Single-Chip CMOS-MEMS Dual Mode Scanning Microwave Microscope

We present the design, fabrication and experimental validation of an integrated Scanning Microwave Microscopy (SMM)/Atomic Force Microscopy (AFM) system that does not require the use of a conventional laser-based AFM. Microfabricated SMM probes are collocated with piezoresistive strain-based sensing AFM probes in a CMOS-MEMS process, and are actuated by integrated electrothermal scanners. Integration of AFM enables dual mode imaging (topography and electrical properties) and more importantly, it enables control over tip-sample distance, which is crucial for accurate SMM imaging. This design is unique in the sense that the tip can be scanned over the sample in 3 degrees of freedom, over a
$20\ \mu{\rm m}\times 10\ \mu{\rm m}\times 30\ \mu{\rm m}$
scan range in the x, y, and z directions respectively. We fabricate our device by using a standard foundry CMOS process followed by in-house maskless MEMS post processing to release the devices. Single-chip SMM/AFM devices with integrated 1-D and 3-D actuation are thus obtained. These devices can be used to modulate the tip-sample separation to underlying samples with a periodic signal, improving immunity to long-term system drifts. We also investigate the effect of tip-sample separation on the resolution of the instrument. To increase measurement sensitivity, a single-stub matching network has been used to match the high tip-to-sample impedance to the 50 ohm characteristic impedance of a performance network analyzer. Measurement results of the CMOS-MEMS SMM are presented to verify the proposed concept.