Figure 7. Schematic structure of PS-based quasiballistic electron emitter. At that applied voltage, the mean energy of output electrons becomes higher than 2 eV. The corresponding electron temperature is far from the thermal equilibrium. The mean energy of emitted electrons can be tuned well by the applied voltage while keeping narrow energy dispersion. Both the output electron energy distribution and the emission angle dispersion become significantly narrow even at room temperature.
The energy distribution becomes more monochromatic at a low temperature of around K. The relatively low operation voltages and the compatibility with silicon planar processing make it possible to drive the emitter array under an active-matrix mode. Being the energetic, directional, planar, and uniform emission, the quasiballistic emission from PS is insensitive to vacuum pressure, in contrast to the conventional cold cathodes such as field emitters and metal-insulator-metal ones.
Hybrid Nanostructured Porous Silicon-Silver Layers for Wideband Optical Absorption
Far from it, the PS emitter operates in gases and even in solutions. The application studies have been carried out in vacuum flat panel display, multibeam parallel lithography, high-sensitivity image sensor , in atmospheric pressure gases negative ion generation, non-discharge VUV emission , and in solutions H 2 gas evolution, pH control, thin film deposition Koshida, a. As specific approaches, two topics on the development that demonstrates the characteristic feature of the PS emitter are presented here.
In advanced silicon device technology, a high resolution below 10 nm , high throughput, and cost-effective nanofabrication process is strongly required. Though electron beam EB is a very attractive exposure source from a viewpoint of the resolution, the conventional focused EB writer has a seriously limited throughput.
Porous Silicon In Practice Preparation Characterization And Applications
If a practical multibeam exposure scheme could be possible, the usefulness of mask-less EB direct-write should be dramatically enhanced. Its major possible applications are photomask fabrication and mask-less direct-write exposure. Specifications of multibeam parallel lithography systems under development are summarized in Table 2. In the conventional systems, thermionic emitter or thermally assisted field emitter is used as an electron source. Since the employment of active-matrix drive is difficult in that case, broadened electron beam is spatially switched by aperture blanking method for generating multibeam Rio et al.
In contrast, the PS approach is characterized by active-matrix drive of arrayed emitters Esashi et al. The PS emitter array can be fabricated on a Si-wafer substrate by planar processes. Figure 8. A Schematic configuration of electron multibeam exposure system using arrayed nanocrystalline PS emitters. B SEM photographs of emitter array. C Delineated resist patterns. The compatibility of the implemented LSI with the active-matrix operation was confirmed, including the basic function for the electron emitter process variation compensation and the test of integrated devices.
The evaluation was performed with the exposure test system, in which an EB-resist coated target wafer was placed at about 3 mm distance from the emitter surface Figure 8B and the exposed resist pattern is shown in Figure 8C. It has been demonstrated that the integrated nc-Si emitter array is compatible with the active-matrix drive for multi-beam massive parallel exposure, and that the selected emitter pattern is delineated corresponding to the activated emitters. In accordance with the results of beam optics simulation in the prototype system, the miniaturized electron optics is suitable for 10 nm order EB writing.
For the practical use with a throughput comparable to extreme ultra-violet EUV lithography, criteria of the electron beam number and the resolution target to be pursued are 10 6 beams and 5 nm, respectively. From a chemical viewpoint, the PS emitter can be regarded as a supplier of electrons with highly reducing activity. Its direct application is liquid-phase thin film deposition of metals and semiconductors under an electron incident mode Suda et al.
Michael J. Sailor: Porous Silicon in Practice. Preparation, Characterization and Applications
The deposition process is illustrated in Figure 9. Output of quasiballistic electrons of the nc-Si emitter impinges onto the target substrate on which an extremely small amount of salt solutions such as CuCl 2 , SiCl 4 , and GeCl 4 was coated in advance with a thickness of nm. The experiments were done in a N 2 -gas filled glove box. Figure 9. Process of liquid-phase reductive thin-film deposition promoted by direct incidence of quasiballistic electrons emitted from nanocrystalline PS cold cathode.
After the emitter operation for a few minutes, residual solutions were removed, and then thin Cu, Si, and Ge films are formed on the incident area as shown in Figure According to the structure and compositional characterizations of deposited thin films, every film consists of nanoclusters. No contaminations were detected by X-ray photoelectron spectroscopy XPS. Obviously thin films are deposited with no byproducts. Thin films can be deposited at room temperature on varied substrates including insulating layers i. Figure Incident electrons with energy of 10 eV can penetrate 10 nm deep in solutions Emfietzoglou et al.
