Principle of die self-alignment: (a) sketch of the droplet and the die; (b) sketch of the die before and after evaporation; and (c) view of aligned dies on a wafer (Ref. 9).
View of the boundary between the hydrophilic and hydrophobic regions; the bonding zone has been functionalized by oxygen plasma.
Dicing marks (left) permit control of the alignment: a good angular alignment is reached, but a slight shift may occur.
The four different modes (and possible reasons for misalignment): lift, twist, shift, and tilt.
(a) After a horizontal displacement, the die is restored to alignment by capillary forces; (b) EVOLVER calculation of the pullback.
Restoring force vs shift for different values of the droplet volume.
Comparison between approximate analytical model (plain line) and EVOLVER (dots). Left: surface energy; right: restoring force.
Die realigns after an initial twist.
Energy (left) and torque (right) as functions of the twist angle, for three values of the gap. Dotted lines are SURFACE EVOLVER results and plain lines correspond to the analytical model.
The die regains its stable position after a lift. Top: after an initial lift; bottom: after an initial compression.
Surface energy as a function of the vertical gap: The continuous line corresponds to the analytical model and the dotted line to EVOLVER results.
Restoring force vs vertical gap. Continuous line: analytical model; dotted line: EVOLVER results.
Two morphologies of the liquid having the same surface energy.
Die tilts to form a dihedral: (a) and (b) large liquid volume; (c) and (d) small liquid volume.
Comparison between EVOLVER numerical program and analytical model: in case of tilt, the interfacial energy varies extremely slowly.
Tilt torque vs tilt angle: continuous line indicates a zero torque and the EVOLVER results (dotted line) show a small tilting torque.
Bulging out/in shape of the surface in the two cases: left, parallel plates; right, dihedral.
The chip slides after a tilt in the case of a large volume of liquid and/or a heavy chip weight.
Left: corners are never totally wetted. Right: picture of the chip after a tilt with the dewetted corners.
Coupled modes: (a): tilt and roll; (b) tilt and twist; (c) shift and twist. The only unstable coupling must include a tilt and/or roll; the other modes are automatically corrected by the capillary forces.
Misalignment due to water spreading on the wafer outside the hydrophilic pad. The die makes a 10° twist, plus a shift with the pad.
Canthotaxis limits for planar wafer and wafer with relief.
Sketch of the shift: left, at alignment; right, after a shift.
Sketch of the twist. The point M is describing AB while is describing . Note that .
Three-dimensional view of the twisted surface (only one twisted surface is shown in the figure).
Sketch of the surface.
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