Sub-femtoFarad MOS varactor characterization tools

Nowadays with capabilities offered by advanced silicon technologies both for design above 60GHz and for high performance Digitally Controlled Oscillator, the use of sub fF varactor is mandatory. One of the challenge to develop this device is to be able to characterize it accurately for process optimization and modeling. This article highlights the metrology requirements for this kind of characterization. In high and above all low frequency range, this type of measurement has to face with the problem of high impedance characterization and mismatch regarding 50Ω. Several limitations as noise or current detection limit appear when the capacitance of the device is very low compare to the measurement environment. In this paper several emerging techniques are evaluated for silicon sub fF capacitance analysis. The tool architecture which seems to be the best to cancel these limitations is the lock-in amplifier. In the high and low frequency range, several equipments use this architecture. It is the case of AH2700A marketed by Andeen-Hagerling. This equipment operates at 1KHz and characterizes an AMOS sub-femtoFarad varactor with a precision about 20aF (20E-18F). In the millimeter wave frequency range, Agilent develops two equipments called SMM (Scanning Microwave Measurement) and DPMM (Doping Profile Measurement Module). These tools characterize very low capacitance with an external module added to a VNA (Vectorial Network Analyzer). We emphasize a relative precision about 50aF with the DPMM.