An Approach to Lithographically Defined Self-Assembled Nanoparticle Films

Both 2D and 3D colloidal particle “crystals” and patterned nanoparticle arrays deposited from colloidal suspensions are the subject of intense study owing to their potential applications in electronics, photonics, biological and chemical sensors, and catalysis. Colloidal particle crystals and patterned colloidal particles are most often formed using processes such as gravity sedimentation, spin-coating, electrostatic self-assembly, convective deposition, and self-assembly under physical confinement. Most reports have focused on template-directed colloidal self-assembly where the template is defined before the nanoparticles are deposited. A few reports have examined lithographically defined approaches for cutting and growing carbon nanotubes (CNTs) into microand nanoscale patterns. It remains challenging to fabricate large-area mesoscopic patterns of nanoparticle films on flat surfaces with high quality and uniformity. Improved control of the morphology of nanoparticle films over large areas is important for many macroscopic film properties, particularly those important in sensor, catalysis, and optical applications. Here, we report a simple, inexpensive approach that combines bottom-up selfassembly and top-down lithography to fabricate nanoparticlefilm patterns with submicrometer periodicity. In contrast to previous approaches, the nanoparticles are uniformly deposited onto the substrate by spin-coating, and post-deposition interferometric lithography (IL). Reactive-ion etching (RIE) steps are used to define the patterns with dimensions as small as 100 nm. 1D and 2D patterns of nanoparticle films have been successfully fabricated. This approach overcomes the “edge-high effect” (thickening of the film at the edges of the pattern features due to the dynamics of the spin-coating process over topology) and the difficulty in filling nanoscale holes in template-directed self-assembly. This novel sequence of operations combined with subsequent RIE produces more uniform, nearly defect-free, larger-area patterned films with well-defined shape, structure, and size. Spin-coating processes for the self-assembly of colloidal particles have the advantages of being inexpensive, rapid, inherently parallel, and compatible with standard microfabrication processes. IL can produce nanoscale periodic patterns over large areas. In particular, IL, as a top-down technique, is quite fast, low cost, reliable, and scalable—both to large areas and ultimately to volume manufacturing. RIE has been widely used not only for semiconductor processing but also for the control of surface morphology, surface chemistry, and the shape and size of nanoparticle arrays. Here, we report the use of the anisotropic-etch capabilities of RIE to remove unprotected silica nanoparticles. We selected silica nanoparticles for these experiments because they are commercially available, inert to most organic solvents, and compatible with both silicon microfabrication and biological processes. There are four main steps in the fabrication process, as illustrated in Figure 1: spin-coating, IL, RIE, and photoresist (PR) removal. A spin-coating process is used first to self-assemble uniform, large-area nanoparticle films with a desired thickness by controlling the concentration of the colloidal suspension, the number of cycles of spin-coating, and the spin speed. IL is then used to define PR patterns atop the nanoparticle films. The periodicity, pattern, and duty factor (line/space ratio) are easily controlled in the IL process. Subsequent RIE with a gas mixture of O2 and CHF3 was used to etch away the C O M M U N IC A TI O N S