Acoustic components used for filtering

The modeling of elastic-wave components has been the subject of a great deal of work, from the development of the first volume-wave devices to the most complex structures currently being developed, combining Bragg mirrors and other multilayer guides, for example.

One of the unique features of modeling guided elastic wave devices is the extraordinary simulation-measurement agreement that can be achieved. This is particularly true for surface wave devices. Here are a few figures to illustrate the point:

• the precision of resonance frequency calculations: a few ppm ;

• accuracy of insertion loss prediction: 0.1 dB ;

• the predictive accuracy of a filter's rejected levels (1 dB for levels of 40 dB).

We could add many other points of agreement between simulation and measurement, such as the prediction of the frequency position and level of spurious modes, for both volume and surface modes, as well as the sensitivity of frequency to quasi-static thermal variations.

There are few areas of physics where simulation-measurement agreement reaches this level.

There are two main reasons why this agreement is possible:

• the first is that the substrates used in the passive piezoelectric filter industry (quartz, lithium niobate, lithium tantalate) are single crystals, which means that their physical properties are remarkably stable. The metal layers (most often aluminum, but also molybdenum, platinum, gold, tungsten or chromium) that form the electrodes (facing solid surfaces, interdigitated combs) are also precisely characterized, although they are subject to more variability than the substrates themselves due to their mode of production (chemical or physical deposition);

• the second is the near-perfect definition of device geometry, thanks to microelectronics technologies. Shadow lithography processes on large wafers (at least 4" in diameter) are used to define the planar metallization patterns. The photorepeaters used for this purpose have largely submicron resolutions (the industry standard in this field is typically 0.3 µm) for acoustic wavelengths of between 50 and 1 µm. In addition, deposition machines (evaporation, sputtering, vapour deposition) enable layer thickness to be controlled to within one percent of the nominal value. The resulting film geometries can therefore be defined with great precision (see