Compositions – Light transmission modifying compositions – Inorganic crystalline solid
Reexamination Certificate
1999-11-05
2001-11-20
Tucker, Philip (Department: 1712)
Compositions
Light transmission modifying compositions
Inorganic crystalline solid
C117S003000, C423S593100, C423S598000
Reexamination Certificate
active
06319430
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to improved crystals of lithium niobate and lithium tantalate and methods of preconditioning such crystals for use in electrical applications such as surface acoustic wave filter devices.
BACKGROUND OF THE INVENTION
Lithium niobate (LN) and lithium tantalate (LT) are widely used in a variety of electronic applications including surface acoustic wave (SAW) signal processing, guided-wave optic modulation and switching, and electro-optic laser Q-switching, and modulation. The physical basis for the suitability of LN and LT crystals for these types of applications is their atomic-scale crystal structure, which results in the crystals' natural piezoelectric response useful in SAW-based devices, electro-optic response useful in integrated optic devices and pyroelectric response useful in pyroelectric detectors. Another characteristic of LN and LT that may be important in some applications is the optical absorption of the crystal. For example, integrated optic devices require a relatively small optical absorption, while other devices; such as SAW filters do not require a low optical absorption. In some instances, this natural physical response of the crystals can complicate crystal processing and adversely effect performance of devices in which the crystals are incorporated.
A crystal's pyroelectric or piezoelectric response causes the external surfaces of a fabricated crystal to become electrically charged in response to a change in temperature of the crystal or in response to a mechanical stress applied to the crystal. These electrical surface charges can spontaneously short, with associated sparking causing dramatic processing or performance failure, or even crystal fracture. One common example of such performance failure is an unacceptably high bit-error-rate of LN-based SAW filters used in digital radio applications. In order to avoid such types of failures, current production protocols for these types of filters include extensive and costly device testing designed to eliminate those filters prone to such spurious pyroelectric or piezoelectric induced failures.
The process of incorporating LN or LT crystals into electronic devices often includes steps that result in exposing the crystals to conditions that invoke an untimely and unwanted pyroelectric or piezoelectric response. In an effort to reduce the risk of problems associated with the unintended build up of surface charges, for example catastrophic discharge of these charges during manufacture, device manufacturers have had to take steps that add significantly to the cost, time, and complexity of incorporating the crystals into devices.
For LN and LT crystals manufactured by conventional methods, surface charges can eventually decay with time as they are neutralized by the movement of free charge from within the crystal itself or from the surrounding environment. This natural decay occurs after the surface charge develops and does nothing to mitigate or minimize the degree to which the surface of the crystal becomes charged as a result of the crystal's natural piezoelectric or pyroelectric response.
In view of the increasing demand for reliable LN and LT crystals for applications such as surface acoustic wave filter devices, guided wave optic modulation and switching, and electro-optic Q-switching and modulation, the need exists for LN and LT crystals that continue to exhibit properties that make them desirable for such applications and that do not suffer from the drawbacks associated with the buildup of excessive spurious pyroelectric or piezoelectric surface charges.
SUMMARY OF THE INVENTION
In accordance with the present invention, crystals of LN or LT are preconditioned to increase the ability of the crystal to reduce electrical charging of the crystal surface. Crystals of the present invention are able to reduce electrical charging by effectively reducing or dissipating the buildup of surface electric charges (caused by the natural pyroelectric or piezoelectric response of the crystal) preferably as fast as the charges are generated.
The present invention also relates to a method for preconditioning or an LN or LT crystal to increase the crystal's ability to reduce electric charging of the crystal's surface. In a preferred embodiment of the method aspect of the present invention, the ability of the crystal to reduce electric charging of the crystal's surface is increased by exposing the crystal to a combination of heat and a chemically reducing atmosphere to increase the free electron density within the crystal, followed by a controlled quenching of the resulting crystal to a temperature ranging between about 250° C. to about 100° C. After the crystal is quenched it may be cooled to room temperature under either an oxidizing or a reducing atmosphere.
The present invention provides crystals which can effectively minimize the buildup of electric charges on the crystal surface that are induced by the crystal's natural pyroelectric or piezoelectric response, particularly those surface charges that build up as a result of mechanical stress or temperature change not associated with the normal operation of the device in which the crystal is incorporated. In a preferred embodiment, the preconditioned crystal is able to reduce surface charging by neutralizing or dissipating these charges as fast as they are generated.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The LN (the compound formed from Li
2
O and Nb
2
O
5
) and LT (the compound formed from Li
2
O and Ta
2
O
5
) crystals of the present invention exhibit piezoelectric and pyroelectric responses that make the crystals suitable for applications such as surface acoustic wave (SAW) signal processing, guided-wave optic modulation and switching, and electro-optic laser Q-switching and modulation. The present invention is described below in the context of LN crystals; however, the description is believed to be equally applicable to other types of crystals such as LT.
LN and LT crystals can be grown by a number of techniques, the best known of which is the Czochralski technique. A summary of the Czochralski technique can be found in Current Topics and Material Science, Vol. 1, E. Kaldis editor, North Holland Publishing Co., 1978, ch. 7 by Dr. Armin Räuber, p. 545-48, which is herein incorporated by reference. LN crystals grown by the Czochralski technique are achieved by pulling LN from a melt. Nearly any kind of conventional crystal pulling apparatus can be used. The crucible in which the LN is heated can be platinum. There are no special demands for the atmosphere, with air in many situations being preferred.
As noted in the background of the invention, LN crystals exhibit a natural piezoelectric and pyroelectric response. As a result, as illustrated in the Comparative Example set forth below, LN crystals in the form of wafers build up surface charges when subjected to a temperature change, such as those encountered when such crystals are processed and incorporated into electrical devices, or when the devices are used. These surface charges, which can take extended periods of time (e.g., 15-20 hours) to dissipate naturally, can cause sparking or shorting, which can lead to device malfunction or failure or even crystal fracture. In contrast, as described in the Example that follows the Comparative Example, preconditioned LN wafers of the present invention are able to effectively reduce the buildup of at least a portion of surface charges resulting from the crystal's natural pyroelectric response. By effectively reducing surface charging, (1) the risk of sparking is reduced, and the risk of device failure can be reduced; and (2) the need for costly and time intensive processing steps normally used to reduce the buildup of surface charging is avoided.
While not intended to be limited to any particular theory, it is believed that the observed reduction in surface charging for preconditioned crystals of the present invention is a result of the increased electrical conductivity of the p
Bordui Peter F.
Jundt Dieter H.
Norwood Richard G.
Standifer Eugene M.
Crystal Technology, Inc.
Fish & Richardson P.C.
Tucker Philip
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