Seminar Series - 2002-2003

Piezoelectric Crystals: Material Constant Issues for RF Device Design

John A. Kosinski
Senior Technologist
AMSEL-RD-IW-S
Bldg. 600 (McAfee Center)
U.S. Army Communication-Electronics Command
Fort Monmouth, NJ 07703-5211

Date:  Tuesday, November 19, 2002
Time:  3:30 p.m.
Place:  W183 Nebraska Hall


Piezoelectric devices are widely used in commercial and military electronics for the purposes of frequency control and selection, and precise timing and synchronization.  These devices encompass both bulk acoustic wave (BAW) devices where plane waves propagate through the interior of the crystal and typically reflect off the major surfaces, and surface acoustic wave (SAW) devices of various types wherein acoustic waves propagate along the major surfaces and typically reflect off of some corrugation formed in or on the surface.  Such devices are used for resonators, delay lines, and filters from frequencies as low as 32 kHz for quartz wristwatch tuning fork resonators to 2.5 GHz and higher for SAW filters for mobile phones.

The design of piezoelectric RF devices requires knowledge from numerous disciplines in crystal physics.  The most basic level of knowledge involves the zeroth order understanding of acoustic wave propagation in piezoelectric crystals, i.e. understanding of the room temperature dielectric, piezoelectric, and elastic properties of the substrate material.  However, devices designed at this level are typically deficient in most applications, and knowledge of the first order effects such as the temperature coefficients and thermal expansion are typically required for a successful design.  In more demanding applications requiring high stability, or conversely for certain sensor applications, the knowledge must extend to higher order effects including nonlinear elastic and piezoelectric effects.

However, even with a theoretically suitable design, yet more information is required to achieve a producible and hence commercially viable product.  Here, the angular variations of the various design parameters must be considered to evaluate the sensitivity of the design to manufacturing errors.  Further, the physical chemistry aspects of the substrate material, the proposed metalization, and any packaging or hybrid circuit embodiment must be considered: how should the substrate be cleaned, will the metal stick to the substrate, is it compatible with photolithographic processing, etc.

In this paper, the field of piezoelectric RF devices will be reviewed in order to delineate the full range of crystal physics and materials science information required for successful design.  The paper will begin with the zeroth order device design, and then will proceed to introduce progressively higher orders of complication and aspects required for device fabrication.  Recent examples of limitations in device design due to limitations in the knowledge of device properties will be presented.


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