Lift-Off Patterning of Nano-Crystalline Quantum Dot Films

The ultra-thin films formed using colloidal solutions of nanocrystalline quantum dots (NQD) show good promise in display/LED, as well as nano-labeling applications between others (e.g. [1]).

Regardless of application, to become a part of the functional device the NQD film must be patterned accordingly. The conventional top-down patterning sequence involving photolithography and etching cannot be used in this case as the process would results in the critical damage to the NQD film. Hence, alternative patterning technologies suitable for NQD films patterning need to be pursued. The goal of this contribution is to continue exploration of an approach to NQD film patterning based on the lift-off principle [2]. 

In electronic/photonic device manufacturing the lift-off process is a method allowing patterning of the thin-film using a sacrificial material, most commonly a photoresist. The lift-off patterning is employed to created desired geometries in the films which are either very difficult to etch (e.g. gold) or are not sufficiently structurally coherent to be subject to a conventional photolithographic process without enduring permanent damage. The thin-films comprised of nano-scale materials such as nandots, nanowires and nanotubes fall into this last category. The principle of the NQD film pattering using lift-off process is demonstrated in Fig.1.

In this study NQDs deposited were CdSe/ZnS core/shell NQDs featuring average diameter of 6.5 nm and corresponding red (620 nm) emission wavelength. Two deposition methods used in this study was a mist deposition and a conventional spin coating. The method of mist deposition converts colloidal solution, in this case CdSe/ZnS NQDs suspended in toluene, into a fine mist of sub-micrometer sized particles using an atomizer. The mist droplets are carried from the atomizer to the deposition chamber by ultrapure N₂carrier gas and then coalesce on the substrate in the presence of an electric field. The same colloidal solution was used in the preparation of the samples by spin coating.

Silicon wafers used as substrates are first covered by spin coating with a layer of positive 1811 photoresist to the thickness of about 1 µm and then cured at 100^oC. The resist covered wafers were subsequently exposed through the standard Cr-photomask in an aligner/exposure tool following which photoresist was develop using in 351 developer (1:4) diluted with Di water for 1 minute (Fig.1a). Then, CdSe/ZnS QD film was blanked deposited during 20 min. long mist deposition process and thermally cured (Fig. 1b). Then, the samples were exposed for 20 min. to 20% O₂in nitrogen plasma at the pressure of 1 Torr at the power of 200 watts (Fig. 1c). In another experimental run instead of plasma ashing resulting in the partial NQD film lift-off, the photoresist removal was carried out by immersion of the wafer in acetone.

The results obtained in this study demonstrate effectiveness of the lift-off process in pattern definition of NQD films. Figure 2 shows patterns created in NQD films using proposed technique.

In the full account of this work a complete set of experimental results obtained in the course of this investigation is presented and possible applications of the processes under investigation is considered.

  References

[1] A. Kshirsagar, et al., ECS J. Sol. St. Sci.Tech., 2(5), R87, (2013).

[2] A. Sabeeh, et. al.,  7^th Intern. Symp on Low-Dimensional Nanoscale Electronic and Photonic Devices, ECS Fall Meeting, Cancun, Mexico, Oct. 5-10, 2014.