The manufacturing process can be summarized as follows:

• Starting from the specification (i.e. the required frequency response, and other related characteristics), the designer chooses the device type and the substrate material. The specification is usually modified to take into account temperature effects and manufacturing tolerances.


• The device is designed with the aid of complex purpose-written proprietary software, which is also used to simulate the performance and check for compliance with the specification. The design often needs to compensate for second-order effects such as diffraction, and sometimes for stray components introduced by the package. The result is a design (electrode positions and lengths and other geometric information, etc.) expressed as a file of computer data.

The design data is translated into information specifying the geometry of the actual device pattern. This data, usually in a standard format, can be used as input for a mask pattern generator to produce the photomask. The mask will be an optical ‘object’ for the projection printer, and may include more than one device.

• The substrate crystal, in the form of a 4” diameter wafer, is coated with an evaporated aluminum film, and the required pattern is produced by photolithography.


• The rf responses of individual devices can be tested on the wafer by using probes and a network analyzer. Devices that may be out of specification can be marked so that they will be automatically rejected at a later stage, thus economizing on packages etc.


• The individual devices are divided by sawing, after which they are called die, and are ready for packaging. Die size can range from 1 mm2 to several cm2 so there can be a few devices or thousands of devices on one wafer.


• Individual die are placed in packages and secured by adhesive. Bond wires are attached between pads on the device and the package, giving connections to the external terminals. The packages are sealed.


• Packages range from special purpose metal housings for military or space applications to ceramic surface mount (SMT) packages used in commercial electronics. There are applications where multiple SAW filters are used such as in multi-band cellular handsets, and because of consumer demand for smaller and lighter handsets, the SAW filter packages begin to dominate from a cost and size point of view. Alternate technologies such as flipchip (where the SAW die is turned over and bond wires are eliminated) and others are often used in these cases.


• External impedance matching components are sometimes needed, and these are usually added by the customer on the PCB. These components may be needed to tune out the transducer capacitances, and also to transform the input and output impedances to a value suitable to the PCB designer. The exact component values for a particular filter design are be provided by COM DEV. Note that this is not just a question of matching the device to obtain low loss: the components often need to be specified by the designer in order to minimize passband amplitude and phase ripple. At this stage, it is also important to minimize feedthrough, which can degrade the stop-band rejection. Feedthrough can be caused by inductive or capacitive coupling between the input and output, or by a ground loop. At high frequencies, considerable care is necessary, and COM  DEV can offer advice to the PCB designer in this regard.


• Testing of the completed devices is done using a network analyzer to check for compliance with the specification. This is done after adding the matching components, if required. However, it may also be done before adding these components, using software to simulate the performance with the components. This helps to identify devices which are out of specification at this stage, so that further unnecessary work can be avoided.