Nanosphere lithography (NSL) is an inexpensive, material specific and high-output nanostructure fabrication process which can systematically produce a 2-D array of periodic structures. It makes use of placing nanospheres in a tightly packed pattern on a substrate in order to create a mask for pattern transfer. By removing the spheres after thin film deposition or etching, the remaining 2-D array on the substrate has triangular shaped nanostructures in a hexagonal pattern, often called a Fischer pattern.

Nanosphere Lithography can be performed using several techniques, each with different variables that will affect the overall size, shape, and uniformity of the nanostructures. In order to minimize the feature size, a mask more complicated than a single layer of nanospheres is required. As the mask increases in complexity, the parameters that need to be adjusted in order to create a uniform pattern also increase. Regular patterns include the vertical deposition method, spin coating and mere dipping. The vertical deposition method yields a better quality of ordered nanosphere array compared to other methods.

Known for its good orderliness and simplicity, the vertical deposition method has been adopted in many research studies employing nanosphere lithography. The general setup for vertical deposition method consist of an apparatus for maintaining a constant ambient temperature such as a dry-bath, or a basic oven, that is capable of providing a stable stream of air or gas, and a vial for holding the nanosphere in solution form with the sample to be coated lying upon the sidewall of the vial. The solution typically consists of nanospheres suspended, and a water-alcohol mixture that controls the evaporation rate. Surfactants are sometimes used together to reduce the viscosity. As the nanosphere solution dries up, the nanosphere will be self-assembled into hexagonal close-packed pattern, adhering to the sample. Unfortunately, cracks often exist in the resultant nanosphere coating, regardless of the degree of order. The reason for the persistent formation of cracks is identified to be caused by the movement of “water line” as the water level changes during evaporation of solution. Also, since the concentration increases as the solution dries up, the thickness of the nanosphere coating, or the number of nanosphere layers, will usually be thicker towards the lower end of the sample than that at the upper end.

Flow-controlled vertical deposition method

The problem of uneven layer distribution along the sample can be resolved by a modified vertical deposition method, namely the flow-controlled vertical deposition method. Instead of waiting for the solution to be evaporated, this modified method proactively scans the water level along the entire sample by adjusting the relative position between sample and vial, while keeping the concentration changes to a minimum. This allows a much shorter processing time which also minimize possible sedimentation of nanospheres that will aggravate the unevenness during the deposition process. Thus, this method is ideal for maintaining a more even thickness of coating across the sample.

For addressing the cracking problem, there have been attempts to use a chemical method in conjunction with vertical deposition method. Crack-free colloidal crystal can be obtained by means of hydrolysis of a silica precursor during vertical deposition. Silica species formed acts like a glue among nanospheres to avoid crack formation during evaporation. Other crack-free approach includes self-assembly at air/water/air interface which will form a free standing nanosphere films, although this is not an approach suitable for lithography. After deposition, a hexagonal close-packed nanosphere pattern can be observed under scanning electron microscope (SEM).

Due to the limitations in nanospheres synthesis and deposition, defects among nanosphere arrays are inevitable. Nevertheless with a low defect density, nanosphere lithography is adequate for many applications unless extremely high accuracy is required.

One distinct advantage of nanosphere lithography is that nanospheres that self-assemble Into a hexagonal close-packed pattern can produce features of nano- or even sub-nano scales, provided that the required dimensions of nanosphere are available.