Cross-section diagram of ideal flat punch indentation geometry with cylindrical punch diameter and film height . It is assumed that the punch height is sufficiently high that no contact with extruded material is made to surfaces other than the punch face and sidewalls.
Gallery of complex imprint dies and imprints formed into polymer thin films. A single crystal iridium barlike stamp [(a) and (b)] produces quasi-plane-strain flow (c). A grid stamp (d) ion milled into silicon has deep trenches with wide walls (e) to produce high resolution patterns in polymer (f). A large array of wide, pitch lines in silicon [(g) and (h)] produces a corresponding pattern in a gold coated polymer film (i). The variation in line pitch is due to drift in the FIB during the die mastering process.
Fabrication of an imprint die at the apex of a mounted, doped, single crystal silicon sphere of diameter. The sphere is glued to a nanoindenter mount (a) and then placed in the indenter under high stress with a thin film polymer sample where rapid, small amplitude (tens of nanometers) oscillatory shear strains are applied (b). This leads to a fretting wear of the polymer surface (c), transferring material to the sphere surface (d). The inking ring is used to center concentric focused ion beam milling cuts of progressively refined quality [(e) and (f)] that removes material to allow free contact of a cylindrical flat punch (g). The punch is decorated with fine grooves (g) which can be used to verify contact with polymer surfaces by post-facto AFM inspection. (i) provides a zoomed out view of the final die fabrication.
Roughness and curvature of flat punch surface. A flat punch milled from the apex of a mounted silicon sphere (a) reveals residual polishing scratch relief in a high contrast SEM image (b). Corresponding AFM images of this punch [(c) and (d)] also reveal this residual relief. The topographic profile of the diameter punch face corresponding to the red line of (d) is shown in (e), where the residual curvature and roughness can be seen. The expected curvature of the silicon sphere is shown by the black line in (e). The three-dimensional (3D) topograph of the surface [dashed outline of (d)] provided in (f) shows the long wavelength nature of the residual roughness across the surface.
Definition of parameters used to characterize deviations from the ideal shown with cross section along the axis of maximum misalignment (see main text for full description). Magnitudes of the parameters are highly exaggerated for clarity.
Room temperature aligning of a diameter flat punch to within with a thick polymer film (10:1 aspect ratio). Mechanical signatures corresponding to maximum residual misalignment depths of 120, 30, and (, 4.7, and ) are shown throughout in red, blue, and black, respectively. A family of topographic profiles in the AFM image (a) are shown for progressively improved alignment in (b), with improvement to the direction of maximum misalignment shown in (c). Inverted topographic AFM images of residual impressions for the initial and improved states of alignment are shown in (d)–(f). In (g), the effect of the large misalignment angle produces an approximate offset from the more well aligned cases in otherwise congruent load vs displacement curves. The effect of even small 4.7 and misalignment is shown clearly by the harmonic displacement signal (h), zoomed in (i) to show the offset from contact before a full stiffness is established.
Tilt stage stiffness. (a) Modification to the load train of standard nanoindentation. The diamond indenter tool is replaced by a polished, single crystal silicon sphere placed into a cylindrically hollowed, blank tip mount piece providing knife-edge seating. The indenter tool itself consists of a local pattern formed from the monolithic silicon crystal (flat punch shown here) that is supported by a silicon half-space with finite stiffness. The sample mount is modified to include a dual-axis tilt stage with a total compliance of near its fulcrum. The measured effect on overall frame stiffness of moving away from the tilt stage fulcrum is shown in (b).
Scanning electron micrograph of a thermally instrumented silicon die. A diameter silicon sphere is epoxied to a mount. Three surface mount resistors soldered in series are epoxied to the sphere. Resistors are powered via diameter copper wire. A thermocouple of wire diameter of is epoxied to the sphere. Focused ion beam milling shapes the apex of the sphere into a flat punch die. An AFM scan shows the silicon flat punch die replicated in polymer.
Thermocouple temperature calibration. To determine the die temperature resulting in isothermal contact with the sample stage, independent batch indentation tests are performed sweeping the die temperature above and below the stage temperature set at . (a) The geometry of instrumented imprint displays a sensitive heat flux dependence on separation distance of die from sample stage. Die-side heating/cooling occurs when the sample stage is hotter/colder than the die. Isothermal conditions are determined when the die temperature does not change when approaching the surface. (b) Load vs displacement curves for squeeze flow polymer testing at stage temperature with die temperature of . Flow of heat and associated thermal expansion during nonisothermal conditions impacts measurements.
Effect of temperature on contact mechanics. Flat punch squeeze flow measurements in polymer with local heating from . In (a), load vs displacement curves show the decrease of yield stress and the transition from elastic deformation to postyield flow to fully viscoelastic flow with increasing temperature. (b) shows the evolution of displacement and phase angle signals with time before and after contact with the sample surface for temperature up to . A zoom in of the phase angle behavior with displacement before and after contact is shown in (c). Small amplitude dynamic stiffness vs displacement into the film following contact as a function of temperature is plotted in (d).
Thermal drift correction. Displacement vs time for the entire load history of two identical indentations of a diameter flat punch into a polystyrene film is shown uncorrected (light trace) and corrected (heavy trace) using the slopes at peak load shown by dashed lines (a). The corresponding corrected load vs displacement curves (b) show good agreement.
Thermal instrumentation validation by indentation of a Berkovich tip into silicon from . (a) Load vs displacement curves indenting silicon to depth loading at . Indentations from show identical loading behavior. The difference in unloading between indents at and indents from is due to influence of thermal drifts during depth-limited loading. (b) Drift-free measurements of silicon modulus approach the bulk value of for indentations from .
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