Compliant Tactile Sensors for High-Aspect-Ratio Form Metrology

Robust manufacturing of micro parts at high throughput needs tailored concepts and methods to assure unchanging work piece quality. An actual challenge is the dimensional metrology with meso-to-microscopic high-aspect ratio features of micro-system components, e.g. micro gears for watches and micro motors, ferrules of optical fibers, hollow microneedles for painless drug delivery into living cells, planar reflective lenses for X-ray optics, densily packed arrays of square channels in lobster-eye optics and through silicon vias (TSV) for vertical electrical interconnects of 3D integrated micro systems and circuits (Baron et al., 2008; Engelke et al., 2007; Hon et al., 2005; Li et al., 2007; Meyer et al., 2008; Muralikrishnan et al., 2005; Simon et al., 2008; Vora et al., 2008; Weckenmann et al., 2006). Form and roughness of deep and narrow micro holes play an important role for the function of nozzles designed for liquid injection. The inlet roundness of the hole, e. g., mainly determines whether flow is detached from the wall resulting in a constricted liquid jet or the fluid is atomized into small droplets. Detached flow can be observed with hydroentangling nozzles of 80 to 150 μm in diameter and 1 mm in depth (Anantharamaiah et al., 2007) while in the case of fuel injection nozzles atomization of the fluid occurs. The form of the spray holes was found to be crucial for optimal fuel injection, i.e. for combustion at low consumption and exhaust gas emission (Jung et al., 2008). Diameter, surface roughness, and inlet chamfer radius have a direct influence on fuel spray atomization, air entrainment and fuel-air mixing. Improved fuel economy, better soot oxidation and reduced NOx emission are benefits observed at optimized hole machining conditions. Furthermore, fuel penetration and spray angle were found to depend on the conicity of the hole which is attributed to the fuel atomization near the hole exit (Bae et al. 2002). While several techniques exist for micro deep hole machining at high throughput, e. g., micro drilling, electro-discharge machining (EDM), laser beam machining (LBM) and deep reactive ion etching (DRIE) (Diver et al., 2004; Hon et al., 2005; Jung et al., 2007; Krüger et al., 2007; Li et al., 2007; Seo et al., 2008), access to the inside surface of deep and narrow micro holes for metrology is very limited. In Fig. 1 the schematic and scanning electron microscopy (SEM) photographs of the seat of a VCO (valve covered orifice) of direct injection (DI) Diesel engine nozzle is shown comprising six spray holes which are 1.1-mmdeep and have a diameter of 165 μm. In the right part of this figure a close-up of one hole by SEM as well as by digital microscopy are shown which display the nozzle surface around O pe n A cc es s D at ab as e w w w .in te ch w eb .o rg