Takeshi Kitajima

Two-dimensional periodic alignment of self-assembled Ge islands on patterned Si(001) surfaces

Two-dimensional alignment of Ge islands is obtained by molecular beam epitaxy of Ge on lithographically patterned Si(001) surfaces composed of periodic arrays of square Si mesas. When the period of the Si mesa arrays is reduced to 140 nm, a “one island on one mesa” relationship is achieved. The Ge islands have an average base width of 85 nm and take on the shape of a truncated pyramid with four {114} facets and a (001) top. The patterning also serves to improve the island size uniformity. The dependencies of the island morphology on the sizes of the Si mesas and Ge coverages are examined to clarify the mechanism of preferential nucleation of Ge islands on the tops of Si mesas.

Two-dimensional CT images of two-frequency capacitively coupled plasma

Two-dimensional images of two-frequency capacitively coupled plasma (2f-CCP) in Ar and Ar/CF4(5%) in an axisymmetric parallel plate reactor are investigated by using 2D-t optical emission spectroscopy. Spatially averaged electron density is obtained by microwave interferometry. Results are presented in the form of 2D profiles of the net excitation rate of Ar(3p5)(eex=14.5 eV) and Ar+(4p4D7/2)(eex=35.0 eV) used as a probe. Large area uniformity of plasma production driven at very high frequency (VHF) (100 MHz) and that driven at high frequency (HF) (13.56 MHz) at low pressure (∼25 mTorr) are compared and discussed under a low frequency (LF) (700 kHz) bias voltage on the wafer. The time modulation of the net excitation rate and the electron density indicate that the LF bias is considerably influential in the production of the plasma and in the confinement of high energy electrons at HF. Functional separation between plasma production in a gas phase and ion acceleration to the wafer is achieved in 2f-CCP exc...

Increased O(D1) metastable density in highly Ar-diluted oxygen plasmas

Enhancement of the growth rate of SiO2 with a rare gas diluted O2 plasma is of interest for application to various microelectronics fabrications. The key is the oxygen metastable atom (D1) density, which has the potential for surface activation. We used vacuum ultraviolet optical absorption spectroscopy to detect O(D1) and found a twofold increase in the density of O(D1) due to the dilution with Ar. The density increase is reasonably explained by the increase of the electron density, the oxygen dissociation fraction, and the Ar metastable density, that are experimentally obtained for low O2 fractions.

Negative charge injection to a positively charged SiO2 hole exposed to plasma etching in a pulsed two-frequency capacitively coupled plasma in CF4/Ar

As microchips become smaller, the threat of damage to the device elements of Si semiconductors by charging during plasma etching will become significant. It will be of first importance to establish a plasma-etching technique that does not involve charging, i.e., “charging-free plasma processing.” Here, we utilize the effect of negative charge acceleration under a double layer in order to neutralize the charge inside a microstructure exposed to plasma etching. We have constructed a dual measurement system consisting of an emission in an interface and a contact hole charging on a SiO2 wafer during etching in two-frequency capacitively coupled plasma (2f-CCP) in CF4/Ar and pure Ar. A reduction in charging voltage is measured in the pulsed operation both of the plasma power source and of the wafer bias in the 2f-CCP in electronegative gases, CF4/Ar.

Diagnostics for low-energy electrons in a two-frequency capacitively coupled plasma in Ar

An experimental procedure to investigate the spatiotemporal characteristics of electrons in the vicinity of mean energy in a radiofrequency plasma is proposed by using optical emission and absorption spectroscopy in Ar. The method employs optical kinetics of long-lived metastable atom Ar(1s5) and short-lived excited Ar(2p9). The electron density distribution ne(z,t) is demonstrated in a typical condition in a two-frequency capacitively coupled plasma, excited at 100MHz and biased at 500kHz in pure Ar. The density shows almost time independent characteristics in the bulk plasma and a strong time dependence in the sheath in front of the bias electrode.

Experimental Study of Very-High-Frequency Plasmas in H2 by Spatiotemporally Resolved Optical Emission Spectroscopy.

