Optical beam lithography beyond the diffraction limit

Ever-increasing demand for information technology has boosted the ongoing development of functional optical devices. Examples are photonic crystals (PCs) for integrated photonic circuits¹ and optical data storage facilities for storing big data.² Optical beam lithography (OBL) provides a powerful tool to fabricate these devices with specific geometric configurations.^{1, 2} Compared with other nanofabrication techniques, the unique advantage of OBL is its ability to fabricate 3D structures with relatively high spatial resolution. However, the fabrication resolution of conventional single-beam OBL is limited by the nature of light to around half the wavelength of the chosen laser. Recently, we have developed a 3D diffraction-unlimited OBL technique with a two-beam strategy that overcomes this limit.^3–5

Figure 1. Linewidth decrease and resolution improving by using 3D diffraction-unlimited two-beam optical beam lithography (OBL). (a) Nanowire linewidth versus the inhibition beam power, under the exposure of the writing beam power of 20mW at the scanning speed of 160μm/s. (b) Resolution improvement versus the power of the inhibition laser beam. For both (a) and (b), the blue scatter dots with error bars represent the experimental results. The red curves are best fits with separate formulas. The insert pictures show scanning electron microscope images of points A to L with a scale bar of 100nm.

Figure 2. Photonic bandgap tuning for woodpile photonic crystals (PCs). Schematics of the geometries of woodpile PCs with lattice constants (a) 400nm, (b) 350nm, and (c) 300nm, which have photonic bandgaps at different wavelengths. (d) The calculated transmission spectrum for the PCs in the stacking direction.

In this new method, one of the two beams is used as the lithography writing beam. The other, with a doughnut shape and at a different frequency region, inhibits the lithography process at the outer ring of the focal spot for the writing beam. When the two beams are superimposed at the focal region, we improve resolution and reduce lithographic feature size by tuning the light intensity ratio between the two beams.^3–5

To implement this strategy, we developed a unique photoresin with two chemical activation channels.⁵ It includes an initiator that is highly photosensitive to two-photon absorption from the writing beam (which allows for 3D fabrication even with low-intensity light from that source). The inhibition beam prevents the two-photon polymerization (2PP) process. Cross-excitation between the writing and the inhibition beams, that is, excitation of the inhibitor by the writing beam or vice versa is avoided. The threshold intensity for the writing beam to generate 2PP is also as low as possible to avoid photodamage and overheating. In addition, it provides sufficient mechanical strength for structures fabricated at the near-threshold condition to survive the wash-out developing process and the inevitable stress it entails.

Figure 3. Bit size and data capacity in single layer for different optical data storage techniques.

By using our new photoresin, we achieved a smaller feature size and finer resolution for the two-beam lithography technique compared with those obtained without the inhibition beam (see Figure 1). The smallest feature size and best two-line resolution we achieved are 9nm and 52nm (points F and L in Figure 1, respectively). These dimensions are 1/88 and 1/15 of the writing beam wavelength of 800nm, respectively. For this 3D diffraction-unlimited OBL, the linewidth and resolution, which decrease as the intensity of the inhibition beam increases, satisfy formulas independent of the wavelength, and show the diffraction-independent property of this technique.

Usually, the diffraction limit in OBL restricts the working wavelength of photonic structures fabricated by single-beam OBL to the IR.⁶By using 3D diffraction-unlimited OBL, two lines with the separation down to 300nm or less are realized, which demonstrates the feasibility of fabricating 3D photonic structures with working wavelength in the visible or even UV. So-called woodpile PCs, which have successive layers rotated 90° to each other, with different lattice constants, have photonic bandgaps at different wavelength ranges (see Figure 2). For instance, a woodpile PC with a lattice constant of 300nm exhibits a UV photonic stopgap (where the propagation of light of a specific wavelength is stopped in certain directions).

Our 3D diffraction-unlimited OBL method also enables ultra-high capacity optical data storage up to petabytes per disc. With this technique, data can be recorded with bit separation down to about 50nm in-plane, which is less than one sixth that for the currently commercially available blu-ray disc (see Figure 3). In this regard, the data capacity can be boosted to 1000GB from 0.6GB of a CD, 4.7GB of a DVD, and 25GB of a blue-ray disc for single-layer recording. Based on the axial resolution of 80nm,⁵ this technique facilitates high-density multi-layer recording which makes it possible to put a petabyte data into a single optical disc with the size the same as the commercial CD or DVD.

In conclusion, we demonstrated 3D diffraction-unlimited OBL using a doughnut-shaped inhibition beam to prevent photopolymerization at the outer ring of the writing beam. By breaking the diffraction limit in OBL, we can fabricate 3D structures with a smaller feature size and higher resolution, down to the nanometer range. Our 3D diffraction-unlimited OBL provides a new tool to fabricate photonic structures that can work in the visible or even UV wavelength range. It can also facilitate petabyte optical data storage, which builds up a technical platform for future information technology.

Currently, we are working on the fabrication of 3D photonic devices for UV optics and photonics using this diffraction-unlimited OBL technique. We are also applying this technique to obtain ultra-high density optical data storage.