Infiltration and Inversion of Holographically Defined Polymer Photonic Crystal Templates by Atomic Layer Deposition

Abstract

The demonstration of a practical technology for 3D optical microfabrication is a vital step in the development of photonic-crystal-based optical signal processing.[1] However, the extension of the optical methods that dominate integrated electronic circuit fabrication to three dimensions is a formidable materials-processing challenge: such a process must be capable not only of sub-micrometer pattern definition in three dimensions, but also of the transfer of this pattern into a homogeneous dielectric with an appropriately high refractive index. In a companion paper, [2] we show that two optical methods, holographic lithography [3] and direct two-photon laser writing, [4][5][6] can be combined to create a rapid and flexible method for the definition of photonic crystal device structures in photoresist. In this communication, we report a further essential step towards the creation of devices operating within a full photonic bandgap: we have used atomic layer deposition (ALD), itself an established semiconductor processing technique, to create high-index TiO 2 inverted replicas of holographically defined photonic crystals, followed by removal of the polymeric template by plasma etching. A range of techniques for 3D optical lithography has been demonstrated. A 3D photonic crystal structure can be written by holographic lithography, [3] which makes use of a periodic interference pattern generated by a multiple-beam interferometer to expose a thick layer of photoresist. 3D microstructures, both periodic and aperiodic, can also be generated by point-by-point exposure of the resist by two-photon absorption at a laser focus. [4][5][6][7] Two-photon laser writing is a serial process; point-by-point fabrication of a 3D photonic crystal is necessarily slower than holographic lithography, which is capable of defining the entire periodic structure in a single laser pulse.[3] The two techniques are complementary: two-photon laser writing can be used to modify a holographic exposure.[8]We have shown that, by imaging the distribution of photochemical change induced by holographic exposure, it is possible to align a subsequent two-photon exposure with the 3D photonic crystal lattice to achieve the precise registration that is required of a device structure embedded in a 3D photonic crystal. [2] This hybrid technique is rapid and flexible, but the polymeric resists used for 3D microfabrication have refractive indices n in the range 1.4-1.6, which is too low for most device applications. Devices based on waveguides and microcavities embedded within a photonic crystal [1] are designed to operate at frequencies within a complete (omnidirectional) photonic bandgap in order to suppress radiative loss; [9] to create a complete photonic bandgap, even in an optimized air-dielectric structure, a refractive contrast of at least 1.9 is necessary.

Periodic nanostructures for photonics

“…Thus, infiltration with a high-index material is desirable. First results on the infiltration of TiO 2 via CVD at room temperature have recently been presented [276]. Unfortunately, typical deposition temperatures for silicon via CVD (see Section 2.2.3.2) are not compatible with the glass temperature of the resist.…”

Introducing Defects in 3D Photonic Crystals: State of the Art

“…Advances in templating silicon with polymeric structures [55,56] indicate possible routes to create structures with enhanced refractive-index contrast. Also, very recently, Summers and coworkers [92] demonstrated the use of atomic layer deposition to replicate the structure of a holographic PC into TiO 2 , a highrefractive-index material which is transparent in the visible region. Finally, since the exposure wavelength is directly proportional to the lattice parameter of the crystal, the chemistry of the photoinitiating system may need to be changed to tune the spectral position of the optical features.…”

Optimized nanoporous materials.

“…A scanning electron microscope (SEM) image of a Ni inverse opal is shown in Figure 6-4. The Al 2 O 3 dielectric was deposited using atomic layer deposition (100 cycles), which was previously shown to be conformal within 2D [139] and 3D porous structures, [140][141][142] assuming enough time is allotted to allow the precursors to diffuse into the structures. However, we were unable to directly observe the conformal dielectric layer in the SEM due to the thickness of the Al 2 O 3 (ca.…”