Antennaplexers and other multiplexers often use notch filters to create a wider passband than achievable with a standard bandpass filter. When using a notch filter in this way, it is often required to provide multiple reject bands to provide isolation to other ports. Previously this has been achieved by using multiple acoustic notch filters or by adding an external tank circuit using SMDs. While the tank inductor can be embedded within the module laminate, the capacitor typically cannot and requires an SMD. Implementing this capacitor on the BAW die as a MIM allows increased integration and enables multiple reject bands on a single die. The BAW MIM capacitor has previously been described for use as a de-coupling capacitor and matching element, but this is the first known application to notch filters for a multi-notch configuration. This can be accomplished using a reflector or passivation layer MIM capacitor.
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Qorvo US, INC., "Multiple Notch Filter Using On-Die BAW MIM Cap", Technical Disclosure Commons, (December 12, 2022)