Sputter Deposition of SiO2 for Temperature Compensation of SAW Filters
Temperature compensated surface acoustic wave (TC-SAW) frequency filters have opened a new field of applications in mobile communication. As standard SAW filters show a temperature-dependent frequency drift of -20 ppm/K up to -40 ppm/K, many high-frequency applications as well as communication in narrow RF bands would not be possible. The application of silicon dioxide (SiO2) for temperature compensation (see Fig. 1) allows the reduction of the temperature-dependent filter frequency drift (TCF) of a TC-SAW filter down to 0 ppm/K. However, the filter performance strongly depends on the quality of the deposited SiO2 as it contributes to the acoustic wave propagation. Especially, a dense and closed layer growth on the interdigital transducer (IDT) is challenging due to the high aspect ratio and varying seed material. In parallel, the industry requirement of high wafer throughput has to be fulfilled.
The required SiO2 for TC-SAW filter can be deposited using a scia Magna 200 reactive sputter deposition system. Magnetron sputtering from a silicon target in an argon/oxygen gas mixture allows the stoichiometric deposition of SiO2 with a high deposition rate. The use of a RF substrate bias significantly influences the layer growth on the SAW IDT structures. Without the RF bias, a strong formation of seams and voids is observed for the grown SiO2 as shown in figure 2. However, the impact of the SiO2 on the filter properties depends on its acoustic properties such as acoustic velocity and acoustic damping as well as the SiO2/IDT interface. The introduction of the RF bias during the SiO2 deposition changes the layer growth dramatically. By using a high enough bias voltage, a uniform layer without the formation of voids and seams is observed which is shown in figure 3. The acoustic wave can effectively interact with the SiO2 which leads to the desired SAW filter properties.
The effect on the temperature-compensation can be seen in figure 4: As the temperature varies, a non-compensated SAW filter strongly changes its resonance frequency. In contrast, a TC-SAW filter with a temperature compensation layer deposited by the scia Magna 200 shows no significant drift. A TCF of -3 ppm/K was achieved. At higher frequencies, the reduced IDT pitch is challenging for the SiO2 layer growth. For example at 1.7 GHz, an insertion loss of 1 dB at a TCF of -23 ppm/K was achieved, which shows the capability of the scia Magna 200.
Besides the SiO2 quality, wafer throughput is a strong requirement in the SAW industry. The SAW filters are built on lithium niobate (LN) or lithium tantalate wafers. Those materials are very fragile and sensitive regarding fast temperature changes, which can occur for high sputter power processes. The scia Magna 200 is equipped with an efficient wafer cooling which allows higher sputter power, and thus higher wafer throughput.
Fig. 1: Schematic view of a typical TC-SAW stack and functional principle.
Fig. 4: Frequency drift of a SAW filter as a function of temperature for a non-compensated SAW filter and TC-SAW filter realized by silicon dioxide deposited with the scia Magna 200.
Fig. 2: SEM cross-section of an IDT electrode covered with silicon dioxide, without substrate bias.
Fig. 3: SEM x-section of an IDT electrode covered with silicon dioxide, with substrate bias.
Acknowledgment:
The research has been funded by the European Union and the Free State of Saxony.
Funding reference: 100206216/10020621618.
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Related Product - scia Magna 200
- Cluster configuration with automated wafer handling, cassette loading
- Recording of process data, production log and component lifetime counter
- Easy standard maintenance e.g. target and sputter shield exchange
- Fab interface possible (e.g. SECS II/GEM)