High-density, low-loss MOS capacitors for integrated RF decoupling

Passive integration can be highly rewarding in terms of miniaturization, but also in terms of performance. A very convincing example is the integration of supply-line decoupling in RF front-end modules. It will be shown that a more powerful decoupling concept can be implemented by using one single low-loss capacitor on silicon, replacing conventional SMD-based decoupling using discrete ceramic capacitors. High-density MOS capacitors with 1-1000 nF capacitance, and 20-100 nF/mm² specific capacitance have been fabricated in 6-inch macroporous Si-wafers, containing over 1 billion macropores with 3.5 μm pitch and 2 μm diameter. Two different techniques (wet and dry) were used to etch the macropores. The 20 to 100-fold enlarged Si-surface thus obtained serves as a substrate onto which MOS capacitors are formed. This capacitor type enables fully MOS-compatible processing (e.g. doping, oxidation, LPCVD of in situ doped poly-Si, etc.) and direct chip attach for RF decoupling applications. The potential of this novel Si capacitor technology has been analyzed in comparison with conventional SMD-capacitors. Superior decoupling in the relevant GHz range was shown by transmission measurements on a 50 Ω microstrip line decoupled by 2.2-380 nF MOS capacitors, made from dry-etched porous silicon. Typically an ESR less than 100 mΩ and an ESL less than 25 pH were found for capacitors over 10 nF. This novel concept can be an important step forward in improving the stability of power-amplifier modules by replacing conventional SMD technology.