With few exceptions, all SAW devices consist of thin-film metal structures fabricated on the surface of a piezoelectric substrate. For example, the interdigital transducer (IDT) is an array of narrow parallel electrodes connected alternately to two bus bars. The substrate material is usually a single crystal of quartz, lithium niobate or lithium tantalate.

The generated wave is guided along the top surface of the substrate, with its amplitude decaying into the depth. In order for the wave to behave as a true surface wave, the substrate needs to be at least a few wavelengths thick, so that the wave amplitude is negligible at the lower surface.

Practical substrates satisfy this need easily and these usually have a thickness of 0.5 or 1.0 mm., chosen to give adequate strength to tolerate stresses involved in fabrication and operation. However, there is market demand for very thin SAW filters driven by the need for smaller cellular phones, and the use of 0.35 mm piezoelectric substrates is becoming common.

In all SAW devices, the critical structures are arrays of narrow parallel metal strips, with spaces between the strips comparable to the width of the strips. In the IDT, parallel electrodes are connected alternately to two bus bars (wide strips at the sides, serving to connect the electrodes to the device terminals). SAW reflectors are made from arrays (gratings) of metal strips, usually connected together by bus bars.

Such reflectors are used in most types of SAW resonators or resonator filters. Another common component is the multistrip coupler (MSC), which also consists of an array of metal strips, but not connected together. In all these cases the spacing of the strips is /2 or less. The strip width is typically half of this, about  /4.

The strips often have a function which is purely electrical, i.e. they serve simply to apply or detect electric fields at the surface. For this purpose, the film thickness only needs to be enough to minimize losses due to resistivity. Typically, the thickness will be in the range 300 to 3000 A. However, in some devices mechanical effects are used - the mechanical motion of the electrodes causes a disturbance of the wave which sets up reflections. This effect is employed in resonators and in SPUDT filters, and in these cases the film thickness is more critical. The metal film is nearly always aluminum, chosen because it has acoustic properties fairly similar to those of the substrate materials. Titanium is sometimes used under the aluminium to improve the power handling characteristics of the device.

SAW devices are made using photolithography, which gives accurate control of the electrode geometry— a very important consideration. This is one reason for the success of the technology, another being that the crystal substrates have very reproducible properties, so that devices with very predictable performance can be produced. The electrode width is a crucial factor. Photolithography is limited by the wavelength of the light used to reproduce the pattern on the substrate. The minimum linewidth is in the region of the light wavelength. For ultraviolet light, as used in large-scale production, the minimum linewidth is about 0.3 µm. This corresponds to  /4, and for a typical velocity of 3500 m/s it gives a maximum frequency of about 3 GHz. Narrower linedwidths are obtainable using special techniques, for example electron beam exposure, but these are not applicable to large-scale manufacturing.

Photolithography is the key part of the fabrication process. Three types are in common practice: wet etching, lift-off and reactive ion etching (RIE). For illustration, the following describes the wet etching process, illustrated in Fig.1.

The substrate, a circular 4" wafer of crystalline quartz, lithium niobate or lithium tantalate, is first carefully cleaned, and then a thin film of aluminium is deposited. This deposition is done at COM DEV by evaporation. The thickness of the aluminium is very important and is a predetermined parameter arising from the design; the thickness uniformity across the profile of the wafer is also very important to ensure consistent behaviour from device to device and COM DEV is able to hold this tolerance to 1% maximum. The wafer is then coated with a photoresist solution and is spun at high speed so that the resist becomes a thin film on the wafer. Baking then solidifies this film. Often an anti-reflective coating is applied under the photoresist. The wafer is then exposed to ultraviolet light through a photomask or reticle which defines the geometry of the pattern to be produced on the wafer. The reticle pattern is fabricated to be several times larger than the desired pattern to be exposed on the wafer and the exposure process is done by projection on a machine called a Stepper, through a high quality lens system.

The Stepper can also be programmed to expose a variety of different device designs on one wafer. The exposure process is critical, and the wafer must be held perfectly flat since the depth of focus may be as small as one µm. Following this, the resist is chemically developed in the wafer track so that regions exposed to light are removed, leaving exposed areas of metal. At COM DEV, the remaining areas of exposed aluminium are dissolved chemically using a reactive ion etch process, often called “dry” etch.

The remaining photoresist is then stripped, and this is the last step in the photolithography process; the result is a patterned wafer containing the SAW devices. The number of devices on a 4” wafer can vary between one or two (for specialized devices or very low frequency filters) to several thousand in the case of high frequency low loss rf filters.

In the process described above, it can be easily understood that virtually any arbitrary shape can therefore reproduced in the metal film on the substrate. The types of patterns are only limited by the designer’s imagination.

As with IC fabrication, the SAW manufacturing process is well suited to high-volume production, and this can bring substantial economies of scale. Metal film deposition is done on up to 20 wafers at a time, and on each wafer the lithography produces many devices simultaneously. Also, processes for die sawing, packaging, wire bonding, testing etc. can be highly automated.

Primary input materials are the wafers, masks and pack ages. These are all available and kept in stock at COM DEV. New designs are synthesized quickly with proprietary in-house software and masks can be made rapidly. A variety of package styles and sizes is available, including DIL and nine standard COM DEV SMT packages are available. (SM3030 through SM24690)

It should be borne in mind that SAW devices vary substantially. For example, typical lengths vary from 2 mm to around 10 cm, and costs tend to vary accordingly, as well as being dependent on the performance and testing required.