substrate, which may be a ceramic backing or may be a pre-fabricated package. A bi-layer encapsulation system employs controlled particles that may contact the active surface of the SAW device, and another layer to lock the first layer in place. The particles are selected based on such characteristics as surface energy, Zeta potential, shape, dielectric constant, and conductivity. To that end, in several preferred embodiments, glass microspheres are employed. A layer of glass microspheres may be disposed upon the upper surface of the device to cover the acoustically active area. Then, an epoxy or other suitable potting material covers the glass microspheres and bonds to the acoustic wave device and to the supporting substrate. The glass spheres may be solid glass but preferably these are balloon-like microspheres, of a nominal 50 micron size, and are filled with nitrogen or another gas. The size of the microspheres is related to the frequency of operation of the device. A second layer of glass microspheres may be applied over the first layer of microspheres and around the edges of the first layer. This layer may contain an appropriate B-stage epoxy, or may consist of a commercially available material known as syntactic foam. The bonding wires may be embedded in the material to protect them from vibrations and shock. This technique can be carried out in a preformed enclosure, or a subsequent layer of material may be molded around it to achieve a molded package.
In the interest of economy of manufacture, the SAW product may be handled robotically in wafer form up until the packaging itself is completed. This process of manufacturing packaged acoustic wave devices first involves patterning a first wafer with a plurality of active acoustic wave devices at respective locations on an upper surface of the first wafer. Currently, a manufacturing lithographic mask can be made with 100 patterns, and can potentially yield 100 devices on the wafer using a lithographic process. Dots of a die-mounting adhesive are placed on a second, ceramic wafer, and the first wafer is laminated onto the ceramic wafer. The dots of bonding material contact the lower surface of the first wafer at the respective locations of the acoustic devices. The top wafer only is cut, so that streets and alleys are removed from the top wafer between the acoustic wave devices. This leaves the ceramic wafer intact. At this stage, the devices are in the form of dies adhered to the ceramic wafer with regions of the ceramic wafer exposed between the devices. Conductive areas, i.e., traces, are formed on the exposed areas of the ceramic substrate. These conductive traces may be present prior to the bonding of the top substrate to it. Then devices are wire bonded to the conductiv
Andersen Laboratories, Inc.
Hall Carl E.
Molldrem, Jr. Bernhard P.
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