Thermodynamic investigation supports that the incident electron energy meets the requirement for preferential nucleation of atoms rather than their out-diffusion Suda et al. The theoretical analysis based on the reaction diffusion equation suggests that the deposition rate depends mainly on the incident electron current density J e , and that it reaches a stationary value within 0.
This is consistent with the experimental results. Typical thin film deposition techniques are summarized in Table 3. The most widely used dry processes chemical and physical vapor deposition are established by precise control of temperature, vacuum pressure, and gas flow rate Seshan, The wet electroplating, based on exchange of thermalized electrons at the working and counter electrodes, proceeds at room temperature with gas evolutions.
It is mainly used to deposit thin metal films Schlesinger and Paunovic, Electron-beam-induced deposition EBID , on the other hand, has been studied to form cluster, metal nanowires, thin films, and nanostructures Kiyohara et al.
Preparation, Characterization and Applications
The focused electron beam with high-energies of 10—50 keV in the conventional scanning or transmission electron microscope is transmitted through membranes and then hits the absorbed gases or ionic liquids on the substrate leading to decomposition of molecules. The key issue is to reduce carbon and other contaminations in deposited thin films.
The ballistic electron incidence mode mentioned above is based on the mechanism different from EBID. Unilateral reduction proceeds with neither gas evolution nor by-product generation. In addition, the deposition of thin metal and group IV semiconductor films is available for varied substrates. The PS acoustic devices are composed of a thin-film surface heater electrode, a PS layer, and a c-Si wafer.
At the same time, its volumetric heat capacity C is also significantly decreased. A significant sound pressure amplitude is produced without any mechanical vibrations. Due to the sound emission from still surface, the frequency response covering a fully wide range is free from the mechanical resonance.
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The theoretical limit of frequency response is 1 GHz. No resonant peaks are observed in the whole range of available frequency. The broad-band flat emissivity of the PS device is useful for reproducing complicated ultrasonic communication calls and male-female interactions between mice Kihara et al. Conventional ultrasound emitters cannot be utilized for this application because of a resonant frequency response and a bulky size larger than mice.
As previously demonstrated, mouse mothers were attracted by pup ultrasonic vocalizations USVs reproduced by an nc-Si emitter, while they did not respond to other synthesized sounds. It was also found that the response to pup USVs was enhanced by social experiences Okabe et al. Recent study on mutual recognition between mother and infant suggests that pup USVs looks to have an individual signature used in pup differentiation by mouse mothers, similar to acoustic communication between human mothers and their infants Asaba et al. Regarding thin metal film heaters and underlying thermal insulators, many studies have been conducted by using varied combinations: suspended Al wires-air Niskanen et al.
The basic characteristics of these devices are consistent with the theoretical analyses of the thermo-acoustic effect and its key factors Hu et al. Making use of the non-resonant and broad-band emissivity with no harmonic distortions, possible applications have been pursued to audible compact speaker under a full digital drive, probing source for 3-dimentional object sensing in air, acoustic pressure generator for noncontact actuation, directivity control under phased array configuration, loud speaker, noise cancellation, thermoacoustic tomography, and thermoacoustic sound projector Koshida, c ; Aliev et al.
Including photonic visible luminescence, emerging functions of nanostructured PS has extended to electronics, biometrics, biomedicine, acoustics, thermology, and energetics. In the quantum-size silicon, especially, the emissive properties of photons, electrons, and sound are activated. From a technological viewpoint, cost- and power-effective production of luminescent nc-Si powder or colloid is desired for wide applications. As one practical approach, high-yield fabrication of strongly luminescent colloidal nc-Si dots has been developed by employing in-situ self-regulated process for pulverization of anodized PS by pulsed laser irradiation.
A multiplier tunneling transport through nc-Si dots, on the other hand, induces quasiballistic electron emission. The potential of nc-Si cold cathode has been made clear by using monolayer graphene as a surface electrode. This makes the foundation more solid for applications to massively parallel EB lithography under an active-matrix drive and to reductive thin film deposition of metals and semiconductors.
Based on specific thermal properties of PS, on the other hand, thermos-acoustic device has been developed. Observed broad-band non-resonant sound emission from a compact PS device provides standard ultrasound source for researches in the bio-acoustic communications.
These studies meet in the direction and requirements for diversification of silicon technology. NK: overview, ballistic electron emission, thermo-acoustic device.
TN: visible luminescent Si quantum dots. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The author NK would like to thank Prof. Esashi, Prof. Kikusui, Dr. Kojima, and Dr. Suda for their support and cooperation.
Abderrafi, K. Silicon nanocrystals produced by nanosecond laser ablation in an organic liquid. C , — Adelung, R. In situ nanoscale observation and control of electron-beam-induced cluster formation.