[16a-B10-1~13] 13.4 Si wafer processing /Si based thin film /MEMS/Integration technology

[16a-B10-7] Printing deposition of thin group IV films using ballistic hot electron emitter

〇Ryutaro Suda¹, Mamiko Yagi¹, Akira Kojima¹, Jun-ichi Shirakashi¹, Nobuyoshi Koshida¹ (1.Tokyo Univ. of Agri. & Technol.)

Keywords:Nanocrystalline silicon, Ballistic hot electron, Printing deposition

Nanocrystalline silicon (nc-Si) diode acts as a supplier of highly reducing hot electrons. In salt solutions, injected electrons induce direct reduction of positive ions leading to the growth of thin films. Under a situation that a small amount of CuSO₄, NiCl₂, CoSO₄, ZnSO₄, SiCl₄, GeCl₄, and mixture SiCl₄+GeCl₄ solutions is dripped on the emitter surface, thin Cu, Ni, Co, Zn, Si, Ge, and SiGe films are uniformly deposited, respectively, on the emitting area. For enhancing the practical usefulness of this effect, we have developed a printing mode, where a target substrate is separately located in close proximity to the emitter, and then emitted electrons impinge upon a solution-coated substrate. As previously reported, when Cu-salt solution coated substrates are irradiated with emitted electrons, reductive reaction of Cu²⁺ ions efficiently proceeds followed by the growth of thin Cu films. We report here the results of the printing deposition of thin Si and Ge films. On Cu substrates coated with SiCl₄+solvent mixture solutions, we observed uniform deposition of thin Si films in the same way as metals. The results indicate that impinging energetic electrons preferentially reduce Si⁴⁺ ions at the solution surface followed by the nucleation for the growth of thin Si film. Similar thin film deposition was obtained from GeCl₄ solution. We estimated the free energy of a cluster formation, as a function of incident electron energy on a basis of thermodynamic nucleation analysis. The analyses of ballistic electron effects suggest that there is a critical energy (about 10 eV) for promoting preferential reduction of target ions within the penetration depth in solutions followed by the nuclei formation for the growth of thin films. The mean energy of ballistic hot electrons emitted from nc-Si diodes fits well the criterion for triggering the cluster formation. As being a clean, low-temperature, damage-less, and power-effective process, the unilateral reduction mode presented here provides alternative means of thin film deposition on versatile substrates.