Johannes de Boor, Volker Schmidt
Another powerful tool for the production of ordered and periodic nanostructures on large areas is laser interference lithography. With laser interference lithography patterns with hexagonal and quadratic symmetry are possible, and the created structures adhere strict short and long range ordering, i.e. they show no domains. Amongst others interference lithography is used at the MPI for the fabrication of large area arrays of silicon nanowires. The fabrication steps are displayed schematically in figure 1 and are explained in more detail below.
Fig. 1: Fabrication scheme for the fabrication of silicon nanowires with laser interference lithography and metal assisted etching.
1. Silicon wafer with the desired properties are coated with photoresist by spin coating. Adhesion is improved with a primer.
2. A scheme and a photograph of the setup used for exposure can be seen in figure 2. The sample is brought into an expanded laser beam. By interference of the directly incident laser beam and the beam reflected from the mirror, a sinusoidal intensity pattern is created in the photoresist. The distance between two intensity maxima, the periodicity p, is related to the wavelength of the laser λ and the angle between normal of the substrate and laser θ and given by:
It can be adjusted by a simple rotation of the sample holder.
Fig. 2: Scheme and photograph of the setup used for laser interference lithography. (Ran Ji, PhD thesis 2008, MPI Halle)
3. After exposures, the photoresist is wet developed. If a negative resist is used the unexposed part of the resist is washed away by the developer and an array of resist lines remains. Arrays of holes or dots are created by rotation of the sample after the first exposure and applying a second exposure and developing afterwards. If the sample is rotated by 60o between the two exposures patterns with hexagonal symmetry are obtained, while a rotation of 90o yields structures with quadratic symmetry. Two example SEM images of arrays of photoresist holes and pillars are shown below. Whether pillars or holes are obtained depends on the chosen exposure time; for the fabrication of silicon nanowires arrays of photoresist pillars are necessary.
Fig. 3: Photoresist pillars with a periodicity of 350nm and a diameter of approximately 200nm.
4. Approximately 20nm of gold or silver are deposited on the photoresist structure by thermal evaporation or sputtering.
5. The photoresist and the redundant metal are removed from the substrate so that the silicon substrate covered with a metal film remains. This metal film has holes with approximately the size of the (now removed) photoresist pillars.
Fig. 3: Gold film on Si after removal of photoresist and redundant metal. The holes in the film have the same diameter as the photoresist posts in figure 2. One can also see some metal caps that have not been washed away with the photoresist. They do not disturb further processing.
6. The silicon nanowire arrays are obtained by etching the sample in an HF/H2O2 solution. While position and diameter are determined by the photoresist mask, the length of the wires is an approximately linear function of the etching time. Two SEM images of arrays of Si nanowires are shown below.
Fig. 4: Silicon nanowires obtained from the mask in figure 3 (left), with a diameter of around 200nm. The right hand side shows silicon nanowires from a different sample which exhibit perfect ordering in some areas but bunching in others. At aspect ratios above 10 the individual wires tend to bunch together, probably due to surface forces.
Properties of the nanowires
The distance of the nanowires is given by the periodicity of the photoresist pattern, which is
With λ=244nm the principal minimum is p=122nm. Experimental reasons set the practical limits to 140nm<p<1500nm. The size of the photoresist pillars (and therefore the diameter of the wires) is a function of the exposure time. The resist posts can also be downsized by plasma etching with oxygen plasma. With these two parameters the diameter of the wires d can usually be chosen between 0.4p<d<0.75p, i.e. silicon nanowires with diameters between several hundred and 50nm are obtainable with this method.