Masashi Yamage

Optical Emission and Microwave Field Intensity Measurements in Surface Wave-Excited Planar Plasma

A large-planar (22 cm diam.) high-density ( ~2×1012 cm-3) plasma is produced in argon gas at 140 Pa by 2.45 GHz–1 kW discharges, using a microwave launcher of small slot antennas. The two-dimensional distributions of optical emission intensities as well as microwave field intensities are measured near the plasma surface irradiated with microwaves. Both the optical emission and the microwave field clearly show stationary patterns of azimuthal mode m=3 and radial mode n=3 at higher pressures (140 Pa), while a mode change to m=6 and n=2 is observed at lower pressures (44 Pa). These patterns are attributed to the excitation and absorption of standing surface waves near the cutoff layer.

Inductively Coupled Plasma Source with Internal Straight Antenna

An inductively coupled plasma source with an internal straight antenna was developed. By inserting an antenna into plasma, the induction of a strong electric field in the plasma and the efficient transmissions of power to plasma is enabled. However, there was a practical problem in that antenna sputtering occurred. Suppression of antenna sputtering and methods of insulating the antenna were studied. Consequently, it was found that sputtering impurities were reduced by covering the straight antenna with a quartz pipe. Furthermore, the amount of quartz pipe etching could be reduced to as little as 1/10th the original value. As a result of fabricating and evaluating the plasma source in which four straight antennas were arranged in parallel, electron density was determined to be as high as 1011 cm-3 even at a pressure as low as 4 mTorr. When the processing performance of the plasma source was evaluated, the ashing rate of the photoresist and the etching rate of the poly-Si were, respectively, 4.8 µm/min and 450 nm/min. These values are at practically applicable levels.

End-point Detection of Reactive Ion Etching by Plasma Impedance Monitoring

For the practical application of end-point detection of etching using plasma-impedance monitoring, the factors determining the impedance were clarified using an electric circuit model of a reaction chamber. In the model, plasma is approximated by a conductor, and the floating capacitance and wafer of a powered electrode, as well as the powered-electrode sheath (the sheath formed between the plasma and powered electrode), are approximated by capacitors. Calculated values obtained using the model agree well with measured values obtained by an impedance monitor installed between the powered electrode and the matching network. Based on this result, it was inferred that the impedance depends on the powered-electrode sheath-voltage and electron density, as well as on the floating capacitance and area of the powered electrode, the dielectric constant and thickness of the wafer, and the electron temperature. Next, the end-point detection method of etching by impedance monitoring was applied to reactive ion etching of SiO2 films, and the following finding was confirmed: the change in the impedance significantly depends on the RF power and the exposed area ratio (the ratio of etched area to wafer area) on the wafer. In addition, the feasibility of detecting the point of change of the exposed area ratio on the wafer, where the area size varies during etching, by detecting microchanges in the impedance, was demonstrated. As a result, the possibility of highly accurate end-point detection of etching by impedance monitoring was confirmed.

Wall Temperature Dependence of Boronization Using Decaborane and Diborane

A new boronization technique based on pyrolysis of boron hydrides on hot walls was investigated in a laboratory experiment. The deposition rate of boron films through pyrolysis of decaborane was high enough to apply the pyrolysis to actual fusion devices bakable to 300°C. The hydrogen concentration of boron films prepared by the pyrolysis or conventional plasma-assisted boronization at various temperatures was measured with 15N nuclear reaction and/or IR absorption methods. The hydrogen content markedly decreased with increasing wall temperature, where decaborane always gives higher H content than diborane. Oxygen gettering and hydrogen recycling in a boronized vessel were evaluated by pulsed glow discharge in O2/He and D2, respectively. The results were compared between the pyrolysis and the plasma-based boronizations in decaborane or diborane at various wall temperatures.

Measurement of Electron Density of Reactive Plasma Using a Plasma Oscillation Method

We measured the electron density of reactive plasma using a plasma oscillation probe and analyzed the mode change of power coupling during etching. The unique behavior of plasmas containing reactive negative ions is clarified. The change in the plasma reactor mode during silicon dioxide etching, from the inductive mode to the capacitive mode with increasing pressure, is investigated.

High speed cleaning of boronized wall with a CF4 containing plasma

Rapid cleaning of boron-thin-film deposited walls with use of a CF4 containing plasma was demonstrated. An extremely high removal rate (~40 A/min) of the films was obtained by a dc glow discharge of a CF4 and H2 mixture. Moreover, a 2-3% addition of oxygen enhanced the removal rate by a factor of two, which is about two orders of magnitude larger than that obtained by a conventional H2 discharge. The mass spectroscopic measurement revealed that the film is removed as BF3. The time evolution of gas composition during the cleaning suggests that the oxygen addition suppresses the surface coverage with carbon layers.

New Inductively Coupled Plasma System Using Divided Antenna for Photoresist Ashing

We have developed an inductively coupled plasma (ICP) system with a small chamber for 300-mm-diameter-wafer processes, and a good uniformity of ashing, and both low substrate temperature and low pressure were achieved. The features of this ICP system are substrate temperatures lower than 60°C in order to suppress chemical reactions, and low pressures of 3–5 Pa to suppress the both oxidation of Cu wiring and the degradation of low-k films. Furthermore, the antenna is divided plurally and capacitively coupled. This new antenna can achieve good uniformity in a small chamber because the capacitive coupling to the chamber through a quartz glass window can be easily controlled by reducing series impedance, even when the radio frequency (rf) power is very high. Moreover, the damage to the quartz glass window can be decreased by controlling the series impedance of the antenna, resulting in a long-lasting quartz window. The chamber structure was also optimized by performing an original plasma simulation to improve the uniformity of ashing rate. As results for 300-mm-diameter wafers in the 460-mm-diameter chamber, an average ashing rate of 848 nm/min with a uniformity of ±5.5% was obtained for photoresist films under the following conditions: an O2 gas flow rate of 200 sccm, a substrate temperature of 60°C, a gas pressure of 3 Pa and an rf power of 4 kW.

Formation of Ammonium Salts and Their Effects on Controlling Pattern Geometry in the Reactive Ion Etching Process for Fabricating Aluminum Wiring and Polysilicon Gate

The role of N2 gas addition in the reactive ion etching (RIE) processes for aluminum wiring and polysilicon gate fabrication was investigated. With an increase in the amount of N2 gas added to the etching gas, the pattern profile changed from a reverse to an ordinary taper and the pattern width increased. By AES analysis, nitrogen was detected in the pattern sidewall passivation layer when N2 gas was added. Optical emission spectroscopy of the plasma revealed that hydrogen was supplied from the decomposition product of the photoresist in RIE of aluminum with BCl3/Cl2. XPS, FT-IR and TDS analyses were carried out to study the structure of the passivation layer. Consequently, it was confirmed that nitrogen combined with hydrogen to form N–H bonds in NH4+, and that NH4+ coupled with Cl-, AlCl4- and SiF62- during aluminum and polysilicon RIE, respectively. It was also found that ammonium salts were deposited on the pattern sidewall, and played a major role in controlling the etching profile and pattern width.

Investigation of plasma-induced damage in silicon trench etching

In this study, we investigated the plasma-induced damage in silicon trench etching. The damage was measured by detecting the dark current, which was a very small leakage current thermally generated from silicon crystal defects. The results indicated that both the amount and depth of the sidewall damage in the trenches were almost the same as those of the bottom damage. From the results of analysis and simulation, we considered that the crystal defects inducing isotropic damage were mainly formed by hydrogen ions diffusing in the silicon trenches.