Customized piezoresistive microprobes for combined imaging of topography and mechanical properties
Customized piezoresistive cantilever microprobes with a deflection range of 120 μm and silicon tips of 100 μm height were operated in a Cypher AFM showing their functionality for measuring topography together with viscoelastic properties of thin films. For drop-in mounting in the AFM a holder was developed comprising the piezoresistive microprobe and its voltage-supply and signal-conditioning electronics. With the probe tip in contact to a glass sample we found a vertical resolution of 2.8 nm in a bandwidth of 1 kHz, which is close to the theoretical limit of 3.0 nm at a deflection of 2.5 μm. This resolution could be verified in topographic images of a scratch of approximately 300 nm in depth. Force-volume images with lithographically patterned photoresist (AZ 5214E) of approximately 300 nm thickness on silicon revealed contrast of the resist-covered and bare regions in topography, stiffness and adhesion. With contact-resonance imaging using the Dual AC Resonance Tracking (DART) method, patterned AZ 5214E photoresist of approximately 50 nm thickness could be distinguished from the bare silicon in topography, contact stiffness (indicated by contact resonance frequency shift) and adhesion (indicated by phase shift). Finally, a droplet of lubricant (Lupranol VP 9209) on glass could be detected by force volume imaging revealing a thickness of approximately 90 nm of the liquid layer with a sharp lateral limitation, which was clearly detected. We conclude that the piezoresistive silicon microprobe is a promising tool for emerging tasks of industrial surface metrology on manufacturing machines, including micro-finish of work pieces and elasticity, thickness, adhesion, etc. of thin solid or liquid deposits on top.