Direct imprinting of liquid silicon

T. Masuda, T. Shimoda
Japan Advanced Institute of Science and Technology,
Japan

Keywords: silicon, imprint, printed, electronics

Summary:

Silicon is one of the most important materials for electronics. The most widespread process for microfabrication technology of silicon is photolithography including etching process. However, huge capital outlay and large-scale equipment has been recognized as a large issue to be solved. In order to overcome these inherent problems in the conventional processes, we have been proposing a new silicon patterning method, in which “liquid silicon” is imprinted to make solid silicon patterns. We synthesized a polymeric precursor solution for semiconducting silicon named “liquid silicon”, as show in Figure 1 [1,2]. Either p-type or n-type liquid silicon was also synthesized by dissolving appropriate amounts of white phosphorus or decaborane, respectively, in the solution [3,4]. The liquid silicon was converted to solid silicon via dehydrogenation [5]. Here, the liquid silicon films were directly imprinted to form well-defined and fine silicon patterns with dimensions of several hundreds of nanometers or less. The approach to fabricate silicon patterns using nanoimprinting included the following steps (Figure 2): (a) the liquid silicon film was spin coated onto a substrate, and was cured at 160°C for 5 min on a hot stage; (b) a mold was pressed into the film at 10 MPa at 160°C for 10 min followed by 10 MPa at 200°C for 10 min to solidify the film; (c) the mold was released after cooling down to 160°C; and (d) the resultant patterned film was annealed at 400–800°C for 20 min to complete the conversion to solid silicon. First, the liquid-to-solid conversion was investigated on flat films. Figure 3(a) shows a photographic image of the solution-processed silicon films with a thickness of 80 nm and with various annealing temperatures. The film’s color shows dark brown except for the film with 800°C. This change in color at 800°C is attributed to crystallization, as supported by Figure 3(b), in which Raman spectra of the films are shown. The typical phonon bands of amorphous silicon at 470, 388, 302, and 150 cm−1 are observed in the films annealed below 700°C, whereas a sharp peak corresponding to crystalline silicon at 515 cm−1 was observed in the film annealed at 800°C. The structural, optical, and electrical properties of the silicon films were comparable with silicon films prepared using conventional vacuum-processing. Thermal analysis and scanning electron microscope (SEM) images during the annealing clarified that the imprinted liquid silicon was converted to solid silicon with a volume shrinkage of 53%–56% due to liquid-to-solid conversion. However, the well-defined angular patterns as well as electrical properties were preserved after imprinting process [6,7]. The imprinted crystalline silicon patterns were shown in Figure 4. Direct imprinting of liquid silicon is practical approach to realize solution-processed silicon devices without photolithography process. Moreover, the technique produced well-defined silicon patterns with high resolution and high throughput, demonstrating its potential for next generation silicon devices in a field of nano-structured devices. The elimination of the conventional photolithography process in microfabrication of silicon devices is particularly important for realization of printed silicon electronics.