Yoshihiro Todokoro

Ultrathin silicon dioxide layers with a low leakage current density formed by chemical oxidation of Si

Chemical oxidation of Si by use of azeotrope of nitric acid and water can form 1.4-nm-thick silicon dioxide layers with a leakage current density as low as those of thermally grown SiO2 layers. The capacitance–voltage (C–V) curves for these ultrathin chemical SiO2 layers have been measured due to the low leakage current density. The leakage current density is further decreased to ∼1/5 (cf. 0.4 A/cm2 at the forward gate bias of 1 V) by post-metallization annealing at 200 °C in hydrogen. Photoelectron spectroscopy and C–V measurements show that this decrease results from (i) increase in the energy discontinuity at the Si/SiO2 interface, and (ii) elimination of Si/SiO2 interface states and SiO2 gap states.

Detection and printability of shifter defects in phase-shifting masks. II: Defocus characteristics

One drawback of phase-shifting lithography lies in the high printability of shifter defects. A transparent object on masks (shifter defects) produces lead or lag of the optical phase, which causes an unique printability with respect to focus level. We investigated the printability of shifter defects on phase-shifting masks in out-of-focus conditions. Two types of shifter defects related to the phase angle were investigated: (a) a phase error which is a shifter layer having an incorrect thickness and (b) a phase angle defect which is a defect having a phase angle other than 180°. Phase error causes asymmetrical pattern profiles. It was found that the illumination coherence factor (σ) has a strong influence on the allowance of the shifter thickness (phase error). The effect of phase angle defects located in a large clear area and in small features was investigated. The phase angle defects have an optimum focal plane other than the best focal plane. A defect with a low phase shift located in fine patterns has high printability. In order to establish a mask fabrication process for production-worthy phase-shifting masks, further investigation in the detection of low phase angle shifter defects is necessary.

Detection and Printability of Shifter Defects in Phase-Shifting Masks

Because of the high printability of shifter defects in phase-shifting masks, it is worthwhile to characterize the inspection and printing of the shifter defects. The detectability and printability of shifter defects as a function of size and location have been investigated by experiments and simulation. A test mask with various programmed shifter defects was inspected by means of a die-to-die inspection system and printed in positive resist with an i-line stepper. Corner defects are difficult to detect and have low printability. A defect located in small features has high printability. We have also investigated the detectability and printability of the phase angle defects which have phase angles other than 180°. Defects with 120° to 180° phase angles have high printability. Defects with phase angles below 90° are not printed.

Studies on interface states at ultrathin SiO2/Si(100) interfaces by means of x-ray photoelectron spectroscopy under biases and their passivation by cyanide treatment

The energy distribution of interface states at ultrathin oxide/Si(100) interfaces is obtained using a new method, i.e., x-ray photoelectron spectroscopy measurements under biases between the metal overlayer and the Si substrate of the metal-oxide-semiconductor (MOS) devices. Ultrathin thermal oxide layers formed at 450 °C in oxygen have an interface state peak near the midgap and it is attributed to isolated Si dangling bonds with which no atoms in the oxide layer interact. On the other hand, thermal oxide layers formed at 650 °C have a two-peaked structure, one peak above and the other below the midgap, and they are attributed to Si dangling bonds with which an oxygen or Si atom in the oxide layer interacts weakly. The density of the interface states, especially that near the midgap, decreases drastically by cyanide treatment, i.e., the immersion of Si in a KCN solution for a few seconds followed by a rinse in boiling water, performed before the oxide formation. It is suggested that cyanide ions penetrat...

Decrease in the leakage current density of Si-based metal–oxide–semiconductor diodes by cyanide treatment

Crown-ether cyanide treatment, which includes the immersion of Si in KCN solutions containing 18-crown-6 molecules, is found to greatly decrease the leakage current density of Si-based metal–oxide–semiconductor (MOS) diodes. The decrease by one order of magnitude for the single crystalline Si-based MOS diodes is attributable to the elimination of Si/SiO2 interface states by reaction with cyanide ions and formation of Si–CN bonds. The reduction in the leakage current density by two orders of magnitude is caused for polycrystalline Si-based MOS diodes, and this decrease is attributed to the passivation of trap states in poly-Si as well as the interface states.

