Ion Beam Trimming of POI Wafers for RF Filters

With the evolution of the modern communication standard 5G higher frequencies are used for receiving and sending. RF filters working at such frequencies can be manufactured as a so called “guided SAW device”. This new class of devices utilizes a bonded piezo layer on a standard silicon wafer. Those wafers are called piezo on insulator (POI). The filter device uses a principle known from standard surface acoustic wave (SAW) filters. 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 the IDT.

Building a SAW device on a POI wafer leads to a guiding of the acoustic wave within the piezo layer. This results in a higher coupling factor K² for larger bandwidth filters and built-in temperature compensation for high band density.

Typically, POI wafers consist of two or three different functional layers (see Fig 2). The thickness of each layer has to be adjusted to a certain value by keeping the surface as flat as possible. The demand of layer thickness homogeneity as well as the target thickness accuracy becomes stricter than methods like grinding and chemical mechanical polishing (CMP) can deliver. In this case, ion beam trimming (IBT) can be used to drastically improve surface uniformityas well as desired target thickness accuracy. Thereby, removal up to 1 µm are addressed while maintaining low surface roughness.

The scia Trim 200 uses localized ion beam etching for precise adjustment of film thickness. 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 (see Fig 3). By altering the local dwell time it is possible to precisely adjust the material thickness, and hence the device frequency across and other properties.

Results of a typically trimmed POI wafer can be found in Figure 4a and 4b. Average thickness of 2450 nm was reduced to a target value of 1600 nm with standard deviation improving from 439 nm to 35 nm (improvement factor 13). The thickness distribution after trimming is well centered around the target thickness.

Likewise, AFM images (see Fig. 5) of the wafer before and after the trimming process show that the original surface roughness was kept throughout the trimming procedure.