Seung-Ki Joo

Low temperature poly-Si thin-film transistor fabrication by metal-induced lateral crystallization

A new low temperature crystallization method for poly-Si TFTs was developed: Metal-Induced Lateral Crystallization (MILC). The a-Si film in the channel area of a TFT was laterally crystallized from the source/drain area, on which an ultrathin nickel layer was deposited before annealing. The a-channel poly-Si TFTs fabricated at 500/spl deg/C by MILC showed a mobility of 121 cm/sup 2//V/spl middot/s, a threshold voltage of 1.2 V, and an on/off current ratio of higher than 10/sup 6/. These electrical properties are much better than TFTs fabricated by conventional crystallization at 600/spl deg/C.

Pd induced lateral crystallization of amorphous Si thin films

A thin palladium layer (∼40 A) was selectively formed on top of amorphous silicon films before annealing and the effects of palladium layer on the crystallization behavior of the amorphous silicon films were investigated. It was observed that the amorphous silicon right under the Pd layer could be crystallized to grain sizes of several hundred angstroms by annealing at 500 °C. In addition, the area between the Pd thin pads, patterned by lithography, was found to be crystallized to grain sizes of a few tens of microns in length by the same annealing. Such lateral crystallization was found to reach more than 100 μm in some cases. The lateral crystallization phenomenon might be useful for the fabrication of low temperature polycrystalline‐Si thin film transistors, providing large‐grained Si films.

Plasma-Assisted Atomic Layer Growth of High-Quality Aluminum Oxide Thin Films

Thin aluminum oxide (Al2O3) films were grown by the plasma-assisted atomic layer controlled deposition (PAALD) method using Dimethylethylamine alane [(CH3)2(C2H5)N:AlH3] (DMEAA). Al was deposited by the PAALD method, then the Al films were oxidized into Al2O3 by plasma oxidation in the same chamber without breaking the vacuum. Al2O3 thin films of 15 nm thickness were prepared by repetition of this process. Thus prepared Al2O3 thin films exhibited a refractive index of 1.68. The thickness and the refractive index fluctuation of the film over a 4 inch wafer were ±2.3% and ±1.9%, respectively, for the deposited films. The leakage current density and breakdown field were measured to be about 10-8 A/cm2 at 1 MV/cm and 7 MV/cm, respectively. Considerable improvement of the electrical properties was realized by the post oxygen-plasma annealing at 200°C.

Characterization of sputter-deposited LiMn[sub 2]O[sub 4] thin films for rechargeable microbatteries

Thin films of LiMn[sub 2]O[sub 4] spinel were fabricated by RF magnetron sputtering. The as-deposited films were amorphous but could be crystallized into a spinel structure by rapid thermal annealing in an oxygen atmosphere. The electrochemical performance of 2000 [angstrom] thick LiMn[sub 2]O[sub 4] thin-film cathodes were tested in a LiMn[sub 2]O[sub 4]/1M LiClO[sub 4] in PC + DME/Li cell. LiMn[sub 2]O[sub 4] spinel films prepared at 750 C showed good intercalation kinetics and very promising cycling behavior. Room temperature cycling of these films had capacities of about 50 [mu]Ah/cm[sup 2]-[mu]m at 200 [mu]A/cm[sup 2] and maintained more than 98% of their original capacity after more than 1000 cycles.

Fabrication of high-mobility p-channel poly-Si thin film transistors by self-aligned metal-induced lateral crystallization

High-mobility p-channel poly-Si TFTs were fabricated using a new low-temperature process (/spl les/500/spl deg/C): self-aligned metal-induced lateral crystallization (MILC). With a one-step annealing at 500/spl deg/C, activation of dopants in source/drain/gate a-Si films as well as the crystallization of channel a-Si films was achieved. The TFTs showed a threshold voltage of -1.7 V, and an on/off current ratio of /spl sim/10/sup 7/ without post-hydrogenation. The mobility was measured to be as high as 90 cm/sup 2//V/spl middot/s, which is two to three times higher than that of the poly-Si TFTs fabricated by conventional solid-phase crystallization at around 600/spl deg/C.

