All-Printed Inorganic Logic Elements Fabricated by Liquid Embossing
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ABSTRACT The liquid embossing process, devices made by this process, and their characteristics are presented. Structures fabricated and discussed here include: conductive lines with cross line resistance greater than 100 GQ and resistivities 4 times that of the bulk material, multilayer structures with etched sacrificial materials, vias that conduct through an embossed insulating layer, photodetectors made with nanocrystal solutions of CdSe, and all printed inorganic field effect transistors. EXPERIMENT Process Liquid embossing l121is a physical process used to create features in functional materials. Though this process can pattern a wide range of materials this paper is restricted to a discussion of patterning solutions of nanocrystals, spin on glasses (SOG) (Ohka T7), and insulating polymers (Japanese Synthetic Rubber AL 3046). Our paper gives a brief overview of the process, then descriptions of the types of devices produced with liquid embossing and their characteristics. The devices discussed include conducting metal lines, released mechanical structures, and transistors. A stamp is created by casting an elastomer (PDMS) from a master. The master is typically made of photoresist on a silicon wafer, but can be any surface with raised physical features. As shown in Fig. 1,the stamp is brought into contact with a thin film of functional material in liquid phase. The raised features on the stamp emboss through the thin film to the underlying substrate and remove material. The stamp is then removed while the functional material is still in liquid phase; the material can reflow if embossed again but otherwise will remain patterned. Finally the material is cured, which removes the solvents and sinters or cures the material. Thin films are created by either a drawdown bar or conventional spin coating. In the draw down process a drop of liquid material is placed on a substrate, and a ground metal cylinder is pulled over the substrate. The only pressure on the substrate is the weight of the cylinder. The thin films obtained are very consistent for a range of liquid material properties including viscosity, vapor pressure, and surface wetting.
135 Mat. Res. Soc. Symp. Proc. Vol. 625 © 2000 Materials Research Society
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Figure 1: Schematic of the liquid embossing process: (a) the elastomeric stamp is brought into contact with a thin film of liquid, (b) the stamp contacts the underlying substrate and selectively patterns the liquid film, (c) the stamp is removed and the liquid film remains patterned. Single-Laver Structures We have patterned structures with a wide variety of resolutions and geometries. 200 nm line width gratings were fabricated over areas greater than 4 cm with high reliability, (Fig. 2a). Other structures with non-repeating features have been fabricated over the entire surface of a 4" wafer, (Fig. 2b). Typical solid film thickness range from 50 nm 400 nm.
30~5
nmlm 240 nm
28171nm
1408nm 25 pm
a/b
0
nm
7
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Figure 2: Atomic Force Microscope images of single layers of patterned Au: (a)
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