Surface Trimming for Surface Acoustic Wave (SAW) Devices
Modern mobile communication depends increasingly on frequency filters, since more and more communication standards with often multiple frequency bands have been established. A huge part of these filters are realized by using the surface acoustic wave (SAW) mechanism. SAW filters have a remarkable efficiency and strong suppression of frequencies outside the transmission bands, and thus an extremely high Q factor.
A SAW filter consists of a piezoelectric substrate, such as quartz, lithium tantalate (LiTaO3), or lithium niobate (LiNbO3), and two sets of interleaved metal electrodes called interdigital transducers (IDTs) on top of the substrate (see Fig. 1). Incoming electrical signals at the input transducer generate acoustic waves due to the piezoelectric effect. These waves propagate along the substrate surface and are reconverted at the second transducer. An efficient signal transmission only occurs if the signal frequency f matches the resonance criteria f = v0/λ. Thereby v0 is the speed of the acoustic surface wave propagation and λ is twice the distance between the comb structures of an IDT.
Due to the fact that the available frequency bands used in telecommunications are limited, the 3G, 4G and 5G standards take advantage of carrier aggregation in order to increase the date rate. This means parallel transmitting on multiple bands. To avoid any interference between different bands while using them in parallel, the specifications for bandwidth have become increasingly tighter. This requires a growing precision in manufacturing of SAW filters and an additional temperature compensation layer, which is realized by a SiO2 coating on top of the IDT. Special temperature compensated SAW devices (TC-SAW), typically use an additional Si3N4 passivation layer on top (see Fig. 2), which also requires superior uniformity. Current deposition tools cannot provide a sufficient uniformity of the metal used for the IDT or the temperature compensation layer across a wafer.
A localized trimming process is necessary to improve the uniformity and to maintain a high yield for mass production of SAW filters. Ion beam trimming uses a beam of positive charged ions, e.g. Ar+, to physically etch material from the wafer. The beam with a typical diameter of 7 ‑ 15 mm ensures a sufficient lateral resolution and a high throughput.
During the trimming process a focused broad ion beam moves in a meander-shaped pattern across the substrate surface. By altering the local dwell time, it is possible to precisely adjust the material thickness, and hence the device frequency across the SAW wafer.
The trimming of a SAW device without temperature compensation causes an etching of the metal electrodes and the substrate material. The resulting sensitivity function (see Fig. 3) is non-monotonous, which arises from the etch rate difference of the metal electrode and the substrate. The range for a negative frequency shift can be extended by adjusting the process parameters. For generating higher positive frequency shifts, a minimum removal is required.
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