Frequency Trimming for Bulk Acoustic Wave (BAW) Devices

High frequency filters in mobile com­mu­ni­ca­tion increasingly apply Bulk Acoustic Wave (BAW) principle rather than Surface Acoustic Wave (SAW) principle. BAW becomes advantageous against SAW for frequencies above 2.4 GHz. The devices are very small and have a very good performance at the same time. Also, the productions costs have been reduced over the last years.

A BAW filter uses a piezoelectric film commonly made of aluminum nitride, which is contacted by two electrodes (see Fig. 1). To generate an acoustic resonator, the thickness of the piezoelectric film has to match to λ/2 of the wavelength of the longitudinal acoustic wave. Means the thickness is defined by the acoustic velocity of the piezo material and the target resonance frequency.

Additionally, the resonator needs to be acoustically isolated from the substrate. There are two different types. (Thin) Film Bulk Acoustic Resonator (FBAR) devices use a cavity between substrate and resonator (see Fig. 2). The Solidly Mounted Resonators (SMR), which are shown in Fig. 1, use an acoustic mirror to achieve an isolation from the substrate. The acoustic mirror is made of alternating λ/4 films with high and low acoustic impedance. With typical materials like silicon dioxide and tungsten, only a few layers are needed to achieve a very good isolation.

The final frequency of each device is adjusted by an additional mass load on top, which is deposited on the top electrode, e.g. silicon nitride.

The requirements for the layer uniformity of each material, especially the AlN piezo layer, are very high. Realistic deposition ho­mo­gene­ity deviations of 0.5 % lead to the final device yield of few ten percent only. By using additional tuning steps, as ion beam trimming, the layer uniformities can typically be improved by factor 10 to 20. Thus, final device yields of more than 90 % can be achieved (see Fig.3).

The process of ion beam trimming can be employed for each individual layer in the BAW stack. Thickness adjustments of the AlN piezo layer are most important, because its thickness directly defines the resonance frequency. The frequency is additionally influenced by the mass load of the oscillator. Thus, mass load trimming helps to further improve yield. By trimming the acoustic mirror layers, additional quality factors of the RF device can be optimized.