3D Optical Systems Group

INTRODUCTION

While two-dimensional (2D) nanofabrication techniques, such as electron beam and x-ray lithography, nanoimprinting, dip-pen lithography, etc, can reliably produce features less than 20nm, these methods lack the ability to create non-planar patterns, i.e. three-dimensional (3D) structures. The use of the 3rd dimension will allow micro/nano-scale devices to provide additional flexibility or even enable completely new functions. The 3rd dimension also helps overcome the related challenge of bridging the gap between the nanoscale features of a system and the macroscale (Figure 1).

The Nanostructured Origami™ process overcomes this difficulty of utilizing the 3rd dimension by borrowing ideas from the Japanese art of paper folding, or origami. By first patterning 2D membranes via conventional lithography tools and then folding them into an arbitrary layout, almost any 3D shape can be formed. Of course, each of these folded membranes will have been patterned in the previous step with the desired mechanical, electrical, optical, and other functions (Figure 2 and 3).

Figure 1 Bridging the gap between nanoscale and macroscale.

(a) (b) (c)
Figure 2 (a) Planar fabrication. (b) Membrane folding. (c) Completed device.

CURRENT PROGRESS

FOLDING METHODS

We are currently investigating two main methods for origami-style membrane folding: Lorentz force actuation and residual-stress induced curling.

In the Lorentz force actuation method, a current is applied to a segment to be folded in the presence of a magnetic field. By the Lorentz law, there will be an upward force acting on the outer edge of the segment, as shown in Figure 4.

Another folding scheme that we are investigating treats silicon nitride as “paper.” Crease areas are first lithographically defined, and folding is induced by depositing a stressed metal layer on top of the nitride. We are able to fold the nitride layer to an arbitrary angle by adjusting the size of the crease and manipulating the residual stress of the deposited metal actuation layer.

Figure 5 shows the scanning electron microscope (SEM) image of a layer folded beyond 360o. The radius of curvature is approximately 20 microns, which is approximately the same as the radius of a human hair.

Figure 6 shows an overhead view of curling. The upper layer is chrome and looks white under the microscope. The underlayer, silicon nitride, is a darker green color. One can observe folding from the outer corners.

Figure 4: Membrane folding via Lorentz force

Figure 5: SEM image of 360o fold.

Figure 6: Overhead view of curling.

COMPLETED DEVICES

Several devices have been successfully fabricated. Our first devices were fabricated using silicon as the structural material of the flaps and folded up to 180º using the aforementioned Lorentz force actuation method. Gratings with sub-40nm features were also incorporated into the fabricated devices. The next generation of devices used a polymer called SU-8 as the structural material. Figure 8 shows a two-flap device made of SU-8 that has popped out of the substrate due to the stress-induced folding scheme mentioned previously. Figure 9 shows pyramid structures that are used for maintaining spacing and alignment between different folded layers. Figure 10 shows the device after the folding step is completed.

Figure 7: Gratings of nanoscale feature size integrated into folding device

Figure 8: Two-flap device after partial folding induced
via residual stress.

(a) (b)
Figure 9: (a) Origami using the SU-8 process. (b) Close-up view of the alignment pyramids.

Figure 10: Overhead image of a two-flap device.

KINEMATIC MODELING

Software has been developed for visualizing the folding process and modeling the kinematics of origami structures. Our model begins by attaching a closed “skeleton” structure to the origami segments. The skeleton is then broken into two parts and forward kinematics are performed in the part that contains the actuation mechanism. The second part is constrained by the integrity condition, and so its motion is estimated via inverse kinematics. Singularity in the inverse kinematics of the second part indicates that kinematic integrity is violated in the given structure. Kinematic compatibility is ensured by tracking the segment edges (this function has not yet been implemented in our software tool). The GUI of the developed software is shown in Figure 10, and Figure 11 shows an example of the visualization and kinematic modeling process [1].

Figure 11: Software developed for visualizing and doing kinematic modeling the folding process.

Figure 12: Five-segment origami mechanism (actuation sequence shown clockwise on top) and the folding angles of two of the segments in response to actuation of the diagonal joint.

CONCLUSION AND FUTURE WORK

The Nanostructured Origami™ Fabrication and Assembly Process has been developed for 3D nanomanufacturing using conventional 2D lithography tools. Results presented above show great promise for this technique, and we are already in the process of applying this method to the fabrication of novel 3D nanostructured devices, such as energy storage systems and multi-layer diffractive optical elements.

REFERENCE:

1. T. Buchner, "Kinematics of 3D Folding Structures for Nanostructured Origami," Diploma Thesis, Laboratory for Machine Tools and Production Engineering, RWTH Aachen University (2004).