(Invited) Electronic and Acoustic Applications of Anodized Nano-Crystalline Silicon

Electrochemically anodized nano-crystalline Si (nc-Si) behaves as a functional material with a confined structure. In the electron transport through a chain of nc-Si dots, phonon scattering losses are minimized due to discrete energy levels in the conduction band, and then ballistic electrons are generated at the early stage of injection [1]. This induces efficient ballistic hot electron emission from an nc-Si diode. It is possible to use this emitter not only in vacuum, but also in atmospheric-pressure gases and solutions [2]. Extremely lowered thermal conductivity and heat capacity of the nc-Si layer [3], on the other hand, significantly enhances thermo-acoustic effect. In contrast to the conventional piezoelectric transducer, the thermally induced acoustic emission exhibits no resonant peaks in the frequency response. Here we discuss topics in the studies on these subjects.

The nc-Si emitter is a kind of MIS diode consisting of a thin film surface electrode, an nc-Si layer (~1 μm thick), a crystalline silicon wafer substrate, and a back contact. The nc-Si layer includes chains of nc-Si dots (~3 nm in mean diameter) interconnected with tunnel oxides. When a positive voltage is applied to the surface electrode, electrons are accelerated in the nc-Si layer toward the outer surface, and then some of them are emitted through the surface electrode as ballistic hot electrons with a small angle dispersion (±10^°with respect to the normal). At applied voltages of 12-20 V, the mean energy of output electrons reaches 3-7 eV [2].

One possible application of the nc-Si emitter is the use as an exposure source of multibeam parallel lithography. The emitter array is fabricated on a Si substrate by planar processes. Each emitter with an active area of 10 μm☐ is connected to an active matrix driving circuit through a TSV technique. The evaluation was performed with the 1:1 exposure test system, in which an EB-resist coated target wafer was placed at about 4 mm distance from the emitter surface. The wafer was grounded, and emitter surface was set at -5 kV for accelerating the electron. It has been demonstrated that the selected emitter patterns is delineated well corresponding to the activated emitters [4]. The integrated nc-Si emitter array is compatible with the active-matrix drive for multi-beam massive parallel lithography.

Another application is thin film deposition using the reducing activity of ballistic hot electrons. When the emitter is driven in metal-salt solutions such as CuSO₄, CuCl₂, NiCl₂, SiCl₄, and GeCl₄, injected electrons directly reduce Cu²⁺, Si⁴⁺ and Ge⁴⁺ ions, and then thin Cu films [5] and amorphous Si and Ge films [6,7] are grown on the emitting area with neither contaminations nor byproducts. Based on this observation, a printing scheme has been employed, in which a target substrate is separately located in close proximity to the emitter. A gap between the nc-Si emitter and the target substrate is controlled by a piezoelectric actuator depending the ambient (N₂ gas) pressure. Emitted electrons hit metal-salt solutions on the substrate leading to the reducing reaction. The first printing experiment was sought for deposition of thin metal film, where an extremely small amount of CuSO₄ or CuCl₂ solution was coated on a single-crystalline Si wafer and used as a target substrate. It has been observed that thin Cu films are deposited on the irradiated area.

The broad-band flat emissivity of the nc-Si thermo-acoustic device is useful for developing low-distortion digital speaker [8] and reproducing complicated ultrasonic communication calls between mice [9].

References

1. N. Mori, H. Minari, S. Uno, H. Mizuta, and N. Koshida, Appl. Phys. Lett. 98, 062104 (2011).

2. N. Koshida, T. Ohta, B. Gelloz, and A. Kojima, Curr. Opin. Solid State Mater. Sci. 15, 183 (2011).

3. N. Koshida, Thermal Properties of Porous Silicon, in “Handbook of Porous Silicon”, ed. L. Canham (Springer, 2015), pp 207-212.

4. N. Koshida et al, Proc. SPIE Symp. on Advanced Lithography, Vol. 9423, 942313 (2015).

5. T. Ohta, B. Gelloz, and N. Koshida, ECS Solid State Lett. 13, D73 (2010)

6. N. Koshida, A. Kojima, T. Ohta, R. Mentek, B. Gelloz, N. Mori, and J. Shirakashi, ECS Solid State Lett. 3(5), P57 (2014).

7. R. Suda, M. Yagi, A. Kojima, R. Mentek, N. Mori, J. Shirakashi, and N. Koshida, Jpn. J. Appl. Phys. 54, 04DH11 (2015).

8. N. Koshida, D. Hippo, M. Mori, H. Yanazawa, H. Shinoda, and T. Shimada, Appl. Phys. Lett. 102, 123504 (2013).

9. A. Asaba, S. Okabe, M. Nagasawa, M. Kato, N. Koshida, K. Mogi, and T. Kikusui, PLoS One Vol. 9(2), e87186 (2014).