The Optics Laboratory
Group of Hans Hallen, Physics Department, North Carolina State University

Wafer-level Self Alignment

Self-assembly of structures that are more than tiny in size is a current problem that can have broad applications in new ways to construct the objects we use. As a step in that direction, self-alignment at the wafer level is demonstrated and characterized. Although self-alignment of millimeter-sized objects has been shown, several physical processes become important as the structure size is increased to a whole wafer. These processes are measured and modeled in this paper. Self assembled monolayers (SAM), both natural and modified by oxidation, are used to create surface energy gradients that are used to produce capillary alignment forces. The capillary fluid profile is measured, modeled, and used to predict the alignment forces with no adjustable parameters. Alignment forces are measured and agreement is found as the surface tension of the liquid, hence the generated force, is varied. The considerations govern pattern design rules, which are described and ensure a long capture range, high alignment force, and avoidance of wafer edge dragging. Details and origins of these rules will be discussed.

  • We have shown that self-alignment can be made to occur at the full wafer level to an accuracy approaching that required for realistic wafer stacking. Capillary forces are used as a driver for the alignment, with strategically chosen hydrophilic/hydrophobic patterning to eliminate local minima, insure adequate force near alignment, and provide an even lowering of the upper wafer as fluid escapes to insure alignment is maintained during bonding. A simple model for calculating the alignment force is given, as are measurements of the water profile between the wafers and alignment forces. These are found to describe the process well.

  • More to come once the paper comes out...

  •  More info is in the papers.

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