Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2001-04-26
2004-06-15
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492210, C250S492300
Reexamination Certificate
active
06750460
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to adjusting the properties of devices such as surface acoustic wave (SAW) devices, and, more particularly to a system and method for adjusting the frequency of SAW devices by processing the surfaces of the devices by gas cluster ion beam (GCIB) irradiation.
SAW devices are used in a variety of applications, such as resonators for frequency generation oscillators, delay lines, pressure transducers, or as filters. Setting the frequency of SAW devices precisely for a specific resonator or filter application can be a difficult task, especially if numerous devices are required to be set to a specific frequency within a tight tolerance ~100 PPM or less.
Generally, a SAW device comprises a pair of transducers, but sometimes more, with each transducer having a set of conductive members which is disposed on or recessed within an upper portion of a surface which supports surface acoustic wave propagation. As SAW devices find new applications, the requirements for precision in the frequency characteristics of the surface acoustic wave device increase. Accordingly, in many applications, it is now desired to have the center frequency of the device within ±1 ppm of the design frequency. Many factors contribute to deviations from the design center frequency of a SAW device including the fabrication techniques presently used to manufacture SAW devices. Typically, with present techniques, the after fabricated SAW device has an actual center frequency within about ±100 ppm of the design frequency. Accordingly, the frequency characteristic of the fabricated devices must be modified either upwards or downwards in frequency to meet the design frequency. Typical SAW devices for commercial cellular telephone applications are made up of a set of interdigitated transducers (IDTs) deposited onto quartz substrates using conventional photolithography processes. The IDTs have been formed from a variety of metals including pure aluminum, copper-doped aluminum, titanium-doped aluminum, tantalum, or other metal or combination of metals. The variation in the photolithography definition from substrate to substrate, as well as from device to device on the same substrate can cause large frequency variations in a batch of SAW devices. Several techniques have been used to trim or adjust the frequency of these devices including reactive ion etching (RIE) to reduce the SAW device frequency and ion beam milling. For very tight tolerance, it is difficult to use RIE, and ion beam milling can cause significant damage to the single crystal quartz substrate leaving the device unusable for its intended application.
More specifically, SAW devices are typically formed on quartz single crystal substrates varying in size from 2 to 5 inches in diameter. A photomask is used to print patterns of multiple devices on each substrate. Several substrates are then put into a deposition system to deposit the IDTs. Typically the deposition will be carried out by electron beam evaporation. Variations in the evaporation process from run to run, or within a single run can yield variations in film thickness from substrate to substrate, or from batch to batch. As a result of the variations in the photomask, photolithography process, deposition process, and other processing variables, the frequencies of the resulting SAW devices can vary significantly, making the devices unusable for the intended application without some method of post-fabrication frequency modifying operation.
Several techniques are commonly employed in the art to change the frequency characteristics of a SAW device. One technique known as air-baking involves exposing the SAW device to air disposed at an elevated temperature for a limited period of time to produce an upshift in the center frequency of the device. The utility of air-baking is relatively limited, however, since air-baking has not proven to be a reproducible technique, and furthermore, the amount of frequency shift obtained during the air-baking process is extremely limited particularly at frequencies below 500 MHz.
A second method involves using etching techniques such as RIE. The reactive ion etching techniques involve sophisticated equipment, in which the SAW device is exposed to fluorine ions produced by an r.f. discharge. The fluorine ions selectively etch the surface wave propagation surface. The result of reactive ion etching is to trim down the center frequency of the SAW device. With reactive ion etching, frequency adjustment as much as −500 ppm may be obtained. Reactive ion etching, however, involves the use of relatively expensive and sophisticated equipment and, furthermore, the technique may involve relatively long etching times for devices in which a large frequency adjustment is necessary. Additionally RIE is extremely difficult to use for very tight tolerances. In addition, modifying the frequency has been performed by ion beam milling to remove metal from interdigitated transducers (IDTs) to increase the frequency. However, this technique can cause significant damage to the single-crystal quartz substrate, leaving the device unusable for its intended purpose.
Another technique known in the art is set forth in U.S. Pat. No. 4,243,960 by White et al. and in papers entitled “Fine Tuning of Narrow-Band SAW Devices using Dielectric Overlays”, 1977 Ultrasonic Symposium Proceedings, IEEE, pgs. 659-663 by Helmick et al. and “Observation of Aging and Temperature Effects on Dielectric-Coated SAW Devices ”, 1978
Ultrasonics Symposium Proceedings
, IEEE, pp. 580-585 by Helmick et al. This patent and these papers describe a technique in which a dielectric coating is provided on the surface wave propagation surface and in contact with the electrodes forming the interdigitated transducers, with the amount of frequency shift selected by controlling the thickness of the deposited coating. While the described technique produces frequency variations, these frequency variations come at the expense of a relatively large increase in the insertion loss of the device generally on the order of 1 db to 2 db, as well as, a relatively large increase in the so-called “turnover temperature” of the piezoelectric material which supports the surface acoustic wave propagation.
Some materials that are commonly employed to support surface wave propagation, such as ST-cut and rotated ST-cuts of quartz, exhibit a parabolic surface wave velocity variation as a function of temperature. The maximum of this parabolic variation is referred to as the “turnover temperature”. In many applications, the SAW device is designed to operate close to this temperature, particularly when the frequency stability of the SAW device is of critical importance. Large unpredictable variations in the turnover temperature place the device out of specification for such applications, since the cut of the substrate material is specified for its particular temperature dependent characteristic. Accordingly, the large shifts in the turnover temperature described in the above references make this technique impractical for use in many SAW device applications.
One form of SAW device includes a substrate having a surface for acoustic wave propagation at a predetermined surface acoustic wave velocity characteristic. There are a pair of IDTs coupled to the acoustic wave propagation surface. The two interdigitated transducers are on the substrate surface, and are spaced apart by a region of the acoustic wave propagation surface. A prior art method of adjusting the center frequency of such a SAW device is to adjust the surface wave velocity characteristic of the surface wave device by depositing a thin layer of a nonconducting elastic material, such as aluminum oxide or zinc sulfide onto a portion of the region separating the pair of interdigitated transducers to change the surface wave velocity characteristic of the surface wave device. This method is described in U.S. Pat. No. 4,757,283. A drawback of this technique is that the added (deposited) material has a tendency to undesirably cha
Cohen Jerry
Epion Corporation
Erlich Jacob N.
Nguyen Kiet T.
Perkins Smith & Cohen LLP
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