|Abstract:||Atomic Layer Deposition (ALD) of aluminum oxide (Al2O3), gallium oxide (Ga2O3) and Zirconium oxide (ZrO2), driven by arrays of microcavity plasmas (MALD), has been demonstrated. Microplasma has been drawn attention due to its unique characteristics such as high electron density (>1016 cm-3), low temperature (electron and ion temperature are ~ 5 and 0.1 eV, respectively) and plasma ignition at atmospheric pressure. Especially, the higher electron density of microplasma is directly related to gas dissociation such as oxygen and ammonia which are co-reactants of semiconductor thin film deposition. The ratio of the local electric field strength to the gas number density (E/N) in microplasma is also increased significantly relative to conventional (macroscopic) plasmas, resulting in higher dissociation of co-reactant. A compact ALD system with reduced volume by at least a factor of five was achievable due to the miniaturized microplasma source operating in a lower frequency AC waveform. Due to the complete reaction between precursors, the stoichiometric value of films presented ~ 1.5 in the crystalline state of Al2O3 and Ga2O3, indicating the presence of negligible levels of impurities and oxygen vacancy, which was verified from EDX, RBS and SIMs analysis. The thin films were grown at 300 K by dissociating oxygen in an array of microcavity plasmas with a reactor back pressure of 5 Torr. Metal-oxide semiconductor capacitors (MOSCAP) fabricated from Al2O3 ¬film deposited on p-Si by MALD with Al top ohmic contact and Au bottom electrodes exhibited breakdown electric field strength of 6.1 MV/cm and a hysteresis width of < 1 mV, indicating near-zero trap charges in the film. In addition, dielectric constant of Al2O3 ¬film, ε_ox, can be derived from the C-V accumulation region. The measured ε_ox of MOSCAP is 9.7 and this value is enough for MOS structures application in drastic conditions, in particular, the high thermal conductivity, high chemical stability, high radiation resistance, and low permeability to alkali impurities. These values along with Al2O3 film having 30 nm thickness are higher than conventional values in MOSCAP. This result is noticeable in that MALD process in room temperature is enabled to produce high-quality film with uniformity and conformality. Furthermore, area-selective deposition at specific locations on a surface with uniform and conformal Al2O3 thin film were attainable by MALD due to the room temperature process. A general limitation of lift-off area-selective ALD (AS-ALD) is that the deposition process requires low temperature (< 50 °C) to avoid the passivation layer damage by thermal energy and plasma source. Since MALD operation is completely room temperature deposition without irregular discharge, which causes damage on fragile substrates such as PMMA and PR. Based on this technique use, the lateral dimension 1 - 10 µm of Al2O3 film was uniformly deposited with ~ 68 nm thickness. Thus, all of results show that how well the MALD process can be compatible to semiconductor area-selective patterning with narrow lateral dimension through simple lithography and lift-off. E-beam lithography was used to achieve ~ 250 nm dimension structure without the etching process.
Furthermore, amorphous Ga2O3 (a-Ga2O3) was deposited on Si, quartz, and PET substrate. Since Ga2O3 is a high bandgap semiconductor material, its crystallinity has been analyzed after post-annealing above 800 °C by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Due to the deposition of a-Ga2O3 on PET substrate, a flexible deep ultraviolet (DUV) photodetector was successfully fabricated. Based on the Tauc law calculation with transmittance of grown film as a function of wavelength, a-Ga2O3 bandgap was measured ~ 5.35 eV, indicating higher photo response at 222 nm UV illumination then typical 254 nm. By increasing surface to volume ratio of a-Ga2O3 surface with etching-free patterning process, photocurrent was half-orders of magnitude increased with the same applied bias voltage on the flat surface DUV photodetector. Furthermore, the optical parametric oscillator (OPO) laser was utilized to measure the fastest response time of a-Ga2O3/PET and a-Ga2O3/Si DUV detector. The rise and fall times of a-Ga2O3/PET detector were 0.34 µs and 11.84 µs, and a-Ga2O3/Si were recorded 0.38 µs and 9.98 µs, respectively.
Finally, optical filters based on alternating layers of zirconium dioxide (ZrO2) and Al2O3 dielectric distributed Bragg reflector (DDBR) structure were fabricated. These ZrO2/Al2O3 multi-layers have been successfully deposited on the quartz substrate in order to realize the bandpass with different central wavelengths in UV range. Depending on the design method of the optical filter, the central passband of transmittance spectra varied from 222 nm to 300 nm. Theses DDBR structures have good periodicity, which was verified by scanning electron microscope (SEM) and secondary ion mass spectrometry (SIMs). As a proof of concept, a krypton chloride (KrCl) excimer lamp, having dominant emission at 222 nm wavelength was used to verify the suitability of the wavelength-window-selection of the optical filter.