Researchers have used two types of direct laser writing (DLW) to draw line gratings on a glass substrate at resolutions down to 100 nm. [Image: Qiulan Liu, Zhejiang Lab and Zhejiang University]
Scientists in China have used direct laser writing to create tiny features spaced just 100 nm apart in a monomer material at high speed (Opt. Lett., doi: 10.1364/OL.552034). They say that their new optimized form of printing might in future be used to fabricate a range of optical structures, such as photonic crystals, microolenses and metamaterials.
Skipping the mask
Conventional photolithography involves creating features in a photosensitive material by shining laser light through a patterned mask, and then using etching or other processes to transfer those features to the underlying substrate. Direct laser writing (DLW) does away with the mask by using a very intense laser to convert specific portions of a monomer-based photoresist into polymer, with the material moving in a well-defined sequence beneath the beam. The nonlinearity resulting from the high intensity can yield 3D polymer structures with feature sizes below the diffraction limit.
Numerous research groups have used the technique to produce ever smaller feature sizes, including lines just 50 nm wide and nanowires with features as small as 7 nm. But one sticking point has been improving resolution—the minimum distance between two adjacent features. This objective is hampered by the tendency of intense beams to polymerize regions of the substrate beyond the feature being written.
High resolution, high speed
In the latest work, Xu Liu of Zhejiang University and colleagues in Hangzhou and Chongqing have used two variants of DLW to maximize resolution while achieving high writing speeds. One of these is multiphoton DLW. This technique involves firing laser pulses whose wavelength is not absorbed by the substrate, but which are intense enough to liberate two photons simultaneously. The other approach is DLW with peripheral photoinhibition, which uses a second beam to suppress polymerization in the immediate vicinity of the monomer region being targeted.
Liu and colleagues used a femtosecond pulsed laser at 525 nm to excite the monomer medium and a picosecond laser with a very slightly longer wavelength (532 nm) to inhibit polymerization. They set up their system to perform excitation a fraction before inhibition, introducing a roughly 3-ns delay between the two beams by sending them along different optical paths.
The researchers found that by using both techniques, they could print out line gratings with 100 nm resolution at a writing speed of 100 μm/s
The researchers employed a monomer called PETA that they combined with the substance Bis (2,2,6,6-tetramethyl-4-piperidyl-1-oxyl) sebacate (BTPOS) to try and prevent polymer chains from linking up with one another and causing adjacent features to merge. They also used a spatial light modulator to modulate both beams, while additionally taking steps to correct wavefront aberrations and prevent the laser from losing focus or fluctuating power.
Combining techniques
The researchers found that by using both techniques, they could print out line gratings with 100 nm resolution at a writing speed of 100 μm/s (even if the spacings were somewhat less uniform than they were at 120 nm). They point out that this resolution exceeds the diffraction limit of 128 nm as dictated by the laser's wavelength and the optical system's numerical aperture. They also showed that they could crank out the lines at an impressive 1000 μm/s while still achieving resolutions down to 120 nm.
Liu and colleagues also used multiphoton DLW to create 3D structures in the form of woodpiles—layers of cylinders stacked on top of one another, with each layer at right angles to the one below. They were able to make woodpiles with cylinders spaced just 225 nm apart and with parallel-layer separations of 318 nm—just beyond the 320 nm diffraction limit.
The team also reckons that its system is ideal for printing devices with precisely, closely spaced optical waveguides that could be used in displays providing virtual or augmented reality. The researchers say that the next stage in their work will involve printing nanostructures at still higher speeds, aiming to reach 10 or even 100 mm/s at high resolution. They also intend to make their technique more stable by improving the photoresist.