Modified SAMs and templates for achieving self-alignment of full wafers
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RESEARCH PAPER
Modified SAMs and templates for achieving self‑alignment of full wafers Ako Emanuel1 · Ernest M. Walker III1 · Hans D. Hallen1 Received: 9 February 2020 / Accepted: 17 May 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Self-assembly of structures that are more than a few millimeters in size is a current problem that can have broad applications in new ways to construct the objects that we use. As a step in that direction, fluidic self-alignment at the wafer-to-wafer level is demonstrated, characterized, and modeled. Although self-alignment of millimeter-sized objects has previously 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, both natural and modified by oxidation, are used to create surface energy gradients that are used to produce capillary alignment forces. These forces exceed the wafer-edge forces. The measured capillary fluid profile is modeled to predict the alignment forces with no adjustable parameters. Alignment forces are measured and the trends are predicted as the surface tension of the liquid, hence the generated force, is varied. These considerations govern pattern design rules, which are described and ensure a long capture range, high alignment force, and avoidance of wafer-edge dragging. Graphic abstract
Keywords Capillary force · Fluid self-alignment (FSA) · Hydrophobic · Hydrophilic · Patterned surfaces · Surface chemistry
1 Introduction
* Hans D. Hallen [email protected] 1
Physics Department, North Carolina State University, Raleigh, NC 27695‑8202, USA
For a long time, researchers have been aiming to create materials that will build themselves into structures. The most developed of techniques to create three-dimensional functional objects from two-dimensional parts is wafer stacking or folding (Mastrangeli et al. 2009; Mastrangeli
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2015; Mastrangeli et al. 2011, 2017). Surface energies often dominate these self-assembling schemes when the scale of the meniscus is less than the capillary length (Böhringer 2003). A few not-self-aligning techniques such as ‘SmartView’ and ‘3Dalign’ have emerged to support wafer alignment during stacking (Lee et al. 2011). The problem is that these have been found to be costly and inaccurate when applied to tall stacks (Narimannezhad et al. 2016), and are not self-aligning. A self-aligning process would be useful in electronic packaging, where multiple wafers are difficult to view through at high resolution with IR imaging. The presence of a ground plane makes view-through impossible. The alternative, alignment from both sides, either views both sides at once with a specialized microscope or uses digital techniques to overlay the back-side image while observing the front. Both have relative positioning errors that add up. Wafer stacking has been achieved without self-alignment (Terfort et al. 1997; Ohba et al. 2010), but i
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