Fabrication and characterization of an LiMnO thin-film cathode for rechargeable lithium microbatteries

Abstract Thin films of LiMn2O4 spinel were fabricated by r.f. magnetron sputtering. The fresh films were amorphous but could be crystallized into a spinel structure by rapid thermal annealing in an oxygen ambient. The electrochemical performance of LiMn2O4 thin-film cathode was tested in LiMn2O4 thin-film (2000 A)/1 M LiClO4 in propylene carbonate + dimethoxyethane/Li metal arrangements. LiMn2O4 spinel films prepared at 750°C has a favorable structure and grains for lithium-ion intercalation. Room temperature cycling of these films showed good intercalation kinetics and very promising cycling behavior.

Analysis of grain-boundary effects on the electrical properties of Pb(Zr,Ti)O3 thin films

The effects of grain boundaries on the characteristics of Pb(Zr,Ti)O3 (PZT) thin films were investigated by locating the top electrode within grain, on the line boundary, or on the grain-boundary intersection in a controlled manner. It turned out that, when there was no grain boundary in the area measured, excellent ferroelectric and electrical performance was obtained. On the other hand, serious degradation was observed in terms of polarization, leakage current, breakdown field, and fatigue characteristics when grain boundaries were included in the area studied. It was found that degradation of PZT thin films was closely correlated with the length of grain boundary in the area measured.

High-Performance Low-Temperature Poly-Silicon Thin Film Transistors Fabricated by New Metal-Induced Lateral Crystallization Process

Effect of crystalline defects in metal-induced lateral crystallization (MILC) thin film transistors (TFTs) was studied and a new MILC method was proposed to improve the electrical properties of poly-Si TFTs. Defects at channel, which were formed by Ni in conventional MILC method, could be successfully removed by means of asymmetric Ni-deposition. Since the crystalline defects were removed at the channel, field-effect electron mobility increased by a large value up to 120 cm2/Vs, while the leakage current was reduced. Electrical properties of the TFTs fabricated by the new MILC method were dependent on which side of the channel was deposited with Ni.

I‐V characteristics of electron‐cyclotron‐resonance plasma‐enhanced chemical‐vapor‐deposition silicon nitride thin films

Silicon nitride thin films were fabricated by electron‐cyclotron‐resonance plasma‐enhanced chemical‐vapor deposition (ECR PECVD) at room temperature and current‐voltage characteristics were analyzed. A ledge in the first I‐V curve always appeared in ECR PECVD silicon nitride films and then disappeared in the subsequent I‐V curves. It turned out that the trapped charges caused by injection of electrons were responsible for the ledge in the I‐V curves of fresh samples. A new conduction mechanism for low electric field was proposed, namely trapping current by tunneling. This model turned out to be very successful to explain the low‐field I‐V characteristics in ECR‐PECVD silicon nitride films, such as temperature dependence of I‐V curves and the reverse current phenomenon. Computer simulation suggested the trapping cross section as 1×10−16 cm2 and the trap density as 7×1018 cm−3. The calculated trapping cross section corresponds to that of the neutral trap centers, which agrees well with the experimental results.

Pd2Si-assisted crystallization of amorphous silicon thin films at low temperature

An ultrathin palladium layer was deposited on top of amorphous silicon films and the crystallization enhancement of the underlying amorphous silicon films was observed. The crystallization temperature was lowered down to 350 °C while that of intrinsic silicon films is around 600 °C. The degree of enhancement was found to be dependent on the thickness of the palladium layer as well as the annealing temperature. From the microstructure analysis, the formation of Pd2Si precipitates in the amorphous silicon was observed at the initial stage of crystallization and after further annealing, the crystallization of amorphous silicon was observed with the simultaneous split of the preformed Pd2Si precipitates into many small pieces. The mechanism of this abnormal phenomenon is discussed with the theory of epitaxial growth.