Chemistry and Strain Allow New Microfabrication

Discovery of a new bonding mechanism has enabled the facile fabrication of arrays of semiconductor microstructures.

Image courtesy of Max Lagally, University of Wisconsin-Madison
Scanning electron microscope image of 2D arrays of channels created in a silicon sheet taking advantage of novel interface interactions with the soft substrate. Inset: top-view optical image

The Science

A new mechanism for controlling bonding between a thin semiconductor sheet and an expandable soft plastic substrate has been discovered. Control of the interface chemistry and mechanics led to controllable and reproducible formation of 3D structures.

The Impact

The work expands materials options for micro- and nanotechnologies relevant to energy harvesting, flexible optoelectronics, and microfluidics.

Summary

Unique bond formation between a thin semiconductor sheet and an expandable soft plastic during the application of strain enables the facile fabrication of arrays of identical semiconductor microstructures, and thereby expands materials options for micro- and nanotechnologies relevant to energy harvesting, flexible optoelectronics, and microfluidics.  Elucidation of the bonding mechanism, as achieved in this work, promises an expanded set of applications because the types of microstructures that can be created are tunable with the strain that is created by the bonding. The method combines establishment of stress, via the bonding mechanism, in the thin crystalline-semiconductor sheet during and after submersion in a fluid, lithographic patterning to create regions of high stress, and relaxation of the stress induced in the nanomembranes under compression, via a guided self-organization process. In this approach, lithographically patterned semiconductor nanomembranes are attached to a thick polymer substrate (polydimethylsiloxane (PDMS)); the composite is then immersed in a liquid solvent that causes the polymer substrate to undergo a uniform volume expansion. When the composite is removed from the solvent, the patterning causes the formation of regions of high stress, resulting in buckling of the semiconductor nanomembrane. Weak interactions (but more than just van der Waals bonds) were found responsible for adhesion between nanomembranes and the substrate during solvent immersion and evaporation, but strong bonds formed after long exposure to air for the unbuckled regions.  Unique interface chemistry and mechanics led to controllable and reproducible formation of 3D structures through control of the patterning geometries, thickness of the nanomembrane, and solvent chemistry. The method guarantees a uniform applied expansion or contraction (strain) over large areas and eliminates the need for complex equipment for straining the substrates. The approach can be applied to a variety of hard-material film/soft-material substrate combinations and different chemical straining agents.

Contact

Max Lagally
University of Wisconsin at Madison
lagally@engr.wisc.edu

Funding

DOE Office of Science, Basic Energy Sciences program and Air Force Office of Science and Research.

Publications

Francesca Cavallo, Kevin T. Turner, and Max G. Lagally, Advanced Functional Materials, (2013) [DOI: 10.1002/adfm.201303165]

Highlight Categories

Program: BES , MSE

Additional: Collaborations , Non-DOE Interagency Collaboration