Decrease in gap states at ultrathin SiO2/Si interfaces by crown-ether cyanide treatment

A simple method to passivate interface states at ultrathin SiO2/Si interfaces is developed. In this method, ultrathin SiO2-covered Si is immersed in a KCN solution containing crown-ether, followed by a rinse in water at 25 °C. The conductance–voltage measurements show that the interface state density is decreased to ∼1/10 by this crown-ether cyanide treatment. The capacitance–voltage measurements show that contamination by K+ ions is effectively avoided by the inclusion of crown-ether. These results demonstrate that crown-ether molecules effectively capture K+ ions and consequently CN− ions effectively may react with defect states, probably forming Si–CN bonds. The passivation of interface states by the cyanide treatment improves the electrical characteristics of metal–oxide–semiconductor tunneling diodes.

Mechanism of platinum-enhanced oxidation of silicon at low temperatures

The mechanism of platinum (Pt)-enhanced oxidation of Si below 300 °C has been investigated by means of high-resolution x-ray photoelectron spectroscopy. When a Pt layer is deposited on the ∼1-nm-thick silicon oxide-covered Si, low-temperature heat treatment grows the silicon oxide layer between the Pt layer and the Si substrate, while silicon oxide is formed mainly on the Pt layer in cases where Pt is directly deposited on the Si substrate. Oxidation is enhanced by the application of a positive bias voltage to the Si substrate with respect to the Pt layer during the heat treatment of the specimens with 〈∼4 nm Pt/silicon oxide/Si(100)〉 structure in oxygen, and in this case, a ∼8-nm-thick oxide layer is formed at 300 °C for 2 h. It demonstrates that oxygen ions are the moving species in the oxide layer. The plots of oxide thickness with respect to oxidation time are linear in the oxide thickness region below 3∼4 nm, indicating that the reaction at the interface is the rate-determining step. The activation e...

Nitridation of silicon oxide layers by nitrogen plasma generated by low energy electron impact

Low temperature nitridation of silicon oxide layers by nitrogen plasma generated by electron impact is investigated using x-ray photoelectron spectroscopy (XPS) and synchrotron radiation ultraviolet photoelectron spectroscopy and it is found that a large amount of nitrogen can be incorporated in the layers. The valence band structure of the oxide surface nitrided at 25 °C is similar to that of Si3N4, while that nitrided at 700 °C resembles the mixture of silicon oxide and silicon oxynitride. Measurements of XPS depth profiles show that the nitrogen concentration is high near the surface and the oxide/Si interface.

LOW TEMPERATURE CATALYTIC FORMATION OF SI-BASED METAL-OXIDE-SEMICONDUCTOR STRUCTURE

Si‐based metal–oxide–semiconductor structure is formed at temperatures as low as 300 °C using the catalytic activity of the platinum (Pt) layer. X‐ray photoelectron spectroscopy and transmission electron micrography measurements show that heat treatments of the ∼5 nm‐Pt/∼1 nm‐chemical oxide/Si(100)〉 devices at 300 °C increase the thickness of the oxide layer to 4–4.5 nm and the oxide layer is present between the Pt layer and the Si substrate, but not on the Pt surface. It is found that the thin chemical oxide layer effectively prevents the Pt diffusion and the silicide formation during the heat treatments. Heat treatments in dry‐ and wet‐oxygen result in nearly the same oxide thickness. Oxygen atoms (or oxygen ions) produced at the Pt surface are suggested to be a diffusing species through the Pt and silicon oxide layers.

Phase‐shifting lithography: Maskmaking and its application

The use of phase‐shifting masks (PSMs) allows substantial improvement in resolution and depth‐of‐focus of optical systems. Before the advantages of PSMs can be realized in a production process, several problems of mask technology must be solved. In order to advance PSMs from a development tool to a production tool for very large scale integration (VLSI) devices, the technologies concerning mask fabrication, mask inspection, and mask repair have been investigated. A silicone‐based resist has been developed for simplifying the mask fabrication processes. The printability and detectability of shifter defects have been investigated in order to specify the requirements of systems of PSM manufacture and inspection. The developed silicone‐based resist has been applied for mask defect repair. These technologies can bring PSMs closer to being a practical production tool for VLSI. To verify the effectiveness of PSMs for device manufacturing, PSMs have been successfully applied to gate definition of a high‐speed GaA...