SFIL Templates

(Much of this work was carried out by Motorola Labs.1-4)
SFIL imprint templates are fabricated using an analog of the photomask fabrication process. In a typical Cr-based template process, a 6-in ´ 6-in ´ ¼-in fused silica reticle blank coated with a thin Cr film is spin-coated with an electron beam resist, and baked to drive off excess casting solvent. The plate is then exposed in a high resolution direct-write e-beam tool. The resist is then developed, leaving a resist pattern that exposes selected portions of the underlying Cr film. This resist pattern is used as an etch mask to pattern the Cr with Cl-based RIE. The resist is removed after etching, leaving behind a patterned Cr layer on the silica substrate. A fluorine-based RIE transfers the image into the fused silica substrate to a depth of 100 to 200 nm, depending on design constraints. The Cr layer is typically left on the templates until after the cutting process, which is described below; this facilitates template pattern recognition, and minimizes mistakes in cutting.
Figure 1. Standard Cr-based SFIL template fabrication process.
The Cr layer is not used in the SFIL process as a photon absorber as it is in projection lithography, but it serves only as a charge dissipation layer during e-beam patterning and as a hard mask during the quartz feature definition etch. As a result, the Cr layer needs to be only thick enough to possess a sufficient conductivity and to withstand the SiO2 etching. The template fabrication process was modified to by thinning the Cr layer from ~1000 Å to ~100 Å, and also by using a thinner layer of e-beam resist. These process modifications have provided the ability to fabricate templates with features smaller than 20 nm. Figure 2 shows SEM images of template features down to 20 nm.
a) b) c) d)
Figure 2. SEM images of templates fabricated using the thin-Cr process. a) 60 nm, b) 40 nm, c) 30 nm, and d) 20 nm features.
Template fabrication schemes employing a transparent conducting oxide into the final template have also been investigated. The addition of a blanket conducting layer dissipates charge during SEM inspection of the final templates. One such scheme involves coating the fused silica plate with a film of indium tin oxide (ITO), which is transparent and conducting. The ITO film is then covered with a film of deposited SiO2, followed by e-beam resist. The resist is patterned using an e-beam tool, and chemically developed. The patterned resist is used as an etch mask to pattern the underlying SiO2 film. A convenient result is that the ITO serves as an etch stop for the SiO2 etch process. The resist is finally stripped, resulting in an imprint template of patterned SiO2 features resting on a blanket film of ITO on fused silica substrate. The SiO2 features will define the imprinted features in the SFIL process, while the ITO film is transparent to allow the SFIL process exposure and conducting to allow SEM inspection of the template.
Figure 3. ITO-based SFIL template fabrication process.
An added benefit of this process is that since the ITO serves as an etch stop for the SiO2 patterning etch, the template feature depth is defined solely by the thickness of the deposited SiO2 film. SEM images of templates made using this process are shown in Figure 4.
Figure 4. SEM image of 20 nm template feature fabricated using the ITO process.
A third template fabrication scheme consists of spinning a film of hydrogen silsesquioxane (HSQ) on the ITO layer, as shown in Figure 5. The HSQ is directly written with e-beam lithography, and the unexposed regions are developed away, leaving the cured HSQ topography. In its cured state, HSQ becomes a durable oxide making it a very convenient material for direct patterning of SFIL template relief structures. One benefit of this scheme is that it eliminates the etching processes associated with other template fabrication methods.
Figure 5. HSQ-based SFIL template fabrication process.
This process is by nature a negative-tone process. Figure 6 shows SEMs of an HSQ template demonstrating resolution as small as 20 nm.
a) b) c)
Figure 6. SEM images of HSQ-based template. a) 60 nm, b) 20 nm features.
  1. D.J. Resnick, et al.. "High Resolution Templates for Step and Flash Imprint Lithography." Proc. SPIE 4688: 205 (2002).
  2. W.J. Dauksher, et al.. "Characterization of and Imprint Results using ITO-based Step and Flash Imprint Lithography Templates." J. Vac. Sci. Tech. B 20(6): 2857-2861 (2002).
  3. D.P. Mancini, et al.. "Hydrogen Silsesquioxane for Direct E-beam Patterning of Step and Flash Imprint Lithography Templates." J. Vac. Sci. Tech. B 20(6): 2896-2901 (2002).
  4. T.C. Bailey, et al.. "Template Fabrication Schemes for Step and Flash Imprint Lithography." Microelectron. Eng. 61-62: 461 (2001).