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(Color online) (a) Steps for transfer printing with an elastomeric stamp, where applied shear stresses are used to control the strength of adhesion. Inset: a schematic illustration and critical dimensions of the stamp including post and backing layer. (b) Optical micrographs collected by imaging through the transparent stamp during the printing steps. A stamp “inked” with a silicon plate (100 × 100 × 3 μm) is brought into contact with a silicon substrate, sheared by 12.5 μm (γ = 14%) in the -x-direction, and then slowly retracted to transfer the plate from stamp to substrate. Scale bars correspond to 50 μm.
(Color online) (a) Measured pull-off forces required to delaminate stamps from a flat silicon substrate, as a function of shear displacement. The posts on the stamps have fixed heights of 50 μm and lateral dimensions up to 250 μm; retraction and shear velocities were fixed at 10 μm/s. (b) Normalized pull-off forces, P, from (a) as a function of shear strain (from Eq. (1)) in the post. The data from posts with different sizes collapse, approximately, onto a single line.
(Color online) (a) Calculated normal stress distributions in a 200 μm wide post 1 μm above the stamp-ink interface for shear strains between 0% and 14.8%. (b) Average normal stress as a function of average shear stress at the interface for the pull-off forces in Figure 2. The stresses were determined from measured loads and applied shear displacements using a finite element model. The data collapse onto approximately a single line. (c) Strain energy release rate, G, calculated using finite element analysis, for a stamp with a post width L = 150 μm at different applied shear strains and normal forces. The pull-off force at failure can be determined from the intersection of the curves with the toughness of the interface (black dashed line shows a representative toughness of Γ0 = 50 mJ/m2). (d) Average normal stress vs. average shear stress at failure of the stamp/Si interface predicted from fracture-based finite element calculations assuming Γ0 = 50 mJ/m2. Modeling results exhibit similar behavior to the experimental results in (b).
Demonstrations of printing silicon plates (100 × 100 × 3 μm) using shear to control the adhesion. (a) Yields for transfer printing onto a bare silicon substrate as a function of shear. (b) Examples of plates printed onto a structured (line and space geometry; 3 μm width, 17 μm spacing) PDMS substrate. (c) Plates printed onto a micromachined ledge on a silicon wafer. Inset: cross-sectional magnified view. (d) Overlapping, stacked plates printed onto a silicon wafer surface.
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