3.042 Fall 2008
9/11/08
Processes for 2D and 3D Patterning of MetaMaterials
Background:
Lithography has been a top-down technology enabling Moore’s Law and the Information Age revolution. Key materials of this technology are photopolymers and their processing involves clever use of masks and interference phenomena to create 2D and 3D patterns at the nano-micro scale. MetaMaterials are defined as materials that obtain several of their properties primarily via their structure rather than by the intrinsic properties of their constituent components. Recent examples of metamaterials are structures that can act as “invisibility cloaks.*” Key to fabricating such structures is the use of top-down and bottom-up processing and combining different materials into complex units that are periodically arranged.
Figure 1 (a). Schematic of several unit cells of air cylinders in a solid, (b) Schematic of a 3x3x3 unit cell array of a bicontinuous structure based on Schwartz’s P surface. (c) and (d) Experimental polymer photoresist structures of the 2 model structures shown in (a) and (b) made by Lloyds mirror and by multibeam interference lithography respectively.
Project Goals and Deliverables:
The deliverables for this project will include a physical 2D structure and a 3D structure. Projection masks can be used for 2D patterning, which can be made via 2D printing of a plastic/air design followed by metallization (thermal vacuum evaporator).
Then the projection mask is put into contact with the photoresist, exposed, etc.
In particular, try to vary your procedures to be able to make thick 2D periodic patterns in SU8 (with aspect ratios in the range of 1-10). This pattern can also be used as a mold to create a 2D relief PDMS phase mask**. The phase mask in turn could be placed in conformal contact with a photoresist film to create multiple diffracted beams which will interfere and produce a 3D multibeam interference intensity distribution, that can be developed, leading to a 3D periodic structure (examine resist layers of various thickness, including 50 microns). 3D printing could be used to create 3D prototypes but necessarily of larger length scale. Since photopolymers will not in general have the ability to provide all the right component properties, one could imagine an infiltration of the polymer/air template (e.g. by a sol gel process) to create a dual structure that after burning off the polymer is, for example, a bicontinuous air/ceramic structure.
Targeted applications for consideration are photonic crystals, phononic crystals, dual photonic-phononic crystals. Design of the type of structure, the required geometry and topology and length scale (feature size) will critically depend on the intended application. Target at least a bi-functional design and demonstrate that the structure provides responses that are not simple rules of mixture of the component materials (e.g. the thermal conductivity of the structure is not given by a simple weighted average of the conductivities of the ceramic and air components). MSE foci in this project will include:
1. fabrication and structural characterization of structures to include using 2D and 3D printing and 2D and 3D lithography, 2. rational design of a suitable 2D structure using appropriate materials taking into consideration the ability to actually fabricate the structure in real materials, 3. Preliminary software modeling/simulation of the emergent metamaterial properties, (emphasis in 3.042 should be on topics 1 and 2).
The team should consider simple initial designs to become familiar with processing techniques and SEM imaging for structural characterization of masks and final 2D and 3D structures. Depending on the targeted application, the team should indicate how their structures would need to be modified (choice of component materials, length scales etc) in order to exhibit metamaterial-like behavior for some property of choice.
Motivational and Informational References to get started:
wikipedia: metamaterials, lithography
phase mask: e.g. http://www.stockeryale.com/o/lasiris_pm/
http://www.metamaterials.net/
NSL: MIT NanoStructures Laboratory http://nanoweb.mit.edu/
http://phoenix.phys.clemson.edu/labs/224/diffraction/index.html
Periodic Materials and Interference Lithography, M. Maldovan and E. L. Thomas, Wiley-VCH 2008.
* David Smith Lecture, Friday noon 6-120, MSE Seminar Series October 17, 2009; David is one of the originators of cloaking of EM waves using metamaterials.
“Sound Ideas” – article on phononics