Naoki Mizutani

Realistic Etch Yield of Fluorocarbon Ions in SiO2 Etch Process

The energy distribution and flux of ions striking an rf-biased electrode were measured by using an rf floating ion energy analyzer. Energies of CF1+, which was the dominant species, were distributed over a voltage range of about half the peak-to-peak bias voltage. Energetic ions with neutral radicals, forming the reactive fluorocarbon polymer layer on a SiO2 film, affected etching characteristics such as rate and selectivity. To investigate the chemical activity of the reactive layer, we estimated the etch yield of SiO2 from the given energy distribution of the ions and the etch rate of SiO2. We found that the energy dependence of the etch yield should be controlled by precisely regulating the flux and composition of neutral radicals under a given ion flux, in order to obtain a high etch rate under the actual SiO2 etching conditions.

Ion energy and angular distribution at the radio frequency biased electrode in an inductively coupled plasma apparatus

The incident energy and angle of ions that bombarded the rf biased electrode in an etching apparatus using inductively coupled Ar–O2 plasma, were analyzed for the pressure of 4–20 mTorr. For the energy analysis at the rf driven electrode, the analyzer was set to rf floating, that is, the electric potential reference of the analyzer was equal to the potential of the rf biased electrode. The ion energy distribution (IED) was measured by the retarding method, and the ion angular distribution (IAD) was measured by annular ion collector rings. The measured IED and IAD were qualitatively explained by a simple calculation model that included charge exchange and elastic collision in the sheath. From a comparison between the measured and calculated results, the ion transverse temperature at the ion sheath edge was estimated to be about 1000 K.

Charge Exchange Ion Energy Distribution at the RF Electrode in a Plasma Etching Chamber

Ar ion energy distribution affected by the charge exchange collision in a sheath was measured by an ion energy analyzer in the rf mode at the rf electrode in a plasma etching chamber. Dependence of the energy distribution on Ar gas pressure ranging from 1.4 to 19.3 mTorr was measured. Under high pressure, an extra peak in addition to the saddle-shaped peaks was observed, and the energy distribution of the ions in the low-energy region increased. These experimental features were adequately explained by a simple calculation model. We also calculated energy distributions of neutrals that were generated by the charge exchange collision, and suggested the connection of the high-energy neutrals with the plasma etching process.

Ion Energy Analysis through rf-Electrode

Energy measurements of ions which impinge on an rf-electrode in plasma were taken using two modes, a dc-mode and an rf-mode. We calculated the ion energy distributions which were obtained using these modes and found that, for the dc-mode, the energy distribution broadened, and the center of the distribution varied, depending on the mass number of the ion. The distribution had extra peaks in addition to the normal saddle-shaped peaks. The features of the calculated distribution agreed well with the experimental results. The rf-mode was necessary to obtain the correct ion energy at the rf-electrode.

Etching characteristics of porous silica (k=1.9) in neutral loop discharge plasma

Etching characteristics of a porous silica material (ISM-1.5™ produced in ULVAC, Inc.) were investigated and compared with those of thermal oxide. The etch rate of porous silica in magnetic neutral loop discharge plasma was approximately two times higher than that of thermal SiO2 film when linear saturated perfluorocarbon compounds were used. This may be due to the low film density of the porous silica. However, in the case of C4F8 (octafluorocyclobutane) plasma, the etch rate ratio to SiO2 was about 1.45. When C4F6 (CF2=CFCF=CF2: hexafluorobutadiene) was used, the etch rate ratio was also very low (0.6). So, the etch rate strongly depended on the gas structure, whereas the SiO2 etch rate did not depend on the gas species and was almost constant. Through mass spectrometry and x-ray photoelectron spectroscopy measurements, it was deduced that the fluorocarbon polymer formed in the pore suppressed the etch rate of porous silica in C4F8 or C4F6 plasmas.

Ion energy and mass analyzer at radio frequency electrode in a plasma chamber

There are two modes for ion energy analysis at a rf electrode in a plasma chamber. One is a dc mode in which the electric potential of the ion energy analyzer is constant in time, and another is a rf mode in which the electric potential of the analyzer oscillates with same frequency, amplitude, and phase as that of the rf electrode. For correct ion energy analysis at the rf electrode, the rf mode is necessary. For ion mass analysis in the rf mode, the electric potential of the ion mass analyzer also must oscillate. We fabricated such an ion energy and mass analyzer, and confirmed its performance.

Measurement of Negative Ions Generated on the Si Etched Surface

Negative ions emerging from etched Si surface were measured in CF4+O2 mixed capacitive coupled plasmas (CCP) in order to observe the surface reaction in real time. Carbon containing negative ions were observed in the low O2 concentration mixed plasma, and C2F4- ion was observed in the high O2 concentration mixed plasma. C2F3- and C2F4- ions abruptly increased at around the O2 mixing ratio of 15%.

MASS ANALYSIS OF NEGATIVE IONS IN ETCHING PLASMA

Negative ions in etching plasma were extracted from a plasma chamber and mass spectra were measured. For the extraction of the negative ions, the plasma sheath was broken by a cone-shaped extractor electrode which had an orifice on the tip. To avoid disturbing the plasma, the tip of the extractor electrode was slightly stuck out of an electrically earthed wall. To remove electrons from the extracted negative charged flux, a magnetic field was applied to the flux. By using the extractor electrode and the magnetic field for the removal of the electrons, the negative ions could be extracted efficiently and the mass spectra could be measured with low noise.

Fabrication and Evaluation of Superconducting Quantum Interference Devices with Nb/Al–AlOx–Al/Nb Edge Junctions

We have fabricated superconducting quantum interference devices (SQUIDs) with an inductance of 140 pH by using sub- µm Nb/Al–AlOx –Al/Nb edge junctions. The small capacitance of the junction resulted in the very large voltage swing of 210 µV and gradient dV/ dΦ of 940 µV/Φ0. Consequently, the very low flux noise of 0.61 µΦ0/ Hz0.5 corresponding to the energy sensitivity e of 8.6h at the white noise level was obtained by using a conventional flux-locked loop configuration. The low-frequency noise was also very low, 2.7 µΦ0/ Hz0.5 corresponding to e= 170 h at 1 Hz. We could obtain very low flux noise for the SQUID which had the practical inductance using only a conventional readout circuit.

Fabrication of High-Quality Nb/Al-AlOx-Al/Nb Junctions by a Simple Process

We have fabricated Nb/Al–AlOx –Al/Nb junctions that have extremely low leakage currents by a simple process using only two mask levels. Reducing the thickness of the Al layer under the AlOx improved the quality. For a 20 µm2 junction, the current at 0.5 mV at 0.5 K was 4 or more orders of magnitude lower than that at 4.2 K. For 0.70 µm2 junctions, the current at 0.5 mV below 1.5 K was 3 or more orders of magnitude lower than that at 4.2 K. We have also fabricated ultra small junctions of high quality. These high-quality junctions can be fabricated by a simple process, hence they are appropriate for various applications.