A very-high-frequency (VHF) plasma at 100 MHz is experimentally investigated in parallel-plate configuration in H 2 using spatiotemporal optical emission spectroscopy and current-voltage-power waveforms. The observed optical emission intensity and net excitation rate profiles have strong temporal and spatial variations. The VHF plasma is still capacitive, and is maintained by reflected electrons in the oscillating sheath next to the instantaneous cathode. The characteristics of VHF plasmas, that the excitation rate increases in proportion to the square of driving frequency, and that the minimum sustaining voltage and pressure decrease with increasing frequency, are experimentally confirmed at pressures, ranging from 0.01 to 2 Torr and at the frequency of 100 MHz

Growth modes of InN (000-1) on GaN buffer layers on sapphire

In this work, using atomic force microscopy and scanning tunneling microscopy, we study the surface morphologies of epitaxial InN films grown by plasma-assisted molecular beam epitaxy with intervening GaN buffer layers on sapphire substrates. On smooth GaN buffer layers, nucleation and evolution of three-dimensional InN islands at various coverages and growth temperatures are investigated. The shapes of the InN islands are observed to be predominantly mesa-like with large flat (000-1) tops, which suggests a possible role of indium as a surfactant. Rough GaN buffer layers composed of dense small GaN islands are found to significantly improve uniform InN wetting of the substrates, on which atomically smooth InN films are obtained that show the characteristics of step-flow growth. Scanning tunneling microscopy imaging reveals the defect-mediated surface morphology of smooth InN films, including surface terminations of screw dislocations and a high density of shallow surface pits with depths less than 0.3 nm. The mechanisms of the three-dimensional island size and shape evolution and formation of defects on smooth surfaces are considered.

Imaging of defect-mediated surface morphology of GaN(0001¯) grown on sapphire by molecular beam epitaxy

In situ scanning tunneling microscopy and ex situ atomic force microscopy are used to study the surface morphology of GaN(0001¯) grown on sapphire substrates by plasma-assisted molecular beam epitaxy. Several types of defect-mediated surface morphologies are characterized, including surface terminations of screw and mixed dislocations, defect-induced grooves, and impurity-induced step bunching. In addition to the early theory of dislocation-enhanced crystal growth developed by Frank [F. C. Frank, Acta Crystallogr 4, 497 (1951)] step structures are also found to be critical to understand the formation of spiral growth hillocks around surface terminations of different types of threading dislocations. Growth experiments are also performed under different conditions, e.g., by varying III∕V flux ratio and growth temperature to further investigate the relationship between the final surface morphology and the early stages of growth, such as sapphire nitridation and the transition stage from the three-dimensional...

Influence of driving frequency on oxygen atom density in O2 radio frequency capacitively coupled plasma

The influence of the driving frequency on the absolute oxygen atom density in an O2 radio frequency (RF) capacitively coupled plasma (CCP) was investigated using vacuum ultraviolet absorption spectroscopy with pulse modulation of the main plasma. A low-power operation of a compact inductively coupled plasma light source was enabled to avoid the significant measurement errors caused by self-absorption in the light source. The pulse modulation of the main plasma enabled accurate absorption measurement for high plasma density conditions by eliminating background signals due to light emission from the main plasma. As for the effects of the driving frequency, the effect of VHF (100 MHz) drive on oxygen atom production was small because of the modest increase in plasma density of electronegative O2 in contrast to the significant increase in electron density previously observed for electropositive Ar. The recombination coefficient of oxygen atoms on the electrode surface was obtained from a decay rate in the afterglow by comparison with a diffusion model, and it showed agreement with previously reported values for several electrode materials.

Diagnostics of N2 dissociation in RF plasmas by vacuum ultraviolet emission and absorption spectroscopy

The N2 dissociation degrees in a 50 MHz inductively coupled plasma (ICP) are evaluated by a procedure using vacuum ultraviolet optical emission spectroscopy (VUVOES). The number density of N 4S° atoms (nN) is evaluated from the N2 dissociation degree by determining the N2 rotational temperature that is in equilibrium with the gas temperature. The nN evaluated by VUVOES is crosschecked by the nN measured by vacuum ultraviolet absorption spectroscopy (VUVAS). The comparison reveals a fair proportionality between the nNs evaluated by VUVOES and those measured by VUVAS, suggesting that VUVOES is applicable to the monitoring of N2 dissociation in low-pressure, high-density N2 plasmas. The number density of N 2D° metastables is also measured by VUVAS and found to be comparable to the number density of the ground-level N atoms (4S°). The dissociative recombination of with electrons is suggested as a possible mechanism of N 2D° metastable generation that makes the N 2D° density comparable to the N 4S° density in the pulsed N2 ICP during power-off periods.